geomatica

Vol. 60, No. 2, 2006 GEOMATICA 121

La géomatique est un champ d'activités qui intègre, selon une approchesystémique, l'ensemble des moyens d'acquisition et de gestion desdonnées à référence spatiale requis pour effectuer les opérations sci-entifiques, administratives, légales et techniques dans le cadre duprocessus de production et de gestion de l'information sur le territoire.

Geomatics is a field of activities which, using a systemic approach,integrates all the means used to acquire and manage spatial datarequired as part of scientific, administrative, legal and technical oper-ations involved in the process of the production and management ofspatial information.

124 Notice to CIG Members / Avis de nomination aux membres

128 IPY GeoNorth 2007 / API GéoNord 2007

212 XXIII International FIG Congress

225 Appointment of New Executive Director

227 ISPRS Commission VIII Symposium

230 Call for Papers, Cartography in Canada 2003-2007

ANNOUNCEMENTS / ANNONCES

125/126 President’s Report / Rapport du présidentTerry Tarle

129/130 Executive Directors Perspective /Le point de vue des directeurs exécutifs

Jean Thie136 Directives aux auteurs / Instructions to Authors192 Instructions to Authors Final Manuscript

206 Geomatics and the Law: Root and BranchAlec McEwen

208 Book ReviewsGerald McGrath

213/217 Industry News / New ProductsRobin Becker

220 The International Scene: ICA, FIG, ISPRSJames S. Simpson

222 Field Notes & Office MemosWilliam R. (Bill) Brookes

226 Fifty Years AgoHarold E. Jones

229 Association of Canada Lands Surveyors /Association des arpenteurs des terres du Canada

231 Canadian Council of Land Surveyors /232 Conseil Canadien des Arpenteurs-Géomètres

233 Obituaries: Karl Kraus and James George

234 Calendar of Events / Calendrier des événements

236 Sustaining Members, Profiles and Membership

GEOMATICA1

No. 22006Vol. 60

131/133 Introduction Guest Editor /Introduction Rédacteur en chef invitéRobert Duval

137 Space Geodetic Techniques and the Canadian SpatialReference System Evolution, Status and Possibilities

P. Héroux, J. Kouba, N. Beck, F. Lahaye, Y. MireaultP. Tétreault, P. Collins, K. MacLeod, M. Caissy

151 The Evolution of NAD83 in CanadaMichael R. Craymer

165 A Gravimetric Geoid Model as a Vertical Datum in CanadaM. Véronneau, R. Duval and J. Huang

173 Crustal Motion and Deformation Monitoring of the Canadian Landmass

J.A. Henton, M.R. Craymer, H. Dragert, S. MazzottiR. Ferland and D. L. Forbes

193 Global Geodetic Observing System—Considerations forthe Geodetic Network Infrastructure

M. Pearlman, Z, Altamimi, N. Beck, R. ForsbergW. Gurtner, S. Kenyon, D. Behrend, F.G. LemoineC. Ma, C.E. Noll, E.C. Pavlis, Z. MalkinA.W. Moore, F.H. Webb, R.E. Neilan, J.C. RiesM. Rothacher and P. Willis

FEATURES / ARTICLES DE FOND DEPARTMENTS / CHRONIQUES

The Canadian Expedition Beta Team at the Mars Desert ResearchStation in Utah. (see Field Notes & Office Memos page 222 / voirpage 222)

CANADIAN SPATIALREFERENCE SYSTEM

INTRODUCTIONRobert Duval, Geodetic Survey Division, Natural Resources Canada

Ever wanted to be in two places atonce? We all know it isn’t possible andwe can hardly believe that we could bemisled to think so. But in this modernworld, where spatial geo-referencingusing coordinates (latitude, longitude,and height) is becoming ubiquitous,the likelihood of being confused bydifferent coordinates being associatedto a same location is increasing.Obviously, merging or integration ofgeospatial or georeferenced informa-tion works best when a point or anobject can be associated to a singlecoordinate.

Nowadays, GPS receivers arefinding their way to almost everywhereand expanding georeferencing capabili-ties far beyond the traditional geomaticscommunity: transport and taxi compa-nies use them to track vehicle locations;they are used in farming and forestryoperations for autonomous platformguidance for crop harvesting and otherrobotic field operations; and cars usethem for navigation, with some systemseven able to alert emergency services toyour exact location if you’ve beeninvolved in an accident! GPS technol-ogy is making positioning easier thanever before and consequently impactinghow we organize information, coordi-nate activities, and manage assets. To beuseful though, positions must be com-patible with one another, otherwise theirutility is compromised and confusionfollows. It is a bit like time! Imagine thechaos within a city, a province, or nationif we all operated with different time“systems” and had no means of rec-onciling them? A standard time systemused by all ultimately ensures that theactivities of a society are synchronized

and organized in a way that allows it tofunction coherently.

Today, the Canadian Spatial ReferenceSystem (CSRS) provided by the EarthSciences Sector (ESS) of NaturalResources Canada (NRCan) continues torespond to the Order in Council that creat-ed the Geodetic Survey of Canada in 1909and tasked the then Minister of Interior “todetermine with the highest attainableaccuracy the positions of points throughoutthe country… which may form the basis ofsurveys for all purposes, topographical,engineering or cadastral, and thereby assistin the survey work carried on by otherdepartments of the Dominion Government,by Provincial Governments, and bymunicipalities, private persons or corpora-tions.” This statement, though nearly 100years old, continues to capture the essenceof the fundamental role played by theCSRS today. Not unlike the time standardanalogy, the CSRS provides the funda-mental reference values (or the startingcoordinates) to ensure that positions any-where in Canada can be determineduniquely and remain compatible with oneanother regardless of when they were deter-mined or their originators. The resultingframes of reference, propagated throughprovincial and municipal networks as wellas other governmental services, serve asstandards which ensure the compatibility ofCanadian georeferenced information.

Over the last 20 years, in order torespond to the needs of diverse user groupsand to adapt to technological changes,several new geodetic (or spatial) referencesystems have emerged, each time intro-ducing a new reference frame [a “geodeticreference system” is a theoretical conceptand consist of a collection of prescribedprinciples, fundamental parameters, and

specifications to quantitativelydescribe the positions of points inspace; in contrast, a “referenceframe” is much easier to relate to, asit is the materialization or ‘realiza-tion’ of such a prescription]. In addi-tion, although the theoretical definitionof these reference systems did notchange, new realizations were intro-duced as measurement accuracyimproved. This adds to the potentialfor some confusion within the usercommunity as to what the basis fortheir coordinates should be.

Traditionally, a reference frameconsisted of ground-based geodeticcontrol monuments with adopted coor-dinates. In Canada, to ensure nation-wide coordination and consistency, theestablishment of primary horizontaland vertical networks of geodetic con-trol points has traditionally been a fed-eral responsibility. To facilitate down-stream user access, provincial, munic-ipal, and other governmental agencieshave further densified the network“fabric,” establishing a hierarchicalstructure. For most of the last century,surveyors “connected” to these controlpoints using classical techniques tointegrate their surveys within theadopted reference frame, the standardensuring the compatibility of Canadiangeoreferenced information.

Today, with the advent of GlobalNavigation Satellite Systems, andcurrently GPS in particular, themeans for connecting to the referenceframes have evolved significantly. Avariety of tools and data products arenow available to maximize the effi-ciency and the benefits of these mod-ern technologies. Additionally the


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widespread availability of low-costGPS equipment in the consumer mar-ket is rapidly increasing the user basefor georeferenced information. Alongwith these changes comes theincreased potential for incompatiblecoordinates due to the diversity of ref-erence frames available. It is thereforeimportant not only to facilitate theuser’s access to these reference framesbut also to provide information aboutthem, including their time-evolutionand the related impacts on their georef-erenced information. This is an objec-tive of this Special Issue of Geomatica.

In concert with these new devel-opments, the possibility of integratingdirectly into a stable global referenceframe is also becoming realitythrough applications such as PrecisePoint Positioning. Although this newability is appealing, the requirementto maintain consistency with theadopted reference systems forCanada, the North American Datumof 1983 (NAD83) and the CanadianGeodetic Vertical Datum of 1928(CGVD28), continues to be a priority.The legacy left by the monumentedgeodetic infrastructure still requiresthat relationships between old andnew reference frame realizations beclearly established to maintain coordi-nate traceability over time.Additionally, the desire to maintainstable coordinates for control pointsthat are monumented on the surface ofour “restless planet” requires that ref-erence frame options be available tominimize disruption to the generalusers while also meeting the needs ofmore demanding user communities.

Another fundamental role alsoplayed by the CSRS is its contributionto the determination of underlyingdynamic parameters of the Earth suchas its orientation and rotation rate inspace as well as its gravity field.These parameters, determined fromthe collective effort of the internation-al geodetic community, are not onlycritical to the efficient exploitation ofmodern space-based technology forpositioning and navigation but arealso enablers behind the technologyitself. Furthermore, monitoring varia-

tions in both the Earth’s geometry and itsgravity field, essential to the maintenanceof the reference frames, also contributes toa better understanding of regional andglobal geophysical processes linked tonatural hazards and global change.

The use of reference frames providedthrough the Canadian Spatial ReferenceSystem has extended from traditionalapplications in mapping and cadastral sur-veys to monitoring sea level rise, rectifyingremote sensing imagery, measuring crustaluplift and subsidence, and interpretingseismic disturbances. However, the funda-mental role the CSRS plays has notchanged much over the years, and today itcontinues to provide the means to ensurewe can only be “in one place at one time!”In this Special Issue, papers authoredmainly by personnel from NRCan addressdifferent aspects regarding the evolutionof geodetic reference systems in Canada aswell as provide a look forward.

Space Geodetic Techniques and theCSRS - Evolution, Status and Possibilitieslooks at the evolution of technology overthe past century, with particular emphasison the advent of navigation satellites dur-ing the past decades, and the impact thistechnology has had on reference frameaccuracy and access. Now that the posi-tions (or, in a general sense, the orbits) ofnavigation satellites are continuously esti-mated with centimetre accuracy and sub-sequently made publicly available throughthe Internet, a geodetic control networkhas in essence been created in the sky. As aresult, our dependence on an infrastructureof ground-based monuments is increasing-ly being lessened. This new capability isalso giving rise to the development of newmethodologies for end users to achievegreater efficiency and enhanced accuracyof their positioning information withrespect to a common reference system.

In The Evolution of NAD83 inCanada, the reader will be provided insightsinto our national horizontal reference sys-tem including: the evolution of NAD83from a horizontal to a three-dimensionalreference frame, how it is now linked to thedynamic International Terrestrial ReferenceFrame (ITRF), and how it is maintainedusing active control points and episodicmeasurements of the Canadian BaseNetwork. The improvement in NAD83

accuracy that came along with GPS isshown. Crustal movements driven bygeophysical processes, observed atregional and national scales and capa-ble of affecting reference frame stabil-ity, are now apparent and their impactsare also discussed.

A Gravimetric Geoid Model as aVertical Datum in Canada gives read-ers a glimpse of how the vertical con-trol network has developed nationallyover time and how the advent of spacegeodesy has led the Earth ScienceSector to initiate the “HeightModernization” project towards theadoption of a gravimetric geoid modelas the new vertical datum in Canada.The improvements in geoid modelling,in part attributable to the availability ofdata from recently launched (CHAMPand GRACE) and upcoming (GOCE)satellite gravimetry missions, as well asthe centimetre accuracy available forellipsoidal height determination fromprecisely processed signals from navi-gation satellites, are the major enablersbehind this new direction. The antici-pated operational benefits that stemfrom not having to physically connectto a benchmark for precise heightdetermination are appealing from theperspective of both the user and theprovider of height information.

While geodetic reference framesunderpin the measurements of themotions and slow deformations of theEarth’s crust, these geophysicalprocesses also systematically affect thereference frames provided as standardsfor geodetic surveys. In CrustalMotion and Deformation Monitoringof the Canadian Landmass, some ofNRCan’s efforts to monitor contempo-rary crustal dynamics across Canadaare reported. Progressing from conti-nental to smaller regional scales, therationale, techniques, and results areoutlined in this paper. While therequired observational data and inter-pretations are fundamentally depend-ent on the Canadian Spatial ReferenceSystem, they in turn also contribute tothe incremental improvement of itsdefinition and maintenance.

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Continued on page 135

Avez-vous déjà souhaité être àdeux endroits en même temps? Noussavons tous que c’est impossible etavons du mal à croire qu’on pourraitintentionnellement nous le faireaccroire. Pourtant, en ce monde mo-derne où le géoréférencement spatial àbase de coordonnées (latitude, longi-tude et hauteur) est très répandu, il estde plus en plus possible qu’on attribuedes coordonnées différentes au mêmeendroit. Évidemment, l’intégration oula fusion de données géospatiales ougéocodées fonctionne mieux lorsque lepoint ou l’objet n’est associé qu’à uneseule coordonnée.

De nos jours, les récepteurs GPSsont de plus en plus omniprésents etleurs capacités de géoréférencementsont beaucoup plus avancées qu’aupa-ravant. Les compagnies de taxi et detransport les utilisent pour suivre leursvéhicules; les compagnies forestièreset agricoles, pour le guidage de plates-formes autonomes pour la récolte descultures ou pour les opérations robo-tisées sur le terrain. Ils se retrouventaussi dans plusieurs véhicules automo-bile à des fins d’aide à la navigation.Certains de ces systèmes peuventmême avertir les services d’urgence etleur fournir votre situation géo-graphique exacte en cas d’accident!La technologie GPS rend le position-nement encore plus facile qu’aupara-vant et a des répercussions sur lafaçon dont nous organisons l’informa-tion, coordonnons les activités etgérons nos avoirs. Cependant, afind’être réellement utilisables, les coor-données doivent être compatibles les

unes avec les autres sinon leurs utilité enserait compromise et s’ensuivrait une con-fusion générale. C’est un peu comme letemps! Imaginez le désastre si dans uneville, une province ou un pays, on utilisaittous des systèmes différents pour mesurerle temps et n’avions aucune façon de lesconcilier? L’utilisation d’un système com-mun de mesure du temps permet de s’as-surer que toutes les activités d’une sociétésont synchronisées et organisées de façon àce qu’elle fonctionne de manière cohérente.

Aujourd’hui, le Système canadien deréférence spatiale (SCRS), sous la gouver-nance du Secteur des sciences de la Terre(SST) de Ressources naturelles Canada(RNCan), continue à répondre au décretqui a créé les Levés géodésiques duCanada en 1909 et demandait alors auministre de l’Intérieur de « déterminer, leplus précisément possible, la position despoints partout au pays… qui pourraientformer la base de levés à toutes sortes defins (topographiques, cadastrales oud’ingénierie) et aideraient ainsi tous lesautres ministères du Dominion du Canada,les gouvernements provinciaux, les muni-cipalités, les personnes et les entreprises àmener leurs travaux d’arpentage ». Ceténoncé, bien qu’âgé de près de cent ans,continue à saisir toute l’importance du rôlefondamental joué par le SCRS aujour-d’hui.

Tout comme pour notre exemple dutemps, le SCRS nous offre des valeurs deréférence fondamentales (ou des coordon-nées de départ) afin de s’assurer que lespositions de partout au Canada soientdéterminées de sorte qu’elles soientuniques par rapport à ces points deréférence et demeurent compatibles entres

elles peu importe la date ou la sourcequi les a déterminées. Le cadre deréférence, transmis par les réseauxprovinciaux et municipaux ainsi quepar d’autres services gouvernemen-taux, sert de norme qui assure ainsi lacompatibilité de toutes les donnéesgéoréférencées au Canada.

Au cours des deux dernièresdécennies, on a eu besoin de s’adapterà divers changements technologiques;plusieurs systèmes de référencegéodésique (ou géospatiale) ont étécréés afin de répondre aux besoins desdifférents groupes d’utilisateurs, en yajoutant à chaque fois, un nouveaucadre de référence un « système deréférence géodésique » est un conceptthéorique qui consiste en un ensemblede principes, de paramètres fondamen-taux et de spécifications pour décrire laposition de points dans l’espace demanière quantitative. En comparaison,un « cadre de référence » est beau-coup plus facile à concevoir puisqu’ils’agit de la matérialisation ou de laréalisation d’un tel concept. De plus,de nouvelles réalisations ont été intro-duites au fur et à mesure que la préci-sion des mesures s’améliorait, bien quela définition théorique de ces systèmesde référence n’ait pas changé. Ce quiajoute à la confusion probable de lacommunauté d’usagers sur la nature dela base de leurs coordonnées.

Traditionnellement, un cadre deréférence était constitué de pointsphysiques de contrôle terrestre aux-quels on attribuait des coordonnées.Au Canada, afin de s’assurer de lacoordination et de la cohérence, le


SYSTÈME CANADIEN DERÉFÉRENCE SPATIALE

INTRODUCTIONRobert Duval, Division des levés géodésiques, Ressources naturelles Canada

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gouvernement fédéral était respon-sable des réseaux géodésiques primairesde points de contrôle horizontaux etverticaux. Ensuite, afin de faciliterl’accès aux usagers, les gouvernementsprovinciaux et municipaux et d’autresorganismes gouvernementaux ontdensifié le réseau, établissant ainsiune structure hiérarchique. Au coursde la majorité du siècle dernier, lesarpenteurs se sont rattachés à cespoints de contrôle en utilisant destechniques traditionnelles pour queleurs levés s’intègrent au cadre deréférence adopté; la norme qui permetd’assurer la compatibilité de toutes lesdonnées géocodées au pays.

Aujourd’hui, avec l’avancée dessystèmes mondiaux de navigation parsatellite, et du GPS en particulier, lesmoyens de se rattacher aux cadres deréférence ont évolué de manière signi-ficative. Il existe bon nombre d’outils etde produits pour nous aider à manièrel’efficacité et les avantages de cestechnologies modernes. La based’usagers de données géoréférencéess’agrandit très rapidement, le matérielGPS étant de plus en plus disponible,et à faible coût, aux consommateursmoyens. Le risque de coordonnéesincompatibles augmente donc à causedu nombre de cadres de référencedisponibles. Il est donc importantqu’on facilite non seulement l’accèsdes usagers à ces cadres de référencemais aussi qu’on leur fournisse desrenseignements à leur sujet, incluant leurévolution temporelle et les répercussionssur les données qui leur sontgéoréférencées. C’est précisémentl’objectif de ce numéro spécial deGeomatica.

La possibilité de s’intégrerdirectement dans un cadre deréférence global stable devient aussiréelle par des applications telles quele positionnement par points précis.Bien que cette nouvelle capacité soitintéressante, la priorité demeure d’as-surer la cohérence avec les systèmesde référence canadiens : le Systèmede référence nord-américain de 1983(NAD83) et le Système de référencealtimétrique géodésique de 1928(CGVD28). L’héritage laissé par cette

infrastructure géodésique de points decontrôle nécessite une relation claire entreles réalisations du nouveau et de l’anciencadres de référence afin d’assurer la traça-bilité des coordonnées dans le temps.Puisqu’on désire maintenir des coordon-nées stables pour des points de contrôlephysiques sur notre planète en pleineeffervescence, nous aurons besoin d’op-tions pour ces cadres de référence afin deminimiser les dérangements pour lesusagers moyens tout en rencontrant lesbesoins des communautés d’usagers plusspécialisés.

Le SCRS joue un autre rôle essentielen contribuant à la détermination desparamètres dynamiques fondamentaux dela Terre comme son orientation, sa vitessede rotation dans l’espace ainsi que sonchamp gravitationnel. Ces paramètres,déterminés à partir d’un effort collectif dela communauté internationale géodésique,sont non seulement essentiels à l’opérationefficace de la technologie moderne spa-tiale pour le positionnement et la naviga-tion mais aussi pour soutenir la technolo-gie elle-même. On doit aussi surveiller lesvariations à la géométrie et au champgravitationnel terrestres afin d’assurer lapertinence des cadres de référence. Cetteopération permet aussi de mieux compren-dre les processus géophysiques régionauxet globaux reliés aux risques naturels etaux changements mondiaux.

L’utilisation des cadres de référence duSystème canadien de référence spatiale apermis d’étendre les applications tradi-tionnelles de la cartographie et des levéscadastraux à la surveillance du niveau desocéans, à la rectification d’images detélédétection, à la mesure du soulèvementou de l’affaissement de la croûte terrestreet à l’interprétation des secousses sis-miques. Le rôle principal du SCRS n’acependant pas beaucoup changé au fil dutemps et il continue aujourd’hui à s’assurerque nous ne soyons qu’à un seul endroit à lafois! Les articles de ce numéro spécial ontété principalement écrits par du personnelde RNCan. Leurs articles touchent à diversaspects de l’évolution des systèmes deréférence géodésique au Canada ainsiqu’une vision de l’avenir.

L’article sur les Techniquesgéodésiques spatiales et le SCRS - Évolu-tion, état et possibilités nous offre un

regard sur l’évolution de la technolo-gie au cours du siècle dernier, en met-tant particulièrement l’accent sur l’a-vancement des satellites de navigationau cours des dernières décennies et lesrépercussions de cette technologie surla précision et l’accessibilité du cadrede référence. Maintenant que les posi-tions (c.-à-d., au sens général, lesorbites) des satellites de navigationsont constamment évaluées à une pré-cision d’un centimètre et ensuiteoffertes au public par Internet, unréseau de contrôle s’est en quelquesorte créé dans le ciel. Notre dépen-dance envers une infrastructurefondée sur des points terrestres dimi-nue conséquemment. Cette nouvellecapacité a aussi ouvert la porte àl’élaboration de nouvelles méthodespour les usagers finaux leur permet-tant d’obtenir des coordonnées plusprécises et plus efficaces par rapport àun système de référence commun.

Dans l’article sur L’évolution duNAD83 au Canada, le lecteur auradroit à un aperçu de notre système deréférence horizontal national incluantl’évolution du NAD83 d’un cadre deréférence horizontal à un cadre deréférence tridimensionnel, comment ilest maintenant relié au Repère interna-tional de référence terrestre (ITRF) etcomment il est maintenu par l’utilisa-tion de points de contrôle actifs et laprise de mesures périodiques duRéseau de base canadien. La précisiondu NAD83 a été améliorée grâce auGPS. Le mouvement de la croûte ter-restre résulte des processus géo-physiques observés à l’échellerégionale et nationale et peut se réper-cuter sur la stabilité du cadre deréférence. Ce mouvement est aujour-d’hui perceptible et on discute de sesrépercussions.

L’article intitulé Un modèle gra-vimétrique du géoïde comme systèmede référence altimétrique au Canadadonne au lecteur un aperçu de la façondont le réseau de contrôle vertical s’estétabli à l’échelle nationale et commentles avancées en géodésie spatiale ontmené le Secteur des sciences de le Terreà lancer le projet « Modernisation deshauteurs » qui visait l’adoption d’un

134 GEOMATICA Vol. 60, No. 2, 2006

modèle gravimétrique du géoïdecomme nouveau système de référencealtimétrique au Canada. Cette nou-velle direction a été rendue possiblegrâce aux avancées en modélisationdu géoïde, attribuables en partie à ladisponibilité de données des nou-veaux satellites (CHAMP et GRACE)et du futur satellite (GOCE) en mis-sions gravimétriques et le niveau deprécision, au centimètre près, des hau-teurs au-dessus de l’ellipsoïdeobtenues à partir des signaux très pré-cis envoyés par les satellites de navi-gation. Les avantages opérationnelsanticipés de cette initiative, qui nousévite d’avoir à se rattacher physique-ment à un point géodésique verticalpour déterminer la hauteur précise,sont intéressants pour les usagers etpour les fournisseurs des renseigne-ments sur la hauteur.

Alors que les cadres de référencegéodésique soutiennent les mesuresdes mouvements et des lentes défor-mations de la croûte terrestre, cesprocessus géophysiques modifientaussi systématiquement les cadres deréférence qui servent de normes auxlevés géodésiques. Dans l’article surla Surveillance du mouvement et desdéformations de la croûte de lamasse continentale canadienne, onparle de certains des efforts de sur-veillance de la dynamique contempo-raine de la croûte terrestre au Canadapar Ressources naturelles Canada. Lajustification, les techniques et lesrésultats sont décrits dans cet article,de l’échelle continentale à l’échellerégionale. Alors que les données d’ob-servation et les interprétations néces-saires dépendent essentiellement duSystème canadien de référence spa-tiale, elles contribuent aussi àl’amélioration progressive de sa défi-nition et à sa maintenance.

À notre ère des technologies glo-bales, les cadres de référence modernesdoivent s’arrimer à une « norme »

In this era of global technologies,modern reference frames requirealignment with a global “standard.”Collectively realized by geodeticagencies around the world under theauspice of the International Associationof Geodesy (IAG), the InternationalTerrestrial Reference Frame providesthis foundation. Recognizing the needfor better coordination of internationalefforts to satisfy long-term require-ments, the IAG has initiated the GlobalGeodetic Observing System (GGOS)project and has specifically tasked aworking group to develop a strategy tofurther integrate and maintain the fun-damental geodetic network of instru-ments and supporting infrastructure ina sustainable manner. In GlobalGeodetic Observing System –Considerations for the GeodeticNetwork Infrastructure, a global inte-gration process is proposed thatincludes the development of a networkof fundamental stations with co-locatedgeodetic techniques and preciselydetermined inter-system relationships.The design of this network wouldexploit the strengths of each techniqueand minimize the weaknesses wherepossible. The paper summarizes thepresent state of the infrastructure andprovides a roadmap to future globalgeodetic networks, services, and prod-ucts.

In closing, I would like to thankall contributing authors, includingthose whose papers were not selectedfor publication in this Special Issue.For the papers not presented here, Ilook forward to their possible publica-tion in future issues of Geomatica.Special thanks to Pierre Héroux whohas taken on the coordination of thiseffort within the Geodetic SurveyDivision and without whom this issuewould have not materialized. Finally, Iwould like to extend my thanks toKelly Dean, Editor, Geomatica andCarol Railer, Production andAdvertising Manager, for facilitatingthe publication of this special issueand, despite a very tight schedule, forbeing so accommodating at the finalediting stages of this production. o


globale. Le Repère international deréférence terrestre, fruit des efforts con-jugués des agences géodésiques de partoutau monde sous la supervision del’Association internationale de géodésie(AIG), répond à ce besoin. L’AIG recon-naît la nécessité de mieux coordonner lesefforts internationaux afin de répondre ànos besoins à long terme et a donc lancél’initiative du système global d’observationgéodésique [Global Geodetic ObservingSystem (GGOS)] et a fondé un groupe detravail spécifiquement pour élaborer unestratégie qui visera à intégrer et à maintenirle réseau géodésique d’instruments de baseet l’infrastructure qui le soutient d’unefaçon durable. L’article sur le Systèmeglobal d’observation géodésique - Aspectsimportants de l’infrastructure du réseaugéodésique, propose un processus d’inté-gration global qui comprend l’élaborationd’un réseau de stations de base, des tech-niques géodésiques de colocalisation etdes liens entre les systèmes déterminésavec précision. La conception de ce réseaupermettrait de profiter des forces dechaque technique et minimiserait leursfaiblesses, autant que possible. Cet articleprésente donc un survol de l’état actuel del’infrastructure et un aperçu des futursréseaux, services et produits géodésiquesglobaux.

En conclusion, je tiens à remerciertous les auteurs, incluant ceux dont les arti-cles ne seront pas publiés dans ce numérospécial. Ces derniers paraîtront peut-êtredans de prochains numéros de Geomatica,du moins je l’espère. Je souhaite aussiremercier plus particulièrement PierreHéroux pour avoir coordonné les efforts ausein de la Division des levés géodésiques.Sans lui, ce numéro n’aurait pu être publié.Finalement, je tiens aussi à remercier KellyDean, rédactrice en chef de Geomatica etCarol Railer, responsable de la productionet de la publicité, d’avoir facilité la publi-cation de ce numéro spécial, et ce, malgréun échéancier très serré, et d’avoir été sicoopératives à l’occasion des étapes finalesde mise en page de ce numéro. o

Continued from page 132

The following manuscripts have been printed with permission from ©Her Majesty the Queenin right of Canada (2006)

Les manuscrits suivants ont été imprimés avec la permission de © Sa Majesté la Reine du chefdu Canada (2006).

SPACE GEODETIC TECHNIQUES AND THE CANADIAN SPATIAL REFERENCE SYSTEM EVOLUTION, STATUS AND POSSIBILITIES P. Héroux, J. Kouba, N. Beck, F. Lahaye, Y. Mireault, P. Tétreault, P. Collins, K. MacLeod, M. Caissy Geodetic Survey Division, Natural Resources Canada, Ottawa, Ontario Over the last two decades a revolution has taken place in the field of positioning and navigation. The availability and accuracy of signals from Global Navigation Satellite Systems (GNSS), combined with advances in microelectronics, have greatly improved our ability to georeference information. While positioning was traditionally the business of professionals in the field of surveying and geodesy, it has now become a commodity readily available to a wide range of users, from professional surveyors to recreational amateurs. The Canadian Spatial Reference System (CSRS) has been evolving over this time to facilitate access to the reference frame with innovative products. These new products and associated tools are taking advantage of the widespread availability of navigation satellite signals and the popularity of the Internet to respond to Canadians in a way that enables seamless integration of their geospatial data, on location and in real-time. The move to space geodetic techniques for delivery of the CSRS is presented in this paper. Au cours des deux dernières décennies, nous avons traversé une révolution dans le domaine du positionnement et de la navigation. L’accessibilité et la précision des signaux des systèmes mondiaux de navigation par satellite (GNSS), en plus des avancées en microélectronique, ont grandement amélioré notre capacité à géocoder les données. Alors qu’auparavant, le positionnement relevait de professionnels des levés et de la géodésie, il est maintenant accessible à une grande variété d’usagers, qu’ils soient des arpenteurs-géomètres ou des amateurs de plein air. Le Système canadien de référence spatiale (SCRS) a évolué au cours de ces années afin de faciliter l’accès au cadre de référence par des produits innovateurs. Ces nouveaux produits et outils connexes tirent profit de l’accessibilité globale aux signaux des satellites de navigation et de la popularité de l’Internet afin de répondre aux Canadiens d’une façon qui permet l’intégration parfaite, instantanée et sur place de leurs données géospatiales. Dans cet article, nous vous présenterons la transition du SCRS vers ces techniques géodésiques spatiales. THE EVOLUTION OF NAD83 IN CANADA Michael R. Craymer Geodetic Survey Division, Natural Resources Canada, Ottawa, Ontario The North American Datum of 1983 (NAD83) is the national spatial reference system used for georeferencing by most federal and provincial agencies in Canada. The physical realization of this system has undergone several updates since it was first introduced in 1986. It has evolved from a traditional, ground-based horizontal control network to a space-based 3D realization fully supporting more modern positioning techniques and the integration of both horizontal and vertical reference systems. After a brief review of previous reference systems

used in Canada, the original definition of NAD83 and its subsequent updates are described, focusing on the definition of the current implementation NAD83(CSRS) and its relationship with other reference systems. Official transformation parameters between NAD83(CSRS) and ITRF (including WGS84) are provided for use throughout Canada. Possible future reference systems for Canada and North America are also examined. Le Système de référence nord-américain de 1983 (NAD83) est le système de référence spatiale national utilisé pour la géoréférence par la plupart des agences fédérales et provinciales au Canada. La réalisation physique de ce système a nécessité plusieurs mises à jour depuis son entrée en vigueur en 1986. Le système a évolué d’un réseau de contrôle horizontal terrestre à une réalisation spatiale tridimensionnelle comprenant des techniques de positionnement plus modernes et intégrant les systèmes de référence horizontale et verticale. Après une brève revue des systèmes de référence utilisés au Canada, la définition originale du NAD83 et ses mises à jour subséquentes sont décrites, en se concentrant sur la définition de la mise en oeuvre actuelle du NAD83 (SCRS) et sa relation avec d’autres systèmes de référence. Les paramètres officiels de transformation entre le NAD83 (SCRS) et l’ITRF (incluant le WGS84) sont accessibles aux usagers pour tout le Canada. On examine aussi d’autres systèmes de référence possibles pour le Canada et l’Amérique du Nord à l’avenir. A GRAVIMETRIC GEOID MODEL AS A VERTICAL DATUM IN CANADA Marc Véronneau, Robert Duval and Jianliang Huang Geodetic Survey Division, Natural Resources Canada, Ottawa, Ontario [email protected] The need for a new vertical datum in Canada dates back to 1976 when a study group at the Geodetic Survey Division (GSD) of Natural Resources Canada investigated problems related to the existing vertical reference system (CGVD28) and recommended a redefinition of the vertical datum. The US National Geodetic Survey and GSD cooperated in the development of a new North American Vertical Datum (NAVD88). Although the USA adopted NAVD88 as its datum in the early 90s, Canada did not follow suit because unexplained discrepancies of about 1.5 m were still present between east and west coasts. GSD continued to maintain and expand the vertical datum using the spirit levelling technique; however related cost and inherent deficiencies to this technique has forced GSD to rethink its approach for the delivery of the height reference system in Canada. Meanwhile, advances in space-based technologies and new developments in geoid modelling have emerged and now offer an alternative to spirit levelling. A new project to modernize the vertical datum is currently in progress in Canada. GSD is planning the adoption of a geoid model as the new vertical datum, which will allow users of space-based positioning technologies access to an accurate and uniform vertical datum everywhere across the Canadian landmass and surrounding oceans. Furthermore, this new vertical datum will be less sensitive to geodynamic activity, local crustal uplift and subsidence, and deterioration of benchmarks. Le Canada a senti le besoin de se doter d’un nouveau système de référence altimétrique en 1976 alors qu’un groupe d’étude à la Division des levés géodésiques (DLG) de Ressources naturelles Canada étudiait des problèmes liés au système de référence altimétrique existant

(CGVD28) et avait recommandé une nouvelle définition du système de référence altimétrique. Le National Geodetic Survey des États-Unis et la DLG ont travaillé ensemble à l’élaboration d’un nouveau système de référence altimétrique nord-américain (NAVD88). Bien que les États-Unis aient adopté le NAVD88 et comme système de référence altimétrique vers le début des années 1990, le Canada ne l’a pas adopté à cause de lacunes inexpliquées d’environ 1,5 mètre entre les côtes Est et Ouest. La DLG a continué à maintenir et à améliorer le système de référence altimétrique en utilisant le nivellement de précision (à bulle). Cependant, à cause des défauts et des coûts associés à cette technique, la DLG a dû trouver une autre façon d’établir le système de référence altimétrique au Canada. Entre temps, les technologies spatiales et la modélisation du géoïde ont connu des percées importantes et offrent maintenant une alternative intéressante au nivellement de précision (à bulle). Ce projet est d’ailleurs déjà lancé au Canada. La DLG planifie l’adoption d’un modèle de géoïde en tant que système de référence altimétrique, ce qui permettra aux usagers des technologies de positionnement spatiales d’y accéder pour partout sur le continent canadien et les océans qui nous entourent. De plus, ce nouveau systtème de référence altimétrique sera moins sensible à l’activité géodynamique, au soulèvement ou à l’affaissement local de la croûte terrestre et à la détérioration des repères de nivellement. CRUSTAL MOTION AND DEFORMATION MONITORING OF THE CANADIAN LANDMASS Joseph A. Henton, Michael R. Craymer and Rémi Ferland Geodetic Survey Division Natural Resources Canada, Ottawa, Ontario Herb Dragert and Stéphane Mazzotti Geological Survey of Canada, Natural Resources Canada, Sidney, British Columbia Donald L. Forbes Geological Survey of Canada, Natural Resources Canada, Dartmouth, Nova Scotia The science of geodesy and the corresponding reference systems it develops have increasingly been applied to measuring motions and slow deformations of the Earth’s crust driven by plate tectonics. Improvements to geodetic methodologies have therefore enabled better understanding of the Earth’s systems, including improved modelling and forecasting of changes that may affect society. These geophysical processes also systematically affect the reference frames used as standards for geodetic surveys. Reference frames therefore must not only define the system of coordinate axes (including orientation, origin, and scale), but also characterize the time-evolution of spatial coordinates on the Earth’s surface. When evaluating the effect on reference standards within a given area, it is also important to realize that geodynamic processes operate on various spatial scales. In this paper we summarize some of NRCan’s efforts to monitor contemporary crustal dynamics across Canada. Progressing from continental to smaller regional scales, we outline the rationale, techniques, and results. The observational data and interpretations presented are fundamentally dependent on the Canadian Spatial Reference System yet in turn also contribute to the incremental improvement of its definition and maintenance.

La science de la géodésie et les systèmes de référence correspondants ont été de plus en plus utilisés pour mesurer les mouvements et les lentes déformations de la croûte terrestre causés par les plaques tectoniques. L’amélioration des méthodes géodésiques nous a donc permis de mieux comprendre les systèmes de la Terre en nous permettant, entres autres, de mieux modéliser et prévoir les changements qui risquent de nous toucher. Ces processus géophysiques modifient aussi systématiquement les cadres de référence qui servent de normes aux levés géodésiques. Les cadres de référence doivent alors non seulement définir le système des axes des coordonnées (incluant l’orientation, l’origine et l’échelle), mais doivent aussi définir l’évolution temporelle des coordonnées spatiales sur la surface terrestre. Lorsqu’on évalue leur effet sur les normes de référence dans une zone donnée, il est aussi important de réaliser que les processus géodynamiques se produisent à plusieurs échelles spatiales. Dans cet article, nous résumerons certains des efforts de surveillance de la dynamique contemporaine de la croûte terrestre canadienne par Ressources naturelles Canada. De l’échelle continentale à l’échelle régionale, nous présenterons un survol du besoin, des techniques et des résultats. Les données et les interprétations observationnelles présentées dépendent fondamentalement du Système canadien de référence spatiale tout en contribuant à l’amélioration de sa définition et à sa maintenance. GLOBAL GEODETIC OBSERVING SYSTEM—CONSIDERATIONS FOR THE GEODETIC NETWORK INFRASTRUCTURE M. Pearlman, Harvard-Smithsonian Center for Astrophysics (CFA), Cambridge MA Z. Altamimi, Institut Géographique National, Marne-La-Vallée, France N. Beck, Geodetic Survey Division-Natural Resources Canada, Ottawa, Ontario R. Forsberg, Danish National Space Center, Copenhagen, Denmark W. Gurtner, Astronomical Institute University of Bern, Bern, Switzerland S. Kenyon, National Geospatial-Intelligence Agency, Arnold, MO D. Behrend, F.G. Lemoine, C. Ma, C.E. Noll, E.C. Pavlis NASA Goddard Space Flight Center, Greenbelt, MD Z. Malkin, Institute of Applied Astronomy, St. Petersburg, Russia A.W. Moore, F.H. Webb, R.E. Neilan, Jet Propulsion Laboratory California Institute of Technology, Pasadena, CA J.C. Ries, Center for Space Research, The University of Texas, Austin, TX M. Rothacher, GeoForschnungsZentrum Potsdam, Potsdam, Germany P. Willis, Institut Géographique National, Saint Mande, France Properly designed and structured ground-based geodetic networks materialize the reference systems to support sub-millimetre global change measurements over space, time, and evolving technologies. The Ground Networks and Communications Working Group (GN&C WG) of the International Association of Geodesy’s Global Geodetic Observing System (GGOS) has been working with the IAG measurement services (the IGS, ILRS, IVS, IDS and IGFS) to develop a strategy for building, integrating, and maintaining the fundamental network of instruments and supporting infrastructure in a sustainable way to satisfy the long-term (10 to 20 years) requirements identified by the GGOS Science Council. Activities of this Working Group include the investigation of the status quo and the development of a plan for full network integration to support improvements in terrestrial

reference frame establishment and maintenance, Earth orientation and gravity field monitoring, precision orbit determination, and other geodetic and gravimetric applications required for the long-term observation of global change. This integration process includes the development of a network of fundamental stations with as many co-located techniques as possible, with precisely determined intersystem vectors. This network would exploit the strengths of each technique and minimize the weaknesses where possible. This paper discusses the organization of the working group, the work done to date, and future tasks. Des réseaux géodésiques terrestres bien conçus et structurés permettent de matérialiser les systèmes de référence afin de prendre en compte les changements mondiaux dans l’espace, le temps et les nouvelles technologies à un niveau inframillimétrique. Le groupe de travail sur les communications et les réseaux terrestres (Ground Networks and Communications Working Group (GN&C WG)) du Système global d’observation géodésique (GGOS) de l’Association internationale de géodésie (AIG) a travaillé avec les services de prises de mesures de l’AIG (l’IGS, l’ILRS, le SIR, l’IDS et l’IGFS) afin d’élaborer une stratégie pour édifier, intégrer et maintenir le réseau essentiel d’instruments et d’infrastructures de façon durable afin de répondre aux besoins à long terme (10 à 20 ans) cernés par le Conseil des sciences du GGOS. Le Groupe de travail se prête notamment à l’évaluation du statu quo et à l’élaboration d’un plan pour l’intégration complète du réseau afin de comprendre les améliorations à l’élaboration et au maintien du cadre de référence terrestre, la surveillance de l’orientation et du champ gravitationnel terrestres, la détermination précise de l’orbite et d’autres applications géodésiques et gravimétrique nécessaires à l’observation des changements mondiaux à long terme. Ce processus d’intégration comprend l’élaboration d’un réseau de stations principales intégrant autant de techniques conjointes que possible et de vecteurs, déterminés avec précision, entre les systèmes. Ce réseau exploiterait les forces de chacune des techniques et minimiserait leurs faiblesses. Cet article présente l’organisation du groupe de travail, le travail accompli à ce jour ainsi que ses prochaines tâches.

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THE EVOLUTION OF NAD83 IN CANADA

Michael R. CraymerGeodetic Survey Division, Natural Resources Canada, Ottawa, Ontario

The North American Datum of 1983 (NAD83) is the national spatial reference system used for georef-erencing by most federal and provincial agencies in Canada. The physical realization of this system hasundergone several updates since it was first introduced in 1986. It has evolved from a traditional, ground-based horizontal control network to a space-based 3D realization fully supporting more modern position-ing techniques and the integration of both horizontal and vertical reference systems. After a brief review ofprevious reference systems used in Canada, the original definition of NAD83 and its subsequent updates aredescribed, focusing on the definition of the current implementation NAD83(CSRS) and its relationship withother reference systems. Official transformation parameters between NAD83(CSRS) and ITRF (includingWGS84) are provided for use throughout Canada. Possible future reference systems for Canada and NorthAmerica are also examined.

Introduction

The Geodetic Survey Division (GSD) ofNatural Resources Canada has a mandate to estab-lish and maintain a geodetic reference system as anational standard for spatial positioning throughoutCanada. In general terms, a reference system is anabstract collection of principles, fundamental param-eters, and specifications for quantitatively describingthe positions of points in space. A reference frame isthe physical manifestation or realization of such aprescription. Traditionally, a reference frame con-sists of a network of geodetic control points on theground with adopted coordinates that other surveyscan be tied and referenced to. Since the introductionof the Global Positioning System, this paradigm hasbeen changing.

Mapping, GIS, scientific and other organiza-tions make large investments in georeferenced dataand demand that the integrity of the reference sys-tem be maintained and enhanced to keep pace withthe way they obtain their positioning data.Consequently, GSD is constantly improving the

reference system and periodically publishes newcoordinates effectively representing updated realiza-tions of the reference system. Such updates usuallyresult from densification of the network of controlpoints, elimination of blunders and distortions,improvements in accuracies, and the introduction ofnew positioning methodologies like GPS. At thesame time, continuity must be maintained to ensurelegacy data, based on previous reference systemsand realizations, can be incorporated into the currentreference frame.

The current reference system adopted as anational georeferencing standard by most federaland provincial agencies in Canada and endorsed bythe Canadian Council on Geomatics [CCOG 2006]is the North American Datum of 1983 (NAD83).NAD83 has undergone several updates since its firstrealization in 1986. This paper describes thesechanges, focusing on the current implementation andits relationship with other reference systems. It also

GEOMATICA Vol. 60, No. 2, 2006, pp. 151 to 164

Le Système de référence nord-américain de 1983 (NAD83) est le système de référence spatiale nationalutilisé pour la géoréférence par la plupart des agences fédérales et provinciales au Canada. La réalisationphysique de ce système a nécessité plusieurs mises à jour depuis son entrée en vigueur en 1986. Le système aévolué d’un réseau de contrôle horizontal terrestre à une réalisation spatiale tridimensionnelle comprenant destechniques de positionnement plus modernes et intégrant les systèmes de référence horizontale et verticale.Après une brève revue des systèmes de référence utilisés au Canada, la définition originale du NAD83 et sesmises à jour subséquentes sont décrites, en se concentrant sur la définition de la mise en oeuvre actuelle duNAD83 (SCRS) et sa relation avec d’autres systèmes de référence. Les paramètres officiels de transformationentre le NAD83 (SCRS) et l’ITRF (incluant le WGS84) sont accessibles aux usagers pour tout le Canada. Onexamine aussi d’autres systèmes de référence possibles pour le Canada et l’Amérique du Nord à l’avenir.

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briefly examines possible future reference systemsfor Canada and North America.

Original Realization ofNAD83 – NAD83(Original)

The first continental reference system for NorthAmerica was the North American Datum of 1927(NAD27). It was defined as a reference ellipsoid thatwas positioned and oriented using classical astro-nomical observations to best fit North America. Therealization of this reference system consisted of anetwork of thousands of geodetic control monu-ments (physical makers in the ground) spaced about20 to 100 km apart at locations chosen for intervisi-bility but which were usually inconvenient to access.This was only a horizontal network originally builtup primarily from triangulation surveys in whichsystematic errors accumulated resulting in wide-spread distortions throughout the network. Becauseof the limited computational resources at the time,densification of the reference frame was performedin a piece-wise fashion by holding existing controlpoints fixed to their published values. This furtherpropagated the accumulation of errors by distortingnewer, often more accurate data. For more informa-tion about NAD27, see Junkins and Garrard [1998].

In a cooperative effort to reduce the distortionsin the reference frame and to obtain a system morecompatible with new space-based positioning tech-nologies, Canada, the US, Mexico, and Denmark(Greenland) began a readjustment of the entire con-tinental network using a new reference system calledthe North American Datum of 1983 (NAD83). TheNAD83 system was based on a global referencesystem known as the BIH Terrestrial System 1984(BTS84) together with the reference ellipsoid of theGeodetic Reference System 1980 (GRS80). BTS84was an earth-centred (geocentric) reference frameproduced by the Bureau International de l’Heure(BIH) using spaced-based data from lunar laserranging (LLR), satellite laser ranging (SLR), verylong baseline interferometry (VLBI) and the satelliteDoppler system. It was the most accurate referenceframe available at the time.

Using a relatively dense framework of newDoppler stations across the continent, the NAD83reference frame was brought into alignment withBTS84 using an internationally adopted transforma-tion between BTS84 and the Doppler referenceframe NWL 9D [Boal and Henderson 1988]. Abouta dozen VLBI stations in Canada and the US werealso included to provide a connection to the celestialreference frame. As we shall see below, these VLBI

sites provide the only link between NAD83 andmore modern, stable reference frames. The conti-nental network was then readjusted in 1986 using astepwise methodology known as Helmert blocking.This initial realization is denoted here asNAD83(1986).

Densification

Although the US included their entire hierarchyof networks in the NAD83(1986) adjustment, fromhighest accuracy geodetic to the lowest-ordermunicipal networks, Canada included only its pri-mary control network of about 8000 stations. Thisframework was then densified through subsequentso-called secondary integration adjustments incooperation with the provincial geodetic agencies[Parent 1988]. The first of these was the 1989Eastern Secondary Integration Helmert BlockAdjustment (ESHIBA, now referred to as justSHIBA), that included provincial networks fromOntario eastward. Only a 374-station primary net-work was included in the Maritimes which hadadopted their own new reference system (seebelow). Shortly after, the Western SecondaryIntegration Helmert Block Adjustment (WSHIBA)was completed in 1990 with the western provinces.The same year, NAD83 was proclaimed the officialgeodetic reference frame for federal governmentoperations [EMR 1990]. To assist incorporatinglegacy NAD27 data into NAD83, an official trans-formation and distortion model called the NationalTransformation (NT) was developed [Junkins 1988].

Immediately after the completion of WSHIBA,some western provinces began major GPS surveyingcampaigns to densify and improve their networks.There were also many new federal networks in thenorthern territories. Rather than create confusion byadopting WSHIBA results and subsequently updat-ing them shortly after, it was decided to redo thewestern adjustment with the new data. This newadjustment, completed and made public in 1993, wascalled the Network Maintenance Integration Projectof 1993 (NMIP93).

The ESHIBA and NMIP93 realizations ofNAD83 were the last of the major federal-provincialcooperative adjustment projects and are collectivelyreferred to as NAD83(Original). This network isshown in Figure 1. Based on the ESHIBA andNMIP93 realizations of NAD83, an improvedtransformation from NAD27 was developed.Known as the National Transformation Version 2(NTv2) [Junkins 1990], this new transformationprovided much improved distortion modelling thatadapted to the variations in the spatial density ofnetwork points.

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This furtherpropagatedthe accumu-lation oferrors bydistortingnewer, oftenmore accu-rate data.

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Limitations

At about the same time as these traditionaladjustment projects, a major advancement was tak-ing place in GPS technology and in the realizationof global reference frames. It was at this time thatthe International GPS Service (IGS), through thecooperative efforts of GSD and several other geo-detic agencies around the world, began producingprecise GPS satellite orbits that enabled centimetre-level positioning accuracies in 3D [Beutler et al.1999]. These orbits were computed using a collec-tion of permanent GPS tracking stations on theground, including several in Canada that becamethe Canadian Active Control System (CACS)[Duval et al. 1997]. The number of federal trackingstations has since increased to nearly 50, resultingin even greater improvements in the accuracy of theGPS orbits and positioning results based on them.In essence, the geodetic control network was shift-ing to the GPS satellites in space (see Héroux et al.[this issue]).

At the time of its initial realization, NAD83(and BTS84) was intended to be a geocentric systemand was compatible with the other geocentric sys-tems of the time, including the original realization ofWGS84. However, due to the use of more accuratetechniques, it is now known that NAD83 is offset byabout 2 m from the true geocentre.

Another limitation of the original realizationsof NAD83 was that access to it was provided main-ly through a horizontal control network. Today,many applications of GPS require a 3D referenceframe. Yet another problem revealed by GPS wasthe limited accuracy of conventional horizontalcontrol networks. The significant accumulation oferrors in both the observations and methods of net-work integration were being revealed by the use ofnew GPS survey techniques. Figure 2 illustratesthese errors at points across Canada by comparingNAD83(Original) coordinates to those based onhigh accuracy GPS surveys tied almost directly tothe fundamental reference frame of NAD83. Errorsin the horizontal network are about 0.3 m on aver-age but can exceed 1 m in the northern parts ofmany provinces.

3D Realization of NAD83 –NAD83(CSRS)

In light of the above limitations ofNAD83(Original), a more accurate, true 3D real-ization of the NAD83 reference frame was clearlyneeded which enabled users to relate their positions

more directly to the fundamental definition of theNAD83 reference frame. Together with a highaccuracy geoid (see Veronneau et al. [this issue]), acomplete 3D reference frame would also enable theconvergence of traditional horizontal and verticalreference systems into a single unified system able tosupport all aspects of spatial positioning (see Hérouxet al. [this issue] and Veronneau et al. [this issue]).

Since 1990 the most accurate and stable refer-ence frames available are the successive versions ofthe International Terrestrial Reference Frame(ITRF) produced by the International EarthRotation and Reference Systems Service (IERS).Individual realizations are denoted by ITRFxx,where xx represents the last year for which datawas included in a particular solution. These refer-ence frames are based primarily on SLR, VLBI,GPS and a system called DORIS (Déterminationd’Orbite et Radiopositionement Intégré par

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Figure 1: Traditional horizontal control network comprisingthe original realization of NAD83.

Figure 2: Errors in NAD83(Original) as revealed by high accuracy GPSobservations in NAD83(CSRS).

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Satellite) [Boucher and Altamimi 1996]. A key dif-ference with previous reference systems is thedynamic nature of the reference frame. Coordinatesfor stations are valid for a specific date (epoch) andare accompanied by velocity estimates for propa-gating coordinates to other epochs.

During the first several years, new realizationsof ITRF were introduced on a nearly annual inter-val as significant amounts of new data were added.Now that well over 15 years of data are available,the realizations of ITRF have stabilized to about acentimetre. Consequently, new versions arereleased less frequently. Presently, the last tworeleases were ITRF97 and ITRF2000 [Altamimi etal. 2002]. A new ITRF2005 is due sometime thisyear and is likely to be the last official public ver-sion for several years. Scientific updates areexpected to be released more frequently to densifythe reference frame and improve velocity estima-tion for new stations.

Realizing the benefits of using such a highlystable global reference frame, at the 20th GeneralAssembly of the International Union of Geodesyand Geophysics in 1991, the InternationalAssociation of Geodesy (IAG) adopted ResolutionNo. 1 which, among other things, made the follow-ing two recommendations [IAG 1992]:

“1) that groups making highly accurate geodetic,geodynamic or oceanographic analysis should eitheruse the ITRF directly or carefully tie their own sys-tems to it”

“4) that for high accuracy in continental areas, a sys-tem moving with a rigid [tectonic] plate may beused to eliminate unnecessary velocities provided itcoincides exactly with the ITRS at a specific epoch”

Considering recommendation (4), it wasassumed that recommendation (1) also allowed for theuse of other systems such as NAD83 providing theyare carefully tied to the ITRS. Note that the ITRFcoordinates of points are constantly changing due tothe motions of the individual tectonic plates. It istherefore necessary to specify which epoch ITRFcoordinates refer to and to account for tectonic motionwhen propagating coordinates to other epochs.

Rather than abandon NAD83 altogether infavour of ITRF (as some countries have done), it wasdecided to define NAD83 by its precise relationshipwith ITRF which would comply with the IAG res-olution. A precise connection between ITRF andNAD83 was made possible by the common VLBIstations in both systems. This allowed for the deter-mination of a conformal 3D seven-parameter simi-larity (Helmert) transformation between the two

reference frames. The transformation effectivelyprovides a more accurate realization of the funda-mental NAD83 3D reference frame in terms of theITRF. It also provides any user with convenient andnearly direct access to the highest levels of theNAD83 reference frame. This enables users todetermine accurate positions that are highly consis-tent across the entire continent. Moreover, GPSorbits can be transformed to NAD83 allowing usersto position themselves directly in NAD83 throughapplications such as Precise Point Positioning(PPP) [Héroux et al. this issue].

1996 Realization – NAD83(CSRS96)

The first ITRF-NAD83 transformation adoptedby both Canada and the US was determined withrespect to ITRF89 in the early 1990s [Soler et al.1992]. The scale of ITRF, derived in part fromVLBI stations in Canada and the US, was adoptedfor compatibility with more recent versions ofWGS84 by setting the estimated scale parameterto zero. This realization of the NAD83 referenceframe was denoted as NAD83(CSRS96), where“96” indicates the year the transformation wasintroduced (not any particular coordinate epoch).The transformation was used to produce NAD83coordinates for CACS stations and allowed forGPS orbits to be generated in NAD83.

Unfortunately, following the adoption of thisfirst ITRF-NAD83 transformation, Canada and theUS chose different methods of updating the trans-formation to new ITRF realizations. Canada used theofficial incremental transformations between differ-ent versions of ITRF as published by the IERS. TheUS, on the other hand, recomputed the transforma-tion for each new ITRF, adopting the slightly differ-ent scale of each ITRF. Consequently, the updatedtransformations differed slightly, mainly in scale.This resulted in ellipsoidal height discrepancies ofabout 5 cm along the common borders by the timeITRF96 was introduced in 1998.

1998 Realization – NAD83(CSRS)

In order to reconcile the slightly different real-izations of NAD83 in Canada and the US arisingfrom these different ITRF-NAD83 transformations,a new common NAD83 transformation was derivedwith respect to ITRF96, the most recent at the time.The data used in determining the transformationwere the NAD83(Original) and ITRF96 coordi-nates at 12 VLBI stations in Canada and the US(see Figure 3). These are the only fundamentalpoints in the original definition of NAD83 with 3Dcoordinates in both NAD83 and ITRF96.

154

Unfortunately…Canadaand the USchose differ-ent methodsof updatingthe transfor-mation tonew ITRFrealizations.

Using the ITRF96 coordinates at epoch1997.0, a new seven-parameter similarity(Helmert) transformation was determined[Craymer et al. 2000]. The scale of ITRF96 wasadopted for this realization of NAD83 by settingthe scale parameter to zero after estimation. Thisensures the scale of NAD83 will be compatiblewith the more accurate scale defined by ITRF96and used by other systems such as WGS84. Theestimated parameters are given in Table 1.

In order to correctly account for the tectonicmotion of the North American tectonic plate whentransforming from/to ITRF96 positions at any arbi-trary epoch, the NNR-NUVEL-1A plate motionmodel was adopted [DeMets et al. 1994] as recom-mended by the IERS [McCarthy 1996]. Larson etal. [1997] had shown NNR-NUVEL-1A to be inrelatively good agreement with velocities estimatedfrom GPS in North America at that time (based onmore data it is now known to be slightly biased).The effect of this motion can be treated as addi-tional rotations of the reference frame defined by

(1)

where RX,RY,RZ are rotations about the geocentricCartesian coordinate axes in units of milliarcsecondsper year (mas/y).

To ensure a consistent application of the trans-formation to other ITRF realizations, both Canadaand the US also agreed to adopt the most currentIERS values for transforming between ITRF96 andother ITRF reference frames. The only exceptionwas the incremental transformation between ITRF96and ITRF97 where the GPS-based IGS transforma-tion was used to account for a systematic bias in theGPS networks used in ITRF97.

This new realization of NAD83 was originallydenoted as NAD83(CSRS98) to distinguish it fromthe 1996 realization. Like the 1996 realization,“98” refers to the year it was adopted and not to anycoordinate epoch. However, because theNAD83(CSRS96) realization saw very limited use,the name of the new realization has since beenshorted to just NAD83(CSRS).

The main advantage of this improvedNAD83(CSRS) realization is that it providesalmost direct access to the highest level of theNAD83 reference frame through ties to the CACSand collocated VLBI stations that form part of theITRF network. These stations effectively act asboth ITRF and NAD83 datum points for geospatialpositioning, thereby enabling more accurate, con-venient, and direct integration of user data with

practically no accumulation of error typically foundin classical horizontal control networks.

It is important to bear in mind that the NAD83reference system itself has not changed. It is onlythe method of physically defining or realizing itthat has been updated to make NAD83 more accu-rate and stable, and more easily accessible to moreusers. Any differences between NAD83(Original)and NAD83(CSRS) reflect primarily the muchlarger errors in the original. Each successive updateis generally more accurate than, but fully consistentwith, previous realizations.

Hierarchy of NAD83(CSRS)Networks

The new NAD83(CSRS) realization wasaccompanied by a transition to a new referenceframe structure for Canada (see Figure 4). The tradi-tional horizontal network hierarchy that constitutedthe original realization of NAD83 was replacedwith a more modern framework that takes advan-tage of advanced GPS methods and enables more

RXRYRZ

=0.0532

– 0.7423– 0.0316

mas/y

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Figure 3: VLBI stations included in NAD83(Original) and used in the ITRF96transformation.

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accurate and more convenient access to theNAD83(CSRS) reference frame.

This new reference frame hierarchy is dividedinto active and passive components as illustrated inFigure 4. The active component consists of net-works of continuously operating GPS receivers andproducts derived from them, such as precise orbitsand broadcast corrections. The passive componentis comprised of more traditional monumented con-trol points that users can occupy with their ownequipment.

Active Component

At the top of the “active” reference frame hier-archy are the VLBI and CACS stations that are partof the global ITRF reference frame. Using theadopted transformation, the ITRF coordinates forthese stations can be converted to NAD83 without

156

Table 1: ITRF to NAD83 transformation parameters at an epoch of 1997.0 and their rates of change (mas = milliarcsec,ppb = parts per billion).

TX m TY m TZ m RX mas RY mas RZ mas DS ppbdTX m/y dTY m/y dTZ m/y dRX mas/y dRY mas/y dRZ mas/y dDS ppb/y

ITRF88 0.9730 -1.9072 -0.4209 -25.890 -9.650 -11.660 -7.4000.0000 0.0000 0.0000 -0.053 0.742 0.032 0.000

ITRF89 0.9680 -1.9432 -0.4449 -25.790 -9.650 -11.660 -4.3000.0000 0.0000 0.0000 -0.053 0.742 0.032 0.000

ITRF90 0.9730 -1.9192 -0.4829 -25.790 -9.650 -11.660 -0.9000.0000 0.0000 0.0000 -0.053 0.742 0.032 0.000

ITRF91 0.9710 -1.9232 -0.4989 -25.790 -9.650 -11.660 -0.600WGS84(G730) 0.0000 0.0000 0.0000 -0.053 0.742 0.032 0.000

ITRF92 0.9830 -1.9092 -0.5049 -25.790 -9.650 -11.660 0.8000.0000 0.0000 0.0000 -0.053 0.742 0.032 0.000

ITRF93 1.0111 -1.9058 -0.5051 -24.410 -8.740 -11.150 -0.4000.0029 -0.0004 -0.0008 0.057 0.932 -0.018 0.000

ITRF94 0.9910 -1.9072 -0.5129 -25.790 -9.650 -11.660 0.000WGS84(G873) 0.0000 0.0000 0.0000 -0.053 0.742 0.032 0.000

ITRF96 0.9910 -1.9072 -0.5129 -25.790 -9.650 -11.660 0.0000.0000 0.0000 0.0000 -0.0532 0.7423 0.0316 0.000

ITRF97 0.9889 -1.9074 -0.5030 -25.915 -9.426 -11.599 -0.9350.0007 -0.0001 0.0019 -0.067 0.757 0.031 -0.192

ITRF2000 0.9956 -1.9013 -0.5214 -25.915 -9.426 -11.599 0.615WGS84(G1150) 0.0007 -0.0007 0.0005 -0.067 0.757 0.051 -0.182

Figure 4: Hierarchy of NAD83(CSRS) reference frame.

any loss of accuracy or continuity with previousITRF or NAD83(CSRS) realizations. Moreover,the data for these GPS stations are available to thegeneral public thereby enabling users for the firsttime to tie directly to the highest level of NAD83reference frame. In the old hierarchal networkstructure, users were generally only able to connectto control points in the lower levels of the networkhierarchy with their attendant lower accuracies.

At the level below the CACS sites are addition-al continuously operating GPS receivers, collective-ly referred to as regional ACPs. These regional ACPnetworks were installed in support of specific localand regional projects to determine crustal motionsand monitor sea level rise. They can be considered adensification of the ITRF and IGS global networks.Some examples of these regional networks are theWestern Arctic Deformation Network (WARDEN)and the Western Canada Deformation Array(WCDA) (some of these regional ACPs have recent-ly been incorporated into the ITRF global network)See Henton et al. [this issue] for more discussion ofthese networks.

In addition to the federally operated CACS sta-tions, some provinces have implemented their ownnetwork of active GPS stations that provide data andDGPS corrections to the general public. Someexamples of such systems can be found in BritishColumbia, Quebec, and soon New Brunswick. A fewprivate companies have also installed DGPS systemin various regions. Most of these services charge afee for access to the DGPS corrections. Although theprovincial systems generally tie their DGPS stationsto NAD83(CSRS), not all private systems do. Somesystems have only been tied to the original realiza-tion of NAD83 and thus their accuracies will bedegraded by any local distortions. In some areas thedistortions are fairly coherent enabling accurate rel-ative positioning (cf. Figure 2). However, problemsmight arise if using such services across areas wherethe distortions are quite different.

Note that a Canada-wide DGPS Service(CDGPS) has also been created through collabora-tion between all the provincial and federal geodeticagencies based on NRCan’s wide-area GPSCorrections (GPS•C). Broadcast nationwide viaCanada’s own MSAT communication satellite, thisservice provides sub-metre positions directly inNAD83(CSRS) nearly everywhere in Canada. Formore information about CDGPS and GPS•C seeHéroux et al. [this issue] and the CDGPS web siteat www.cdgps.com.

In addition to providing the link to the globalreference frame, the CACS stations and someregional ACPs contribute to the InternationalGNSS Service (IGS) efforts to produce, among

other products, the most accurate GPS orbits avail-able. Although computed in the ITRF referenceframe of date, these orbits are easily transformed toNAD83(CSRS) like any other coordinates using theadopted ITRF-NAD83(CSRS) transformation. Byusing such precise IGS orbits, users can determinepoint positions directly in NAD83(CSRS). For moreinformation about these products see Héroux et al.[this issue]. In essence, the satellites themselves haveeffectively become an extension of theNAD83(CSRS) reference frame available to users.

Passive Component

In order to assist with the integration of theolder horizontal control networks intoNAD83(CSRS) a new, much sparser but more stablenetwork of “passive” control points was establishedand tied directly to the CACS stations (see Figure 5).Called the Canadian Base Network (CBN), this net-work forms the next level of the reference framehierarchy below the CACS. It is the highest level ofthe passive component of the CSRS reference frame.

The CBN consists of approximately 160 highlystable, forced-centering pillars. This network wasoriginally conceived as an interim measure or transi-tion during the move to a CACS-only referenceframe. However, the CBN has proven to be invalu-able for monitoring the on-going deformation of theCanadian landmass for scientific studies and thelong-term maintenance of the reference frame. Todate, there have been three complete measurementsof the CBN. The quality of these surveys has beenheld in high regard by many scientists because of

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Figure 5: Federal component of the NAD83(CSRS) reference frame.

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the unprecedented spatial detail the results haverevealed about the motions of the Earth’s crust (seeHenton et al. [this issue]). Public interest has alsobeen very high as indicated by much media interest[AP 2004; CanWest 2004; The Globe and Mail2004; The Guardian 2004; The Independent 2004;The New Scientist 2004; UPI 2004; TheWashington Post 2004].

During the establishment of the CBN, theprovincial agencies began densifying the networkfor their own requirements. These densificationsare often referred to as a provincial high precisionnetworks (HPNs). High accuracy ties between theCBN and various HPNs were made during the firstmeasurement campaign of the CBN enabling theprovinces to integrate their traditional networksinto NAD83(CSRS).

The 8000 stations of the primary horizontalcontrol network were not entirely abandoned bythis new reference frame structure. Rather theprovinces assumed the responsibility for theirmaintenance and integration into NAD83(CSRS).Most provinces have readjusted this data togetherwith their own (secondary) horizontal control net-works. These networks provided the main source ofinformation for the development of NTv2 distor-tion models for converting large holdings of geo-referenced data from NAD83(Original) toNAD83(CSRS).

Relationship to OtherReference Frames

ITRF

The transformation between NAD83(CSRS)and any realization of ITRF at any arbitrary epoch (t)can be obtained by combining the definitiveITRF96-NAD83 transformation previouslydescribed together with the incremental between-ITRF transformations and the NNR-NUVEL-1Arotations defining the motion of the North Americantectonic plate. The resulting Helmert transformationcan be written as [Craymer et al. 2000]

(2)

where

XN,YN,ZN are the geocentric Cartesian coordi-nates in NAD83(CSRS)

XI(t),YI(t),ZI(t) are the geocentric Cartesiancoordinates in ITRF at epoch t

TX(t) = TX + dTX ⋅ (t-1997.0) mTY(t) = TY + dTY ⋅ (t-1997.0) mTZ(t) = TZ + dTZ ⋅ (t-1997.0) mRX(t) = [RX + dRX ⋅ (t-1997.0)] ⋅ k radRY(t) = [RY + dRY ⋅ (t-1997.0)] ⋅ k radRZ(t) = [RZ + dRZ ⋅ (t-1997.0)] ⋅ k radDS(t) = DS + dDS ⋅ (t-1997.0) ppbt = epoch of ITRF coordinatesk = 4.84813681 x 10-9 rad/mas

All these parameters are time-dependent due totectonic plate motion and the rates of change ofsome of the incremental transformation parametersbetween different ITRFs. Note that the rotations inthese expressions are given as positive in a clock-wise direction following the non-standard conven-tion used by the IERS. Table 1 summarizes theseparameters for all ITRF realizations available at thetime of this paper (ITRF2005 is expected to bereleased this year). See also Soler and Snay [2004]for further discussion of the ITRF2000-NAD83transformation.

In addition to transforming coordinates, it isalso possible to transform GPS baseline vectors.Because vectors contain no absolute positionalinformation, the translational part of the transfor-mation is not used. Only the rotations and scalechange are applied to the vector coordinate differ-ences as follows:

(3)where

∆XN,∆YN,∆ZN are the geocentric Cartesian coordi-nate differences in NAD83(CSRS)

∆XI(t),∆YI(t),∆ZI(t) are the geocentric Cartesiancoordinate differences in ITRF at epoch t

Although the effect of the rotations and scalechange on baseline vectors is relatively small (ofthe order of 0.1 ppm) and may be neglected in somecases, they systematically accumulate throughout anetwork and can amount to a significant error insome situations. Because the application of thetransformation is relatively simple, it is recom-mended to always transform baseline vectorsunless one is sure they will never be assembled toconstruct larger networks.

Velocities in ITRF can also be transformedinto NAD83(CSRS). This involves only the rates ofchange of the transformation parameters defined in

158

XNYNZ N

=TX tTY tTZ t

+1 + DS t

RZ t– RY t

–RZ t RY t1 + DS t –RX t

RX t 1 + DS t

XI tYI tZ I t

∆X N

∆Y N

∆Z N

=1 + DS t

RZ t– RY t

–RZ t RY t1 + DS t –RX t

RX t 1 + DS t

∆X I t∆Y I t∆Z I t

eqn. (2) and Table 1. These parameters represent theNNR-NUVEL-1A velocity for the North Americanplate and some small drifts in the origin, orientation,and scale of different realizations of the ITRF. Thetransformation can be expressed as

(4)where

VXN,VYN

,VZNare the geocentric Cartesian

velocities in NAD83(CSRS)VXI

,VYI,VZI

are the geocentric Cartesian veloc-ities in ITRF at epoch t

These ITRFxx-NAD83(CSRS) transforma-tions have been implemented in software availablefrom GSD and the US National Geodetic Survey(NGS). GSD’s software is called TRNOBS and willtransform input data files of positions and positiondifferences for GSD’s own GHOST adjustmentsoftware as well as for the commercial GeoLab™software. For US users, the HTDP (Horizontal TimeDependent Positioning) software will transform datafiles of positions and velocities in NGS Blue Bookformat. On-line versions and Fortran source codefor both TRNOBS and HTDP are available at therespective agency’s web sites.

WGS84

The World Geodetic System 1984 [NIMA 2004]is a global reference frame originally developed bythe US Defense Mapping Agency (subsequentlyrenamed the National Imagery and MappingAgency (NIMA) and now called the NationalGeospatial-Intelligence Agency (NGA)). It wasused for mapping campaigns around the world andis the “native” reference frame used by GPS.

WGS84 is unique in that there is no physicalnetwork of ground points that can be used as geo-detic control. The only control points available to thepublic are the satellites themselves, defined by thebroadcast orbits. Because of the relative inaccuracyof these orbits and further degradation prior to May1, 2000 due to the implementation of selectiveavailability (S/A), public users could not get trueWGS84 positions to better than about 10-50 m.Accuracies improved to about 3-5 m when S/A wasturned off and even better accuracies of a metre orless can now be achieved with correction servicessuch as the Wide Area Augmentation System(WAAS). It was at this time that many users began

to notice a systematic bias between WGS84 andNAD83 of about 1.5 m in the horizontal and a metrein the vertical.

Originally, WGS84 was defined in a similarmanner as NAD83. It used a global network ofDoppler stations to align itself with the sameBTS84 reference frame used by NAD83. Thus,WGS84 was identical with NAD83 in the begin-ning. Based on this original realization, NIMAdetermined simple average geocentric Cartesiancoordinate shifts (translations) between WGS84and many local datums around the world. BecauseNAD83 and WGS84 were defined by the sameBTS84 reference frame, the shift between thesesystems was zero [NIMA 2004].

Several years later, in an effort to improve itsstability and accuracy, WGS84 was redefined interms of ITRF [Slater and Malys 1998; NIMA 2004].In doing so, the WGS84 reference frame was shiftedby about two metres and rotated slightly to align itwith the ITRF reference frame. Figure 6 illustratesthe differences between this new WGS84 andNAD83 in Canada for both horizontal and verticalcomponents. This realignment with ITRF occurredthree different times [Slater and Malys 1998;Merrigan et al. 2002; NIMA 2004; NGA 2004].These WGS84 realizations are denoted with a “G”followed by the GPS week the frame was put intouse. Table 2 lists the different ITRF-based realiza-tions of WGS84 giving the particular version ofITRF used and the dates they were put into use. Ofparticular importance to GPS users are the dates usedto produce the broadcast orbits. Users can transformWGS84 positions or baseline vectors to NAD83 bysimply using the parameters for the associated ITRF.

VXN

VY N

VZ N

=VXI

VY I

VZ I

+dTX

dTY

dTZ

+dDS

dRZ⋅k–dRY⋅k

–dRZ⋅k dRY⋅kdDS –dRX⋅k

dRX⋅k dDS

XI tYI tZ I t

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Figure 6: Horizontal (blue) and vertical (red) differences betweenNAD83(CSRS) and WGS84 in the sense NAD83(CSRS) minus WGS84.

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Unfortunately, NGA still considers the G-series realizations of WGS84 to be identical withthe original realization. Thus, the zero transforma-tion with respect to NAD83 has never been revisedin spite of the bias being clearly measurable. Thishas created problems when using WGS84-basedcorrection services and trying to convert results toNAD83. Most receiver manufactures include onlythe original NIMA coordinate shifts (translations)in their receiver firmware, which are zero forNAD83. Consequently, many receivers are produc-ing so-called NAD83 coordinates that are actuallystill in WGS84 and biased by 1.5 to 2 metres withrespect to the true NAD83 reference frame. Greatcare must therefore be exercised when using thetransformations built into receiver firmware andpost-processing software.

NAD83(Original)

To assist in the conversion of large amounts ofdata tied to the original realization of NAD83 andin cases where it is impractical or impossible toreadjust existing NAD83(Original) networks inNAD83(CSRS), many provinces have developedNTv2-type distortion models to convert such datato NAD83(CSRS). This task first involved thereadjustment of provincial networks inNAD83(CSRS). This provided coordinate discrep-ancies between the original and CSRS realizationsof NAD83 with a greater spatial density to bettermodel the distortions. In some cases it was necessaryto perform surveys to provide additional connectionsbetween the old and new realizations.

It is important to emphasize again thatNAD83(Original) and NAD83(CSRS) do not rep-resent different reference systems. NAD83(CSRS)is essentially an updated physical realization (net-work) of the same NAD83 reference system, fullyconsistent with NAD83(Original) but with muchgreater accuracy. The provincial NTv2 distortionmodels therefore do not reflect any changes in thereference system. Rather they represent the errors(distortions) in the networks comprising the originalrealization of NAD83. Because these distortions areabout half a metre on average, users should consider

the accuracy of their georeferencing before decidingwhether data holdings need to be converted fromNAD83(original) to NAD83(CSRS).

NAD27/CGQ77/ATS77

For similar reasons, transformations toNAD83(CSRS) have also been developed for otherolder reference systems. The system with the great-est amount of legacy data was NAD27 and so anNTv2 to NAD83(Original) transformation and dis-tortion model was developed as discussed earlier.This transformation can also be used forNAD83(CSRS). This is because the differencesbetween the original and CSRS versions of NAD83are insignificant compared to the relatively lowaccuracy of the NAD27-NAD83(Original) transfor-mation. These minor differences can therefore besafely ignored without introducing any systematicbias in the results.

In order to reduce the distortions in NAD27,Quebec performed a readjustment of their provin-cial networks based on the NAD27 reference frameseveral years before NAD83 was introduced. Thisrealization was denoted as NAD27(CGQ77) orCGQ77. Quebec developed their own NTv2-com-patible transformations and distortion modelsbetween NAD27, CGQ77, NAD83(Original), andNAD83(CSRS) which are implemented in theirSYREQ software.

At about the same time CGQ77 was imple-mented, the Maritime Provinces introduced yetanother reference system called the AtlanticTerrestrial System of 1977 (ATS77) [Gillis et al.2000]. Unlike CGQ77, this was a geocentric system.It was adopted in New Brunswick and Nova Scotiain 1979 and continued to be used after the intro-duction of the original realization of NAD83. AnNTv2-based transformation between NAD27 andATS77 was developed as was a transformationbetween ATS77 and NAD83(Original). However,the latter used only the federal primary control sta-tions in the Maritimes. When the 1998 realization ofNAD83(CSRS) was introduced in 1998 it was soonadopted or was used unofficially for most position-ing applications. New Brunswick, Nova Scotia, andPrince Edward Island have since developed theirown NTv2-compatible transformations and distor-tion models between ATS77 and NAD83(CSRS).

Maintenance ofNAD83(CSRS)

In general, geodetic reference frames and net-works need periodic maintenance or updating of

160

Version Based on Implemented Implementedat NIMA in Orbits

WGS84(G730) ITRF91 1994-01-02 1994-06-20WGS84(G873) ITRF94 1996-09-29 1997-01-29WGS84(G1150) ITRF2000 2002-01-20 2002-01-20

Table 2: ITRF-based realizations of the WGS84 reference frame.

their coordinates for a variety of reasons. Some ofthese include the addition of new densification net-works, the correction of survey blunders, unstableor disturbed monumentation, the effects of crustalmotion both locally and regionally, and to keeppace with ever increasing accuracy requirements.

Crustal motions are especially troublesomealong the west coast and in central and easternCanada (see Henton et al. [this issue]). Verticalmovements up to 2 cm/y due to post-glacialrebound can quickly make positions outdated. Inthe case of NAD83(CSRS), it is now known thatthe NNR-NUVEL-1A plate motion used in thedefining ITRF-NAD83 transformation is in errorby about 2 mm/y (see Figure 7). Over several yearsthis can accumulate to well over a centimetre whichbecomes problematic for high accuracy and scien-tific applications.

In an effort to ensure the NAD83(CSRS) refer-ence frame keeps pace with future requirements,coordinates are periodically updated as new ver-sions of the ITRF are released. New ITRF coordi-nates for Canadian stations are transformed toNAD83(CSRS) using the adopted transformation.This periodic updating of the reference frame issometimes referred to as a semi-dynamic approachto maintenance where positions are valid for only adefined period of time.

Another method of reference frame mainte-nance is a purely dynamic approach where posi-tions are assumed to be dynamic and are valid onlyfor a specific epoch. Estimated velocities are thenused to propagate the positions to any other date.Such an approach is often required for scientificapplications demanding the highest accuracies. TheITRF is the prime example of a dynamic global ref-erence frame as is the new Stable North AmericanReference Frame (SNARF) discussed below.

Evolving from NAD83

To many, our current NAD83(CSRS) spatialreference system appears to be adequate for mostpositioning activities in North America. However,history has repeatedly shown that reference sys-tems need to evolve to keep pace with the ever-increasing accuracy with which we are able tolocate points on and near the Earth, and to enablethe proper integration of georeferenced data fromvarious sources and from different times.

As previously mentioned, it is now known thatNAD83 is offset from the true geocentre by abouttwo metres. It is therefore incompatible with thenewer realizations of WGS84, the native reference

frame for GPS. As discussed above, this can causeproblems when treating the two frames as the same.In addition, the adopted NNR-NUVEL-1A platemotion model overestimates the magnitude of therotation of the North American plate (see Figure 7).This can accumulate to magnitudes that aredetectible in high accuracy GPS surveys. Finally,intra-plate crustal deformations such as post-glacialrebound can cause coordinates to quickly go out ofdate.

One option of dealing with these problems is tosimply use the most recent ITRF realization as donein some countries (e.g., South America). Theadvantage of this is that it would be completelycompatible with the WGS84 system used by GPS.However, the relentless movement of the NorthAmerican continent due to plate tectonics willslowly but surely ensure that all coordinates sys-tematically change by about 2.5 cm/y. This amountsto a quarter of a metre in only 10 years. If this motionis not accounted for it would result in coordinatediscrepancies at a level unacceptable for most users.

The accumulating coordinate discrepanciesdue to tectonic motion could be somewhat reducedby simply updating the ITRF coordinates on a reg-ular basis following the semi-dynamic approach tomaintenance. However, it would still be difficult torelate data from different time periods. At the veryleast, this approach would require significantefforts to inform and educate the public.

Another approach would be to adopt a versionof ITRF at a specific epoch and to keep this real-ization fixed to North America as recommended by

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Figure 7: GPS horizontal velocities from repeated high accuracy GPS obser-vations with respect to the NNR-NUVEL-1A plate motion estimate for NorthAmerica. The coherent pattern reveals a bias in NNR-NUVEL-1A of about 2mm/y.

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the IAG [1992]. Such coordinates can be related toITRF coordinates at any other epoch using an esti-mate of the motion of the North American tectonicplate as done for NAD83(CSRS). This is theapproach recently used to define the so-calledStable North American Reference Frame (SNARF)[Blewitt et al. 2005; Craymer et al. 2005]. Underthe joint auspices of UNAVCO, Inc. in the US andIAG Sub-Commission 1.3c for North America, aworking group was established with the goal ofdefining such a regional reference frame that isconsistent and stable at the sub-mm-level through-out North America. This reference frame fixesITRF2000 to the stable part of North America tofacilitate geophysical interpretation and inter-com-parison of geodetic solutions of crustal motions.

The SNARF reference frame is essentiallydefined by a rotation vector that models the tecton-ic motion of North America in the ITRF2000 refer-ence frame. The rotations transform ITRF2000positions and velocities at any epoch into theSNARF frame fixed to the stable part of NorthAmerican. Thus, just like NAD83(CSRS), SNARFis defined in relation to the ITRF. The advantage ofSNARF is that it is truly geocentric and also uses arotation vector that more accurately models themotion of stable North America. Previous platerotation estimates have used stations in areas ofintra-plate crustal deformations which can bias theestimation of the rotation vector.

The SNARF plate rotations were determinedusing ITRF2000-based velocities of 17 stations ingeophysically stable areas. The following are therotations adopted for SNARF v1.0 which transformITRF2000 coordinates into the SNARF frame:

(5)

This rotation vector is equivalent to a horizontalsurface velocity of about 2 cm/y in Canada.

Velocities of CBN stations with respect to theSNARF reference frame are plotted in Figure 8. Inthis reference frame the expected outward pattern ofintra-plate horizontal velocities from post-glacialrebound is small but clearly visible. This model ofplate motion is an improvement over NNR-NUVEL-1A for North America (compare Figures 7 and 8).

The first release of SNARF also includes anempirical model of post-glacial rebound based on anovel combination of GPS velocities with a geo-physical model. It has been adopted as the officialreference frame for the Plate Boundary Observatoryof the EarthScope project along the west coast ofNorth America. Over the next few years SNARFwill be incrementally improved and refined andcould become a de facto standard for many applica-tions. Sometime in the future it is possible that, afterfurther analysis and consultation with stakeholders,SNARF or some variation of it may eventuallyreplace NAD83 as the official datum for georefer-encing in both Canada and the US.

AcknowledgementsParts of this paper were based on an in-house

report prepared by Don Junkins, recently retiredfrom GSD. His contribution is gratefully acknowl-edged. I am also indebted to Joe Henton (GSD) forpreparing most of the figures using the GMT soft-ware [Wessel and Smith 1998]. I would also like tothank Allen Flemming (Nova Scotia GeomaticsCentre), Leo-Guy LeBlanc (Service NewBrunswick) and Serge Bernard (PEI Transportationand Public Works) for kindly providing informa-tion about the Maritime networks, and especiallyNorman Beck and Robert Duval (GSD) for theirmany constructive comments and suggestionswhile reviewing this paper.

DisclaimerAny reference to commercial products in not

intended to convey an endorsement of any kind.

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ITRF2000: A new release of the InternationalTerrestrial Reference Frame for earth science appli-

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RXRYRZ

=0.06588

– 0.66708– 0.08676

mas/y

Figure 8: GPS horizontal velocities from repeated high accuracy GPS observa-tions with respect to the SNARF 1.0 plate motion estimate for North America.

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Blewitt, G., D. Argus, R. Bennett, Y. Bock, E. Calais, M.Craymer, J. Davis, T. Dixon, J. Freymueller, T.Herring, D. Johnson, K. Larson, M. Miller, G. Sella,R. Snay, and M. Tamisiea. 2005. A Stable NorthAmerican Reference Frame (SNARF): FirstRelease, Proceedings of the Joint UNAVCO/IRISWorkshop, Stevenson, WA, USA, June 9-11.

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Craymer, M., R. Ferland and R. Snay. 2000. Realizationand Unification of NAD83 in Canada and the U.S.via the ITRF. In R. Rummel, H. Drewes, W. Bosch,H. Hornik (eds.), Towards an Integrated GlobalGeodetic Observing System (IGGOS), IAG SectionII Symposium, Munich, October 5-9, 1998,International Association of Geodesy SymposiaVolume 120, Springer-Verlag, Berlin, pp. 118-121.

Craymer, M., G. Blewitt, D. Argus, R. Bennett, Y. Bock,E. Calais, J. Davis, T. Dixon, J. Freymueller, T.Herring, D. Johnson, K. Larson, M. Miller, G. Sella,R. Snay and M. Tamisiea. 2005. Defining a Plate-Fixed Regional Reference Frame: The SNARFExperience. Dynamic Planet 2005,IAG/IAPSO/IABO Joint Assembly, Cairns,Australia, August 22-26.

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Slater, J.A. and S. Malys. 1998. WGS 84 - Past, Presentand Future. In F.K. Brunner (ed.), Advances inPositioning and Reference Frames, IAG ScientificAssembly, Rio de Janeiro, Brazil, September 3-9,1997, International Association of GeodesySymposia Volume 118, Springer-Verlag, Berlin, pp.1-7.

Soler, T., J.D. Love, L.W. Hall and R.H. Foote. 1992.GPS Results from Statewide High PrecisionNetworks in the United States. Proceedings of the6th International Geodetic Symposium on SatellitePositioning, The Ohio State University, Columbus,OH, March 17-20, pp. 573-582.

Soler, T. and R.A. Snay. 2004. Transforming Positionsand Velocities between the International TerrestrialReference Frame of 2000 and North AmericanDatum of 1983. Journal of Surveying Engineering,130(2), pp. 49-55.

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Author

Mike Craymer has been with the GeodeticSurvey Division of Natural Resources Canada since1991 and is presently head of Geodetic Networks &Standards. He received his Ph.D. in geodesy fromthe University of Toronto and has authored manypapers on a variety of geodetic topics. He is an asso-ciate member of the International GGNS Service andis presently co-Chair of IAG Sub-Commission 1.3con Regional Reference Frames for North America.He is also co-editor of the GPS Toolbox column forthe journal GPS Solutions, editor of the LiteratureReview column for the journal Surveying and LandInformation Science, and publisher of the popularTables of Contents in Geodesy www.geodetic.org. o

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SPACE GEODETIC TECHNIQUES AND THECANADIAN SPATIAL REFERENCE SYSTEMEVOLUTION, STATUS AND POSSIBILITIES

P. Héroux, J. Kouba, N. Beck, F. Lahaye, Y. Mireault, P. Tétreault, P. Collins, K. MacLeod, M. Caissy Geodetic Survey Division, Natural Resources Canada, Ottawa, Ontario

Over the last two decades a revolution has taken place in the field of positioning and navigation. Theavailability and accuracy of signals from Global Navigation Satellite Systems (GNSS), combined withadvances in microelectronics, have greatly improved our ability to georeference information. While position-ing was traditionally the business of professionals in the field of surveying and geodesy, it has now become acommodity readily available to a wide range of users, from professional surveyors to recreational amateurs.

The Canadian Spatial Reference System (CSRS) has been evolving over this time to facilitate access tothe reference frame with innovative products. These new products and associated tools are taking advantageof the widespread availability of navigation satellite signals and the popularity of the Internet to respond toCanadians in a way that enables seamless integration of their geospatial data, on location and in real-time.The move to space geodetic techniques for delivery of the CSRS is presented in this paper.

1. Introduction

The Canadian Spatial Reference System(CSRS) is an abstract collection of principles, fun-damental parameters, and specifications for quanti-tatively describing the positions of points in space,as described by Craymer [this issue]. The referenceframe is the physical manifestation or realization ofsuch a prescription. Traditionally, a reference frameconsisted of a network of ground-based geodeticcontrol points with adopted coordinates that othersurveys were connected and referenced to. Sincethe introduction of space geodetic techniques, thisparadigm has been changing.

Nowadays, the ability to acquire georeferenceddata and merge layers of geospatial informationwith ever increasing accuracy to establish spatialrelationships and extract knowledge has been great-ly improved and is becoming everyday business inmany organizations. As our ability to associatecoordinates to objects of our physical environment

improves along with the precision of our techniques,efficient access to an accurate reference frame mustbe provided and maintained. If relationships are tobe established between objects regardless of theirspatial separation or date of observation, a consis-tent reference frame with sufficient density isrequired to identify and analyze spatio-temporalprocesses of interest. The failure to maintain thiscapability would lead to a reference frame that can-not fulfill its fundamental purpose. In practicalterms, the failure to unify under a common refer-ence frame or the inability to inter-relate referenceframes eventually leads to the existence of differentcoordinates referencing a single object, confusingthe modern coordinate-based users to think thatthey can be in two places at once!

This paper briefly looks back over the past cen-tury and describes the improvement of measuring


Au cours des deux dernières décennies, nous avons traversé une révolution dans le domaine du position-nement et de la navigation. L’accessibilité et la précision des signaux des systèmes mondiaux de navigationpar satellite (GNSS), en plus des avancées en microélectronique, ont grandement amélioré notre capacité àgéocoder les données. Alors qu’auparavant, le positionnement relevait de professionnels des levés et de lagéodésie, il est maintenant accessible à une grande variété d’usagers, qu’ils soient des arpenteurs-géomètresou des amateurs de plein air.

Le Système canadien de référence spatiale (SCRS) a évolué au cours de ces années afin de faciliter l’accèsau cadre de référence par des produits innovateurs. Ces nouveaux produits et outils connexes tirent profitde l’accessibilité globale aux signaux des satellites de navigation et de la popularité de l’Internet afin derépondre aux Canadiens d’une façon qui permet l’intégration parfaite, instantanée et sur place de leurs donnéesgéospatiales. Dans cet article, nous vous présenterons la transition du SCRS vers ces techniquesgéodésiques spatiales.

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techniques for the establishment of geodetic controlnetworks and how they have evolved and movedfrom ‘ground to space.’ It describes the impact oftechnology on precision and reference frame real-ization and access. The approach used to upgradethe CSRS and respond to modern user needs with‘active control networks’ and innovative productscreated through international cooperation areexplained. Finally, future possibilities for real-timegeoreferencing with respect to a consistent referenceframe are considered.

2. The Evolution of GeodeticControl Networks

Looking back over the past century, precisionhas evolved with technology over three major erasthat are referred here as classical (optical and elec-tronic) and space-based (see Figure 1). The classical

era includes the period when optical instrumentswere used to observe angles between control pointsand combine them with baseline measured sparselyalong triangulation chains to transfer coordinatesfrom a reference point. With the limitations of angu-lar measurements and the difficulty of maintainingorientation using astronomical means, systematicerrors accumulated as triangulation chains wereassembled to extend the network. These factors ledto the realization of a continental reference frame,known as the North-American Datum of 1927(NAD27), that we now know was affected by distor-tions of several tens of metres and offset from theEarth’s centre of mass by a few hundred metres[Junkins and Garrard 1998]. This original geodeticfabric underpinned small-scale mapping programsthroughout most of the 20th century.

The introduction of electronic distance meas-uring (EDM) instruments mid-way through the lastcentury significantly improved the precision ofgeodetic surveys by taking advantage of the synergy

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Figure 1: The evolution of geodetic technology and precision.

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between angle and distance measurements. Thiscombination of ground-based measurements wasused to form trilateration chains and graduallyimprove the NAD27 network. It was soon recog-nized that fixing coordinates of the NAD27 realiza-tion to constrain these more precise measurementswas detrimental to network improvement, leading tothe decision to perform a re-adjustment. All observa-tions of the primary national network, supplementedwith a few stations with Doppler observations fromthe emerging TRANSIT satellite navigation system,formed the basis of the NAD83 project. Coordinatedwith North-American partnership and adopted in1986, this new reference system realization wasoriginally believed adequate to satisfy georeferenc-ing requirements for many decades. With a nation-wide average accuracy of 0.3 m and better than 1 min the northern parts of many provinces, this refer-ence frame had an overall relative precision ofabout 20 ppm [Craymer this issue]. Then, naviga-tion satellites from the Global Positioning Satellite(GPS) constellation appeared on the scene in theearly 1980’s.

2.1 The Advent of Space GeodeticTechniques

While station coordinates were being adjustedby national geodetic organizations for NAD83 real-ization in the early 1980’s, an international scien-tific initiative known as the Crustal DynamicsProject, led by NASA, was establishing globalobservatories that used modern space geodetictechniques, mainly Very Long BaselineInterferometry (VLBI) and Satellite Laser Ranging(SLR), to validate the geological evidence for glob-al plate tectonics [Smith and Baltuck 1993]. Thisscientific initiative eventually led to the first real-ization of the International Terrestrial ReferenceFrame (ITRF) released by the International Earthrotation and Reference systems Service (IERS).

The early 1980’s also saw the first GPS satel-lites being launched. The new navigation satelliteswere expected to supersede TRANSIT and becomethe tool of choice for global navigation. GPS alsoquickly became a promising technology for geodet-ic applications given the quality of its coded L-bandsignals and anticipated low cost of future userequipment. Midway through the decade, its relativeprecision of a few parts per million over baselines ofa few tens of kilometres was demonstrated usinggeodetic processing techniques [Goad 1985]. GPSwas soon adopted for control surveys and NaturalResources Canada’s (NRCan) Geodetic SurveyDivision (GSD) acquired its first GPS units in 1985.

While GPS was limited by the precision of itsbroadcast orbits, it presented the operational advan-tage of not requiring inter-visibility betweenground stations, greatly facilitating the operationallogistics of field surveys and allowing for the estab-lishment of control points in locations of convenientphysical access for user occupation. The adoption ofGPS by the geodetic community quickly revealedthe limitations of the recently adopted NAD83 real-ization and led to the realization of NAD83(CSRS),first materialized at active control points and sub-sequently densified with the establishment of theCanadian Base Network (CBN) of about 200 passivemonuments [Craymer this issue].

Sharing the common goal of improving GPSsatellite orbits to respond to the needs of the Earthsciences community, in particular in the field of geo-dynamics, the International GPS Service (IGS) wasformed in the early 1990’s [Beutler et al. 1999]. Withthe objective of providing precise GPS orbit prod-ucts on a continuous basis in a globally consistentreference frame, the IGS stimulated the developmentof a network of globally distributed tracking stationsand the creation of regional and global Data Centres(DCs) for public access. With voluntary contribu-tions from a number of Analysis Centres (ACs) anda cooperative, although competitive, model for prod-uct combination, the quality of GPS satellite orbitsand clocks estimates improved progressively overthe last decade, reaching today’s centimetre levelprecision. These new global products overcame thelimitations of the GPS broadcast orbits and now sup-port positioning with accuracies approaching a fewparts per billion on a global scale. They have alsooffered a reliable means for users to directly connectto the global reference frame using the GPS datathey collect from a single receiver, anywhere onEarth. This new possibility is effectively changingthe way modern GPS users operate.

2.2 From Regional to GlobalReference Frames

The traceability of coordinates over time andspace requires knowledge about the reference sys-tem in which they were computed. Technologiesapplied to realize a reference system determine theaccuracy of some of its fundamental parameters[Pearlman et al. this issue]. For example, accurate-ly establishing a global reference frame origin hasalways been difficult because of the relative insen-sitivity of classical near-Earth measurements to thelocation of the Earth’s centre of mass. The adventof space geodetic techniques has now provided theability to pinpoint the average location of the

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With volun-tary contribu-tions from a

number ofAnalysisCentres

(ACs) and acooperative,

althoughcompetitive,

model forproduct com-bination, the

quality ofGPS satellite

orbits andclocks

estimatesimproved

progressivelyover the last

decade,reaching

today’s cen-timetre level

precision.

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Earth’s centre of mass with centimetre precision.This new capability brings along the option tochoose a single geocentric reference frame with awell defined coordinate system origin.

Historically, different technologies were appliedas they became available for new reference framerealizations, resulting in the creation of a number ofreference frames having different origins. This hasbeen confusing to users who see apparent coordinateshifts as they move between reference frames.Although these shifts can be accounted for in part byapplying transformations that improve compatibility,the recent availability of a global reference system towhich national reference systems can be related ischanging the way in which reference frame areaccessed amd maintained [Craymer this issue].

2.3 From relative to “absolute” accuracy

Until the advent of space geodetic techniques,systematic errors accumulated in control networks astheir spatial coverage was extended. The difficulty incontrolling the propagation of these errors resulted inthe inability to provide absolute accuracy estimates(i.e. accuracy with respect to the reference frame).Therefore coordinates were qualified in terms oftheir relative accuracy (or precision), usually givenwith respect to neighbouring control stations.Today, using signals from navigation satellites, pre-cise orbit products and geodetic processing tech-niques, it is possible to determine coordinates ofstations separated by long distances with very highprecision. Consequently, precision and accuracyestimates now compare favourably and separationbetween markers is less of a factor. Actually, usersanywhere on Earth can now position themselvesrelative to the Earth’s average centre of mass withcentimetre precision and therefore qualify their coor-dinates in term of “absolute” accuracy.

2.4 Options for Integrating Positionsinto a ReferenceFrame

Traditionally, occupation of monumented con-trol points was the only method for integration ofpositioning results within a specific reference frame,with published reference coordinates providing theconnection.

With coordinates of navigation satellites nowreadily available as either broadcast or post-processed orbits, users now have the option to con-nect directly through the satellites. In this case, ref-erence coordinates are provided for the centre ofmass of the satellites which effectively become con-trol points that extend the realization of the referenceframe. For instance, autonomous GPS positioning,

through the GPS broadcast orbits directly connectsusers to the WGS84 reference frame with a globalconsistency of a few metres.

The accuracy of the coordinates for the selectedcontrol point (ground or satellite) characterizes thequality of the realization of the reference frame anddirectly affects the quality of end-user coordinates.Therefore, the control point (satellite or ground)must be carefully selected, along with the neces-sary observations and processing strategy, to meetthe user’s required precision. Not surprisingly, thisis often overlooked since the overwhelming major-ity of GPS users operate in stand-alone mode, rely-ing blindly on the reference frame realization of thesatellite broadcast orbit. The main limitation ofoperating in this mode is that user-positioning pre-cision is limited by the quality of the predictedsatellite orbits.

Users seeking higher precision have generallychosen a relative positioning method and differ-enced satellite observations made simultaneously atone or more surrounding control points with pre-determined (adopted) or user-assigned coordinates.In the later case, despite the control point coordinatepotential uncertainties, a very precise local solutioncan still be obtained as long as the control point andsatellite coordinates are compatible within a fewmetres, which corresponds to the current broadcastorbit precision. This is so because the error in thecoordinates of the satellites and control point causesystematic errors in relative positioning that are afunction of the ratio between the distance separat-ing the user from the control point and the satelliteelevation. Therefore, a combined satellite-stationcoordinate error of a few metres translates into arelative systematic error of a few centimetres overa 100 km baseline, which may satisfy the need forlocal to regional reference frame consistency formany applications. However, while local to regionalconsistency may be achieved, the option to looselyassign control point coordinates gives rise to the pos-sibility of creating a multitude of reference frameswith varying levels of compatibility. Nowadays,with the growing availability of continuously operat-ing reference stations, this risk is increasing and GPSusers could unknowingly be operating within a ref-erence frame that is arbitrarily set by the operator ofa local GPS service. This may significantly impactusers who generally want to work in a referenceframe that does not change or that ensures traceabil-ity of coordinates over time. This issue may becomesignificant as datasets collected using different localreference frames are combined.

While GPS orbit predictions broadcast in real-time have metre-level precision, they can now beestimated in post-mission with centimetre precision.

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…GPSusers couldunknowinglybe operatingwithin areferenceframe that isarbitrarilyset by theoperator ofa local GPSservice.

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Therefore, a global reference frame can now beaccessed through precise orbit and clock productswithout explicitly assigning coordinates to groundcontrol points in the user solution. This new possi-bility is an alternative that significantly impacts theusers as well as geodetic agencies that have tradi-tionally provided coordinates of ground control.

3. Enabling Geodesy from Space

As a result of advances in positioning andcommunication technologies, mainly the advent ofGPS and Internet, the Canadian Active ControlSystems (CACS) was conceived and proposed inthe early 1980’s. This novel idea at the time sug-gested that we could eventually replace dense tra-ditional geodetic control networks, based on ‘pas-sive’ survey markers, with a sparse network of‘active’ sensors or tracking systems operating con-tinuously (see Figure 2). As opposed to passive net-works that required users to physically occupymonumented markers to integrate into the referenceframe, active networks could give users access tothe reference frame through their observed data.Based on signals from space, the ‘active’ conceptonly requires that common space objects be simul-taneously visible from both the user location and

‘active’ sensor. With navigation satellites visiblefrom locations thousands of kilometres apart, theactive control concept offered the possibility to sig-nificantly reduce the number of monumented controlpoints along with their high cost of maintenance.This possibility was particularly appealing to GSDwho at the time was maintaining thousands of con-trol markers across Canada. In addition, mainte-nance of NAD83 could be linked to the most recentrealizations of the ITRF.

3.1 The VLBI Component

GPS was always envisioned as the technologyusers would adopt to access the CSRS throughrelated data and products, as widespread availabili-ty of affordable user equipment was anticipated.Nevertheless, the desire to sustain a referenceframe that would offer long-term stability requiredthat Very Long Baseline Interferometry (VLBI) bepart of the overall solution. While offering an addi-tional level of redundancy, the fundamental roleVLBI plays is to connect the celestial and terrestri-al reference frames through estimates of all EarthOrientation Parameters [NASA 2002]. This is essen-tial for ongoing support of reference frame deliverythrough orbits of navigation satellites. Now thatnavigation and Earth monitoring satellites broad-cast signals with millimetre resolution and that

141Figure 2: From passive to active control networks.

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global reference frames can be realized with cen-timetre accuracy, the requirement to monitor thealignment between the two systems has becomemore important, as stated in the Global GeodeticObserving System (GGOS) project proposal[Pearlman et al. this issue]. Because the GPS satel-lite orbits are conveniently made available in ITRF,a reference frame that is fixed to the Earth androtates with it, it is easy to forget that satellitesorbiting in the Earth’s gravity field are also sensi-tive to the Earth’s spatial orientation and variousforces acting upon them. Therefore, unless orbitcomputations are linked to the inertial celestial refer-ence frame, the connection between GPS orbits andthe earth-fixed reference frame cannot be preciselymaintained in time. VLBI is the sole space tech-nique that can provide reference frame scale andorientation with sufficient precision and stabilityover the long term [Altamimi et al. 2002].

GSD contributes weekly to the core network ofthe global VLBI initiative coordinated by theInternational VLBI Service (IVS). The 46 metreVLBI antenna in Algonquin Park is the most activeCanadian observatory and is complemented with asmaller antenna in Yellowknife and a transportable

system that has been deployed mainly at theDominion Radio Astrophysics Observatory inPenticton and in St-John’s over the past 10 years.These fiducial points are the anchors of the CSRS.

3.2 The GPS Component

By the early 1990’s, NRCan’s GSD beganimplementing the GPS component of CACS (seeFigure 3). The implementation of CACS was inpart influenced by the formation of the IGS. It isworth mentioning that the IGS came along whenindividual countries recognized their inability toachieve their goals without a global cooperativeendeavour. At the time, this was important mainlyto scientists in the field of global geodynamics whohad adopted GPS as their instrument of choice tomeasure plate tectonics with centimetre precision.However, they lacked an accurate and stable globalreference frame on which to express their results.This effort also created the opportunity for geodeticagencies to connect their geodetic networks to theITRF using GPS, leading to further integration anddensification of the global geodetic fabric, at anunprecedented level of accuracy.

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Figure 3: Continuously operating GPS tracking stations of the CACS.

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In an effort to take full advantage of the syner-gies between VLBI and GPS technologies, the firstcontinuously operating GPS Active Control Points(ACPs) in Canada were deployed at the AlgonquinPark and Yellowknife sites where permanent VLBIantennas were in place. Soon after, ACPs were alsoinstalled at the VLBI sites in St-John’s andPenticton. In late 1993, this initial deployment offour ACPs ready for the IGS pilot phase providedminimal spatial distribution of GPS tracking sta-tions over the Canadian territory, but significantlycontributed to the original global IGS networkdesigned to track GPS satellites continuouslythrough their complete orbit, given Canada’s geo-graphical size.

As the core stations of a national GPS networkwere being established, the development of theWestern Canada Deformation Array (WCDA) wasinitiated to address the particular requirements ofgeodynamic studies along Canada’s west coast[Dragert et al. 1995]. The CACS and WCDA ini-tiatives had different objectives, the former provid-ing continuous GPS data for orbit estimation whilethe latter focused on analyzing tectonic movementsin a seismically active area. Nevertheless, theywere strongly linked through GPS orbit productswhich are essential to the recovery of relative sta-tion velocities with mm/year accuracy over base-lines extending several hundred kilometres. TheWCDA has been densified during the last decadeand now consists of over 14 permanent sites[Henton et al. this issue].

The core national network was further expand-ed and upgraded to include a real-time capability inthe mid-1990’s with the intention of providing real-time access to the reference frame through wide-area GPS corrections, applicable Canada-wide.This additional component would allow users toovercome the GPS signal degradation imposed bySelective Availability (SA), which since 1993 hadlimited stand-alone GPS positioning to a precisionof about 100m, insufficient for many geomaticsapplications (SA was eventually removed on May 1,2000). The CACS network was at that time expand-ed to 12 tracking sites giving it the spatial densityrequired to provide simultaneous observations ofall satellites visible over Canada from at least twolocations, creating observation redundancy toincrease correction robustness and reliability.

Additional continuously operating trackingstations were deployed in response to regional stud-ies and are operated through joint funding agree-ments. They include six stations distributed along abroad line running from the Atlantic coast to HudsonBay that covers the portion of the Canadian Shieldmost affected by post-glacial rebound, with uplift

rates in excess of 1 cm/year in north-westernQuebec. Uplift of this magnitude over such a largearea is unique and of interest to operators of spacegravimeters who can infer the corresponding gravitychange to calibrate the long wavelengths of theEarth’s gravity field. This particular geophysicaleffect, most pronounced in Canada, offers an idealnatural “laboratory” for Earth science, helping tolink together time-limited space missions intended toobserve a continuous field. Another eight GPS sta-tions co-located with tide gauges have been installedin the Arctic to monitor crustal motion and sea levelrise in support of climate change studies. Finally,five GPS stations equipped with surface meteoro-logical sensors are contributing to a cooperativenetwork of ground-based GPS receivers co-locatedwith water level gauges around the shores of theGreat Lakes. This network is being used primarily tomonitor the effects of post-glacial rebound on theGreat Lakes water levels and provide access to aconsistent vertical datum. In addition, meteorolog-ical services in both Canada and the US use this GPSdata to recover integrated precipitable water forassimilation into weather forecasting models.

4. Products, Tools andServices for Satellite Geodesy

Although the basic equipment required forpositioning is a satellite receiver (currently GPS),ancillary products and services are needed toensure that coordinates obtained from GPS are con-nected to reference frames that support geodeticapplications. This section describes products andservices, including GPS orbits and clocks, current-ly available to Canadian users to facilitate access tothe reference frame.

4.1 GPS Orbits and Clocks

As already mentioned, the IGS has computedGPS products in support of precise positioningapplications for more than a decade. NRCan’s GSDwas one of the original IGS collaborating agenciesand has since participated as an Analysis Centre(AC) and provided AC Coordination during its ini-tial years of operation [Donahue et al. 2004].

The collaborative global tracking network ofsome 300 permanent, continuously-operating GPSstations provides a rich data set that enables the ACto generate precise products such as GPS satelliteorbit and clock solutions. The IGS GPS orbit andclock products combined from the ACs individual

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Additionalcontinuously

operatingtracking sta-

tions weredeployed inresponse to

regionalstudies and

are operatedthrough joint

fundingagreements.

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contributions [Kouba 2003] come in variousflavours that differ mainly by their latency and theextent of the tracking network used for their com-putations (see Table 1). The IGS “Final” orbit andclock product is usually available the thirteenth dayafter the last observation. The “Rapid” orbit andclock product is combined 17 hours after the end ofday. The latency is mainly due to the varying avail-ability of tracking data from stations of the IGSglobal tracking network, which use a variety of dataacquisition and communication schemes. The evo-lution of the IGS over the past 12 years from a dailyto hourly data availability model, followed by thedevelopment of a sub-daily orbit product, hasresulted in reliable GPS orbit predictions with highprecision available for real-time applications, inparticular GPS meteorology. The sub-daily orbitproduct, and more specifically its predicted portion,continuously updated every three hours, is a directinput to the NRCan real-time correction system andkey to its optimal performance. The estimated por-tion of the IGS “Ultra-Rapid” orbit is equallyimportant for Precise Point Positioning (PPP)applications [Kouba and Héroux 2001].

4.2 Connecting to the NAD83(CSRS)Reference Frame

4.2.1 CACS Network Observational Data

As seen earlier, users may choose to occupy ageodetic control station with coordinates preferablyknown in NAD83(CSRS), collect their own GPSdata and use differential processing techniques toobtain accurate coordinates within this referenceframe. An alternative to the occupation of controlstations directly by the user is to download files andacquire observations available from continuouslytracking stations of CACS. This simplifies field-work logistics and provides reliable and accuratecoordinates for the selected control points. TheCACS GPS stations all collect dual frequencypseudo-range and carrier phase observations atsampling intervals varying from 1 to 30 seconds bycontinuously tracking all GPS satellites in view.Data are retrieved from the sites, translated toRINEX format and stored on a public archive withlatencies varying from 15 minutes to 24 hours.Details about data availability and how to access it

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Table 1: IGS Orbits and Clocks Product Table.

Solution Type Products Precision Latency Updates

Clocks < 7 nsBroadcast (1) real-time 4 times daily

Orbits < 160 cm

Ultra-RapidClocks < 5 ns

Predicted Part real-timeOrbits < 10 cm

4 times dailyUltra-Rapid

Clocks < 0.2 nsEstimated 3 hours

Orbits < 5 cmPart

Clocks < 0.1 nsRapid 17 hours daily

Orbits < 5 cm

Clocks < 0.1 nsFinal 13 days weekly

Orbits < 5 cm

Notes (1): Broadcast orbits and clocks are included for comparison only and are not official IGS products.(2): IGS products increase in precision, reliability and robustness as you move down the Table.(3): A more detailed version of that Table is available at the following address:

(http://igscb.jpl.nasa.gov/components/prods.html)

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is available at: http://www.geod.nrcan.gc.ca/net-work_a/obs_e.php.

Because of the sparse distribution of CACSstations, long baselines may be formed but errors inthe broadcast orbits may cause decimetre errors inposition. This is overcome with the use of preciseGPS satellite orbits available by Internet, althoughtropospheric effects must be properly estimated insoftware for utmost accuracy.

4.2.2 On-Line Precise Point Positioning(CSRS-PPP)

Alternatively, PPP processing can be appliedfor single station solutions without the need tooccupy control stations. This technique fixes theprecise orbit and clocks obtained from an externalsource, such as IGS, to improve the parameter esti-mates, including user position. Typically, with IGSorbits/clocks, positioning is provided directly inITRF at the centimetre precision level. The ITRFpositions can then be easily transformed into theNAD83(CSRS) using adopted transformations[Craymer et al. 2000]. Precise station clock andzenith tropospheric delay solutions as well asinstantaneous (kinematic) positioning at a level of afew centimetres are also possible.

Since the Fall of 2003, a web service, CSRS-PPP [Tétreault et al. 2005], provides a convenientmean to process GPS RINEX observations from a

single GPS receiver in order to obtainNAD83(CSRS) or ITRF coordinates. Table 2 liststhe various PPP modes and accuracies attained afterambiguity convergence. Convergence is achievedonce the estimated carrier-phase ambiguities havereached a constant value that is within the carrierphase noise

When using combined code and carrier obser-vations, ambiguity convergence is critical to the PPPsolution reaching its utmost accuracy, whether GPSis used in static or kinematic mode. Typically, ambi-guities converge within 30 minutes in static modeand one hour in kinematic. Once the ambiguitieshave converged, all parameters can be re-evaluatedby back substituting their final estimates into thesolution to effectively recover optimal estimates forall parameters over the entire observing session. Thisfunctionality is particularly useful to recover optimaluser trajectories in kinematic mode. Otherwise, thePPP solution remains sub-optimal and does not fullybenefit from the full precision of the carrier phases,although it still provides the performance of cor-rected code observations.

For PPP solutions using code-only observa-tions, no ambiguity parameters are involved,although the accumulation of observations over timemay improve the parameter estimates in static modeprocessing, notwithstanding the observation noiseand multipath and the uncertainty in the modeledatmospheric delays. In summary, the accuracy of

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Table 2: PPP Performance after Convergence – Single and Dual Frequency – Static and Kinematic Modes.

Receiver Observation PPP Precision (cm)

Processed Mode

Latitude Longitude Height

Dual Code & Carrier Static 1 1 2

Frequency Kinematic 4 4 10

Single Code Only (1) Static 10 10 100

Frequency Kinematic 50 50 150

Single Code & Carrier Static 2 3 4

Frequency (2)

Kinematic 25 25 50

Notes (1): Quoted PPP code-only performance is for surveying grade receiver. (2): May not yet be available in CRSR-PPP at time of publishing.

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parameters estimated with CSRS-PPP remains afunction of the quality, quantity and type of GPSobservations used. Consequently, it is important tocarefully select the user equipment and proceduresand consider the operating environment to ensurethat the collected GPS data will satisfy the projectaccuracy specifications.

4.3 Orthometric Height Determinationfrom Space-Based Technique

Traditional geodetic techniques separately pro-vided the horizontal and vertical components ofpositions using different surfaces or datums, eitheran ellipsoid or geoid. The traditional separationbetween horizontal and vertical networks originatedfrom the different measuring techniques used forlevelling, either geometric (spirit-level) or trigono-metric (vertical angle/distance measurement). Spiritlevelling was used to establish the national primaryvertical control network while trigonometric level-ling served mainly to determine ellipsoidal heightsof horizontal control points for data reduction.Space-based technologies such as GPS, on the otherhand, provide three-dimensional Cartesian coordi-nates with respect to the geocentre. Once a referenceellipsoid is selected (a uniform mathematical surfacethat approximates the shape of the Earth), Cartesiancoordinates can be readily transformed into ellip-soidal latitude, longitude and height. In contrast,heights above sea level or so-called orthometricheights, are with respect to a surface of constantgravity potential (equipotential surface), a physicalsurface called geoid. In practical terms this meansthat water can only flow in the direction of decreas-ing orthometric height although it could possiblyflow towards a point of higher ellipsoidal height.

Ellipsoidal heights (h) can be transformed tomore practical and physically meaningful orthomet-ric heights (H) by applying the geoid undulation (N)for the location of interest. The Canadian GravimetricGeoid model, once based on observations madeexclusively with land or airborne gravity surveys, hasrecently been updated to include data from the latestsatellite gravimetry missions (CHAMP andGRACE). It has a local relative precision that is com-parable to what can be achieved with spirit-levellingand a nation-wide accuracy that is compatible withthe quality of GPS determined ellipsoidal heights[Véronneau et al. this issue]. This state-of-the-artgeoid model is at the core of the modernization of theheight reference system in Canada, with the objectiveto reduce our dependency on a monumented verticalnetwork. Combining GPS and geoid to recover

orthometric heights is gaining popularity as usersmove to space geodetic techniques for greater opera-tional efficiency. A tool known as GPS-H, availableas a software package or on-line application athttp://www.geod.nrcan.gc.ca/software/gpsht_e.php isavailable for height transformation.

5. Towards Real-Time Geodesy

The increasing availability of wireless commu-nication devices has brought along the possibilityof making GPS corrections accessible ‘in the field.’Building on this capacity, a real-time component tothe CACS was developed to further facilitate end-user access to the reference frame by streamliningusers operations and eliminating the requirementfor post-mission processing.

5.1 Real-Time GPS Data acquisition

During the mid-1990’s, a wide-area model todeliver real-time GPS corrections (GPS·C) Canada-wide and provide access to the NAD83(CSRS)frame at the few decimetre accuracy level wasenvisioned. In 1996, four ACPs of the CACS net-work were upgraded with real-time communicationand the GPS tracking data began streaming to acentral processing facility at 1Hz data rate [Caissyet al. 1996]. Over the next few years, this numbergrew to a total of 12 real-time ACPs covering theCanadian territory.

5.2 Expanding Real-Time Capabilities

Mindful of Internet expansion and interest inreal-time GPS applications, the IGS has recentlyformed a working group to demonstrate real-timeglobal GPS data exchange over the Internet[Muellerschoen and Caissy 2004]. This capabilitycould eventually lead to the production of globalcombined real-time GPS products. Such productscould support a number of correction distributionservices and various scientific objectives, namelyLow-Earth-Orbiters (LEO) orbit determination andatmospheric sounding from satellite occultation.Only a global network can support the productionof orbit products with centimetre accuracy that fullyexploit the high-resolution of the GPS carrier-phasemeasurements.

As evidenced in Table 3, positioning perform-ance is limited when a GPS tracking networkextends over only a national or continental scale. Inorder to achieve the optimal level of positioningperformance, a global tracking network is essentialto continuously track all the GPS satellites in the

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In practicalterms thismeans thatwater canonly flow inthe directionof decreasingorthometricheightalthough itcouldpossiblyflow towardsa point ofhigher ellip-soidal height.

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constellation. NRCan has been working with IGSpartners for the last three years to develop a worldwide real-time GPS tracking network. To date, aglobal network consisting of 40 stations exchang-ing data in real-time is being demonstrated by theIGS real-time working group and a pilot projectexpected to start sometime this year will furtheradvance this initiative. Long-time IGS contributingagencies including the Jet Propulsion Laboratory,the European Space Agency, the GermanGeoForschungsZentrum, and Geoscience Australiaare actively upgrading a subset of their respectiveIGS tracking stations to support real-time datastreaming. GSD is keeping abreast of these develop-ments, sharing expertise developed during GPS·Cimplementation and accessing the global network toimprove the quality of its wide-area corrections.

5.3 Real-Time GPS Corrections(GPS·C)

Software development for the production ofGPS corrections started in parallel with the avail-ability of real-time data streams from CACS sta-tions. A state-space domain scheme, known as WideArea Differential GPS (WADGPS), was preferredover a local differential model since it considerablyreduced the number of real-time ACPs to beinstalled and operated. In WADGPS, separate cor-rections for specific error sources are computed andtransmitted to the users: GPS satellite orbital errors,GPS satellite clock errors and ionospheric delayerrors. Tropospheric delay errors are handled local-ly by users. This development formed the GPS·Ccorrection service [Lahaye et al. 1997].

Within GPS-C the orbit corrections are deter-mined from predictions based on the processing ofhourly tracking data from the global IGS network.The satellite clock corrections are computed in real-time from dual frequency tracking data streamedfrom the real-time ACPs to a central computingfacility. Ionosphere-free, carrier-phase smoothedpseudorange data at two second intervals are com-bined with the latest satellite orbit predictions andtracking station coordinates in a least-squaresadjustment to determine satellite and station clockoffsets with respect to a virtual reference clock keptaligned with GPS time. Observed ionospheric delaysbetween the tracking station receivers and observedsatellites are used to update a single layer grid, in lat-itude versus local solar time. These correction valuesare encoded in messages that provide high-resolu-tion clock and orbit corrections (0.004m) to supportdecimetre level real-time positioning.

As previously mentioned, with more timelyaccess to tracking data from a global network andadvances in satellite orbit prediction, the possibilityto provide users with corrections that support posi-tioning at the sub-decimetre accuracy level is on thehorizon. While this capability has been demonstrat-ed, improvements to the current service still dependon the availability of real-time GPS data from aglobal tracking network on a continuous basis.

5.4 The Canada-Wide DifferentialGPS Service (CDGPS)

In early 2000, Federal, Provincial, andTerritorial geomatics agencies coordinated throughthe Canadian Council of Geomatics (CCOG) formed

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Input - GPS Corrections Computation Positioning RMS* (cm)

Orbit Tracking Observation Lat Lon HgtNetwork

Predicted Wide Code-Filtered 29 26 38

(IGU) Area

Predicted Wide Code-Carrier 11 12 22

(IGU) Area

Predicted Global Code-Carrier 5 6 10

(IGU)

*After ambiguity convergence

Table 3: GPS Corrections - The Network Difference.

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the Canada-wide DGPS Service (CDGPS) to fundthe distribution of the GPS·C over a geostationarysatellite channel (MSAT). Despite the high-cost ofsatellite bandwidth, this approach was selected toseamlessly offer coverage over the Canadian land-mass and territorial waters using a single broadcast.This project included improvements to the GPS·Ccomputation infrastructure and its management toensure high availability (99.97 per cent) of the cor-rections. A robust broadcast protocol withopen/public documentation and software to facili-tate decoding of the over-the-air corrections byGPS equipment manufacturers were developed.Finally, 1000 proof-of-concept radios were manu-factured and distributed to demonstrate the CDGPScapability. The correction distribution elements weredesigned for optimized reception for ground users,even under forest canopy. In late 2003, CDGPSdeclared the Initial Operation Capability status forthe service [CDGPS 2003]. Several GPS receivermanufacturers now offer “CDGPS ready” receiversthat take advantage of this real-time service.

5.5 The Network Transport of RTCMvia Internet Protocol (NTRIP)

As interest in wireless Internet increases andcommunication networks expand, alternate means ofdistributing GPS·C corrections are emerging. Arecently adopted standard known as NetworkTransport of RTCM via Internet Protocol (NTRIP)now facilitates the transfer of GPS data and productsusing the Internet [Weber 2004]. This applicationlevel protocol takes advantage of existing commu-nication services for mobile Internet users mainlyin populated areas. It also enables providers of GPScorrection services to serve their users over theInternet instead of having to deploy and supportexpensive dedicated channels for the disseminationof their product. At this time, NTRIP is beingadopted mainly by GPS users of Real-TimeKinematic (RTK) applications. With NTRIP,providers of correction services have the means tocontrol user access and monitor connection time tosupport a pay-per-use model for cost-recovery. Inareas of cellular coverage, GPS users accessing RTKcorrection streams created by reference stations with-in a few tens of kilometres from where they operatecan position with centimetre relative precision. Thismeets the requirements of many georeferencingapplications at the municipal level. Nevertheless, thelimited area of applicability of RTK corrections andpatchy Internet coverage does not provide a solutionthat can be easily expanded to cover the large expansesof the sparsely populated Canadian territory.

A possible alternative for areas with Internetcoverage that are without RTK correction streamsis GPS·C distribution using NTRIP [Collins et al.2005]. Access to the GPS·C correction stream cancomplement and also extend the CDGPS service,especially in urban areas and at northern latitudeswhere MSAT signal penetration or its low elevationvisibility is problematic.

6. SummarySpace geodetic techniques, in particular the

advent of global navigation satellites and spacegravimetry missions, have revolutionized the field ofpositioning and navigation. By improving the qual-ity of, and access to, a geodetic reference framewhich sustains our ability to work in a ‘coordinated’world, the CSRS has also significantly evolved toremain compatible with new technologies and thepossibilities they bring. Today, a stable global refer-ence frame with centimetre accuracy can be readilyaccessed. Continuously available precise satelliteorbits are simplifying the maintenance of nationalreferences frames such as NAD83(CSRS), facilitat-ing the direct integration of user positioning andenabling the interchange of geospatial information atan unprecedented level of precision on a globalscale. The impact of satellite gravimetry missions ongeoid model improvement is also enabling therecovery of orthometric heights without relying ontime consuming and costly spirit-levelling. Thesedevelopments are leading to the unification of thegeometric and gravimetric reference frames on aglobal scale, further enhancing the usefulness ofgeodetic techniques for multi-disciplinary applica-tions, in earth and atmospheric sciences.

7. AcknowledgementsThe authors would like to acknowledge the

contribution of all GSD personnel who have partic-ipated in transforming the CSRS to better meet userneeds, generating innovative products that promotethe use of space geodetic techniques while facilitat-ing reference frame improvement and maintenance.In particular Dr. Josef Popelar, former Chief of theActive Control Systems Section, is recognized forthe leadership role he played in the early 1990’s.We are also grateful to our colleagues from agen-cies participating in the IGS and IVS of theInternational Association of Geodesy for providinga supportive and cooperative environment in whichto improve the CSRS. Finally, Robert Duval isacknowledged for his contribution to the finalreview of this manuscript.

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Nevertheless,the limitedarea ofapplicabilityof RTKcorrectionsand patchyInternet cov-erage doesnot provide asolution thatcan be easilyexpandedto coverthe largeexpanses ofthe sparselypopulatedCanadianterritory.

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8. ReferencesAltamimi, Z., P. Sillard, and C. Boucher, ITRF2000: A

new release of the International TerrestrialReference Frame for earth science applications, J.Geophys. Res., 107(B10), 2214,doi:10.1029/2001JB000561.

Beutler, G., M. Rothacher, S. Schaer, T.A. Springer, J.Kouba, R.E. Neilan. 1999. The International GPSService (IGS): An Interdisciplinary Service inSupport of Earth Sciences, Adv. Space Res. 23(4),pp. 631-635.

Caissy M., P. Héroux, F. Lahaye, K. MacLeod, J.Popelar, J. Blore, D. Decker, R. Fong. 1996. Real-Time GPS Correction Service of the CanadianActive Control System, Proceedings of the 9thInternational Technical Meeting of The SatelliteDivision of the Institute of Navigation, ION GPS-96, September 17-20, 1996, pp 1787-1791.

CDGPS. 2003. Press Release,http://cdgps.com/e/cdgps_documents/PressRelease_2003-10-14_CDGPSServiceLaunch.pdf, October14, 2003.

Collins, P., Y. Gao, F. Lahaye, P. Héroux, K. MacLeod,K. Chen. 2005. Accessing and Processing Real-Time GPS Corrections for Precise Point Positioning- Some User Considerations”, Proceedings of the18th International Technical Meeting of the SatelliteDivision of the Institute of Navigation, Long Beach,California, September 13-16.

Craymer, M., R. Ferland, S. Snay. 2000. Realization andUnification of NAD83 in Canada and the U.S. viathe ITRF. In R. Rummel, H. Drewes, W. Bosch, H.Hornik (eds.), Towards an Integrated GlobalGeodetic Observing System (IGGOS), IAG SectionII Symposium, Munich, October 5-9, 1998,International Association of Geodesy SymposiaVolume 120, Springer-Verlag, Berlin, pp. 118-121.

Craymer, M. 2006. The Evolution of NAD83 in Canada,Geomatica, this issue.

Donahue, B., Y. Mireault, C. Huot, P. Tétreault and J.Kouba. 2004. NRCan Analysis CentreContributions to the IGS: 1994 – 2004, 10 YearsIGS, Workshop and Symposium, March 1-5, 2004,Berne, Switzerland.

Dragert, H, X. Chen, J.Kouba. 1995. GPS monitoring ofcrustal strain in southwest British Columbia withthe western Canada Deformation Array; J.Geomatica 49(3), 1995; pages 301-313.

Goad, C.C. (ed.).1985. Positioning with GPS,Proceedings of the 1st International Symposium onPrecision Positioning with the Global PositioningSystem, published by the National Ocean Service,Vol. 1-508pp, Vol. 2-438pp.

Henton, J.A., M.R. Craymer, H. Dragert, S. Mazzotti, R.Ferland and D.L. Forbes. 2006. Crustal Motion andDeformation Monitoring of the CanadianLandmass, Geomatica, this issue.

Junkins, D. and G. Garrard. 1998. DemystifyingReference Systems: A Chronicle of SpatialReference Systems in Canada. Geomatica, 52(4),pp. 468-473.

Kouba, J., P. Héroux. 2001. Precise Point PositioningUsing IGS Orbit and Clock Porducts, GPSSolutions, 5(2), pp.12-28.

Kouba, J. 2003. Guide to using International GPSService (IGS) Products, On-Line Publication(http://igscb.jpl.nasa.gov/igscb/resource/pubs/GuidetoUsingIGSProducts.pdf)

Lahaye, F., M. Caissy, P. Héroux, K. MacLeod, J.Popelar. 1997. Canadian Active Control SystemReal-Time GPS Correction Service PerformanceReview, Proceedings of the National TechnicalMeeting of the Institute of Navigation, Santa-Monica, USA, January 14-16, 1997, pp. 695-698.

Muellerschoen R. and M. Caissy. 2004. Real-Time DataFlow and Product Generation for GNSS,Celebrating a Decade of the International GPSService, Workshop and Symposium, March 1-5,2004, Berne, Switzerland, p 69-75.

NASA. 2002. National Aeronautics and SpaceAdministration, Living on a Restless Planet, SolidEarth Science Working Group Report, Pasadena,California, USA, p.8.

Pearlman, M., Z. Altamimi, N. Beck, R. Forsberg, W.Gurtner, S. Kenyon, D. Behrend, F.G. Lemoine, C.Ma, C.E. Noll, E.C. Pavlis, Z. Malkin, A.W. Moore,F.H. Webb, R.E. Neilan, J.C. Ries, M. Rothacher, P.Willis. 2006. Global Geodetic Observing System–Considerations for the Geodetic NetworkInfrastructure1, Geomatica, this issue.

Smith, D.E. and M. Baltuck. 1993. Introduction. InSmith, D.E. and Turcotte, D.L., eds., Contributionsof Space Geodesy to Geodynamics: CrustalDynamics, Geodynamics Series, AmericanGeophysical Union, v. 23, p. 1-3.

Tétreault, P., J. Kouba, P. Héroux and P. Legree. 2005.CSRS-PPP: An Internet Service for GPS UserAccess to the Canadian Spatial Reference Frame,Geomatica, 59(1), pp. 17 to 28.

Véronneau M., R. Duval, J. Huang. 2006. A GravimetricGeoid Model as a Vertical Datum in Canada,Geomatica, this issue.

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Authors

Pierre Héroux leads the Active ControlSystems Technology Team at the Geodetic SurveyDivision, Natural Resources Canada (NRCan). Heholds a BSc from Université Laval (1981) and anMSc from the University of New Brunswick

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1 This is an extension of a paper originally published in: 2005 IAG/IAPSO/IABO Joint Assembly. Cairns Australia.August 22-26, 2005. Series: IAG Symposia, Vol. 130, in press.

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(1997). He has been involved with the use of spacegeodetic techniques and the development of appli-cations and services for post-mission and real-timeprecise positioning in the private and public sectorsfor the past 25 years.

Jan Kouba obtained his first degree from theCzech Technical University in Prague, CzechRepublic, specializing in satellite geodesy. In 1970he obtained his MSc from the University of NewBrunswick and in 1994 DrSc from the CzechAcademy of Sciences. He has been working insatellite geodesy since 1970 (first satellite Dopplerand then GPS). He has held several research posi-tions at the Geological Survey of Canada and theGeodetic Survey Division (GSD) of NRCan.During 1994 to1998 he led the Canadian ActiveControl Technology / IGS Analysis Team at GSDand during this period also served as the firstAnalysis Center Coordinator of IGS. Though hehas retired from GSD in 1998, he has maintainedcontacts with IGS and GSD.

Norman Beck is an Ontario Land Surveyor(Geodesy) and has been working with GPS for over26 years. After graduating from the University ofToronto, he joined Nortech where he developed GPSpositioning and integrated navigation software forthe first available commercial receivers. In the mid1980’s, he joined the Geodetic Survey Division ofNRCan where he has had a variety of duties andassignments including: GPS software development,GPS data processing and analysis, managing fieldsurveys, heading Client Services Section, and morerecently the Active Control Systems Section. He iscurrently chief of this Division.

François Lahaye is a satellite geodesy special-ist within the Active Control Systems TechnologyTeam at the Geodetic Survey Division, NaturalResources Canada (NRCan). He holds a BSc(1987) and an MSc (1991) from Université Lavaland has been involved with the development ofpost-mission and real-time precision GPS applica-tions and services for 17 years.

Yves Mireault has been with the GeodeticSurvey Division since 1988. He holds an MSc inGeodesy (1989) from Université Laval. He hasbeen involved with GPS data processing and analy-sis for the past 20 years. From 1994 to 1998, he wasa member of the Natural Resources Canada

(NRCan) team which, on behalf of the IGS(International GPS Service), performed the combi-nation and analysis of the GPS precise clocks,ephemerides and Earth Rotation Parameters fromthe seven IGS Analysis Centers. Since 1999, he hasbeen involved in the development and productionof the NRCan Ultra Rapid products used in thereal-time GPS Corrections (GPS.C) supporting theCanada-Wide Differential GPS Service (CDGPS)and contributed to the IGS.

Pierre Tétreault obtained a BSc (land survey-ing), in 1983 and an M.Sc (geodesy) in 1987 fromthe University of Toronto. He is currently part ofthe Active Control Systems Technology Team atthe Geodetic Survey Division, Natural ResourcesCanada (NRCan) and works on the computationand application of GPS satellites precise orbits andclocks. He has been responsible for the implemen-tation and expert support of an on-line PrecisePoint Positioning service.

Paul Collins is a Satellite Geodesy Specialist atthe Geodetic Survey Division, Natural ResourcesCanada (NRCan). He holds a BSc from the Universityof East London (1993) and an MSc from theUniversity of New Brunswick (1999). He is involvedwith the development of satellite geodetic techniquesfor real-time wide-area precise positioning.

Ken MacLeod is a member of the ActiveControl Systems Technology Team at the GeodeticSurvey Division, Natural Resources Canada(NRCan). He holds a BSc from the University ofToronto (1985) and obtained and Ontario LandSurveyors License in 1990. He has been involvedwith the use of space geodetic techniques and thedevelopment of applications and services for post-mission and real-time precise positioning and nav-igation for the past 10 years.

Mark Caissy leads the Active Control SystemsOperations Team at the Geodetic Survey Division,Natural Resources Canada (NRCan). He joinedGSD after completing a BSc at the University ofNew Brunswick (1988) and has been involvedmainly with the use of space geodetic techniquesfor precise positioning. He currently manages thereal-time GPS Corrections (GPS.C) infrastructuresupporting the Canada-Wide Differential GPSService (CDGPS) and co-chairs the InternationalGNSS Service (IGS) Real-Time Working Group.o

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A GRAVIMETRIC GEOID MODEL AS AVERTICAL DATUM IN CANADA

Marc Véronneau, Robert Duval and Jianliang HuangGeodetic Survey Division, Natural Resources Canada, Ottawa, [email protected]

The need for a new vertical datum in Canada dates back to 1976 when a study group at the GeodeticSurvey Division (GSD) of Natural Resources Canada investigated problems related to the existing verticalreference system (CGVD28) and recommended a redefinition of the vertical datum. The US NationalGeodetic Survey and GSD cooperated in the development of a new North American Vertical Datum(NAVD88). Although the USA adopted NAVD88 as its datum in the early 90s, Canada did not follow suitbecause unexplained discrepancies of about 1.5 m were still present between east and west coasts. GSD con-tinued to maintain and expand the vertical datum using the spirit levelling technique; however related costand inherent deficiencies to this technique has forced GSD to rethink its approach for the delivery of theheight reference system in Canada. Meanwhile, advances in space-based technologies and new develop-ments in geoid modelling have emerged and now offer an alternative to spirit levelling. A new project tomodernize the vertical datum is currently in progress in Canada. GSD is planning the adoption of a geoidmodel as the new vertical datum, which will allow users of space-based positioning technologies access toan accurate and uniform vertical datum everywhere across the Canadian landmass and surrounding oceans.Furthermore, this new vertical datum will be less sensitive to geodynamic activity, local crustal uplift andsubsidence, and deterioration of benchmarks.

1. Introduction

Height determination within a consistent refer-ence system is at the basis of a large number of eco-nomic activities. These activities range from mapping,engineering, and dredging to environmental studiesand natural hazards; from precision agriculture andforestry, to transportation, commerce and navigation;and from mineral exploration and management ofnatural resources to emergency and disaster pre-paredness. All of these depend on the compatibility

of height information enabled by a common coordi-nate reference system through which all types ofgeo-referenced information can be interrelated andexploited reliably. While the height reference systemsupports numerous technical applications, it is alsoreferred to in many legal documents related to landand water management and safety such as easement,flood control, and boundary demarcation.


Le Canada a senti le besoin de se doter d’un nouveau système de référence altimétrique en 1976 alorsqu’un groupe d’étude à la Division des levés géodésiques (DLG) de Ressources naturelles Canada étudiaitdes problèmes liés au système de référence altimétrique existant (CGVD28) et avait recommandé une nouvelledéfinition du système de référence altimétrique. Le National Geodetic Survey des États-Unis et la DLG onttravaillé ensemble à l’élaboration d’un nouveau système de référence altimétrique nord-américain(NAVD88). Bien que les États-Unis aient adopté le NAVD88 et comme système de référence altimétrique versle début des années 1990, le Canada ne l’a pas adopté à cause de lacunes inexpliquées d’environ 1,5 mètreentre les côtes Est et Ouest. La DLG a continué à maintenir et à améliorer le système de référence altimétriqueen utilisant le nivellement de précision (à bulle). Cependant, à cause des défauts et des coûts associés à cettetechnique, la DLG a dû trouver une autre façon d’établir le système de référence altimétrique au Canada.Entre temps, les technologies spatiales et la modélisation du géoïde ont connu des percées importantes etoffrent maintenant une alternative intéressante au nivellement de précision (à bulle). Ce projet est d’ailleursdéjà lancé au Canada. La DLG planifie l’adoption d’un modèle de géoïde en tant que système de référencealtimétrique, ce qui permettra aux usagers des technologies de positionnement spatiales d’y accéder pourpartout sur le continent canadien et les océans qui nous entourent. De plus, ce nouveau systtème deréférence altimétrique sera moins sensible à l’activité géodynamique, au soulèvement ou à l’affaissementlocal de la croûte terrestre et à la détérioration des repères de nivellement.

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Until recently, Geodetic Survey Division (GSD)of Natural Resources Canada (NRCan) has relied onconventional levelling methods to provide a physicalframework of vertical reference points. The bench-marks established by this method were accessible tousers across the country and used as the basis fortheir surveys. Conventional levelling methods useline-of-sight survey measurements and require thatcrews of surveyors literally walk from coast to coast,taking measurements every 100 metres or so alongmajor roadways. The height reference system estab-lished in this fashion over the last 100 years consistsin a network of more than 80,000 benchmarksspread over approximately 150,000 kilometres oflevelling lines (Figure 1).

In recent years, limitations of the currentheight reference system (instability, distortion, lim-ited coverage, etc.) and its high maintenance costs,combined with opportunities and pressures of newtechnology, forced GSD to re-think the methods ituses to provide height reference in Canada.

Today, new technologies in absolute and air-borne gravimetry and the launch of satellite gravitymissions have greatly enhanced geoid modellingcapabilities, providing an alternative for the defini-tion of the height reference surface. More important-ly, the Global Navigation Satellite Systems (GNSS)

continue to expand and improve in accuracy andease of use, gaining further acceptance as tools ofchoice for geo-referencing in the geomatics andscientific communities. Nowadays, the GlobalPositioning System (GPS) offers a relatively inex-pensive means for users to obtain consistent 3Dpositioning (latitude, longitude, and height) con-nected to geocentric terrestrial reference systemssuch as NAD83 or ITRF as well as providing themeans for geomatics agencies to maintain these ref-erence systems at a lower cost. Unfortunately, thecurrent height reference system is not compatiblewith GPS at the precision level required by users. Amodernization program has therefore been initiatedto fully support and realize the substantial benefits ofGPS and related modern technologies for accurateheight measurement.

The current plan to realize a new vertical datumand replace the Canadian Geodetic Vertical Datumof 1928 (CGVD28) is not a first attempt for NRCan.In 1976-1977, a GSD study group investigatedproblems related to the existing vertical referencesystem (CGVD28) and recommended a redefinitionof the vertical datum. The US National GeodeticSurvey (NGS) and GSD agreed to cooperate on therealization of a new vertical datum for NorthAmerica, to be completed by 1988. This project was

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Figure 1: Physical extent of the first-order levelling network in Canada.

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known as the North American Vertical Datum of1988 (NAVD88). During that period, significanteffort was devoted to improving the precise levellingprocedures and a large portion of the primary verti-cal network was re-observed. Although the US NGSadopted NAVD88 as their new vertical datum in theearly 90s, GSD did not follow suit because of unex-plained discrepancies of the order of 1.5 m from eastto west coasts (likely due to accumulation of sys-tematic errors) and the overall slight improvementthis new datum would have brought. Since then,GSD has reduced physical maintenance of the pri-mary levelling network while concentrating on itsmathematical analysis and the development of agravimetric geoid model as a more advanced poten-tial solution for height modernization.

The recently computed geoid models CGG2000[Véronneau 2002] and CGG05 [Huang andVéronneau 2006], although not yet meeting theaccuracy requirement for a new datum, confirm thepotential of implementing a gravity-based system asa seamless vertical datum covering the wholeCanadian landmass and surrounding oceans. Such adatum would also be compatible with modern posi-tioning techniques for more accessible height deter-mination. Current and upcoming internationalsatellite gravity missions, combined with theoreticalprogress in geoid modelling, will contribute greatlyto improve the accuracy of the Canadian geoidmodel for its adoption as a new datum. Computationof a new geoid model is currently planned for 2008at the earliest, in order to take advantage of datafrom the most recent satellite gravity missions. Anadditional one to two years of work will be requiredprior to the formal implementation of a new systemto validate this geoid model as the basis for the newdatum, to finalize the development of related userapplications, and to carry out the mathematicaladjustment to propagate the new heights through theexisting vertical networks.

Similar to the transition from the NorthAmerican Datum of 1927 (NAD27) to NAD83,which was initiated several years ago and is stillongoing, it is expected that the transition fromCGVD28 to a new datum will span several years oreven decades during which time the two systemswill co-exist. Our experience with the transition toNAD83 is exemplified by the fact that severalorganizations throughout the country are still usingNAD27 today.

Although GSD and the Canadian GeodeticReference System Committee (CGRSC) are wellaware of the technical issues related to the modern-ization of the vertical reference system, there are anumber of practical issues that need to be taken intoconsideration in the development of an implemen-

tation plan. A key concern is that the envisionedmodernization and related transition are conductedin a manner that minimizes negative impacts andmaximizes benefits for users of the reference system.Therefore, a stakeholder consultation is currentlyunderway which should identify users concerns andneeds as well as developing recommendations tofacilitate the transitions to the new datum. Thisconsultation will be a critical input to the heightmodernization implementation plan.

2. Levelling-BasedVertical Datum

The spirit levelling technique is a well-knownapproach that has been used for more than 200years [Gareau 1986] and still provides the mostaccurate method for determining height differencesover short distances. It involves making differentialheight measurements between two vertical graduat-ed rods, approximately 100 metres apart, using atripod mounted telescope whose horizontal line ofsight is controlled to better than one second of arcby a spirit level vial or a suspended prism. Thisprocess is repeated in a leap-frog fashion to pro-duce elevation differences between establishedbenchmarks that constitute the vertical control net-work. Despite upgrades to the instrumentation withnew technologies, the methodology remains costly,time consuming, and laborious.

From 1972 to 2000, the Canadian vertical net-work was almost entirely re-surveyed with about124,000 kilometres of levelling lines observed.Until 1993, GSD carried out an average of 4000 to5000 kilometres of levelling annually.Approximately 65 per cent (~3000 kilometres) wasfor maintenance purposes, the other 35 per cent(~1500 kilometres) related to network expansion.From 1994 to 2000 (following federal governmentProgram Review and associated budget reduc-tions), GSD performed an average of 1200 kilome-tres of levelling annually, with a steady declineover the years. GSD has performed only minimaltargeted levelling since 2001.

To maintain the vertical network on a 25-yearcycle, the observation of approximately 5,600 kilo-metres of levelling would be required annually. Ata rate of $250 to $350 per kilometre, the cost of re-survey alone would range between $1.4M and$2.0M annually. Furthermore, this cost would notinclude repair or replacement of damaged bench-marks ($1000 to $2500 per benchmark dependingon the type), nor the salary costs related to the sur-veys coordination, mathematical adjustment, and

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Current andupcoming

internationalsatellite gravi-ty missions,

combinedwith theoret-ical progress

in geoidmodelling,

will contributegreatly to

improve theaccuracy of

the Canadiangeoid modelfor its adop-

tion as anew datum.

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related data management. Even a skeletal networkof about 30,000 kilometres, once proposed as theminimum vertical framework for Canada, wouldcost about $400,000 per year to maintain andpotentially pre-empt or delay the work essential toestablishing a modernized solution.

The heights currently published are a constructthat results from annual survey observations datingback to 1904. Despite the great care administered tominimize potential error sources, the network wasestablished in a piece-meal fashion by combiningobservations made over successive years andadjusted locally. This resulted in significant region-al distortions in the current published heights.Further degradation of the accuracy of the pub-lished height is attributable to the vertical crustalmotion over time. Comparisons of the heights cur-rently published against more recent scientific net-work re-adjustments and the most recent geoidmodels indicate regional distortions of up to onemetre. Although, the consistency of relative heightsis probably still at the sub-centimetre precisionlevel locally, the regional distortion impede on theapplication of new technology such as GPS toobtain accurate point heights consistent with thecurrent datum.

As an extension of the latter difficulty, the cur-rent published heights were based on the assumptionthat the Pacific and Atlantic oceans were at thesame height. In fact, according to oceanographicevidence, the water level at Vancouver could behigher than the water level at Halifax by 40 to 70cm. This discrepancy causes a national-scale tilt inthe published heights that has significant impact ona number of applications. It also has implicationsfor existing discrepancies in heights along theCanada/US border, where the US Governmentadopted NAVD88.

Due to the extent of the network and the relatedtime required to carry out a full inspection, it is diffi-cult to assess the exact physical state of the network atthis time. We can only extrapolate based on statisticsavailable in our databases or derived from the mostrecent inspections of small sections of the network.

Although only five per cent of the 80,000benchmarks in the GSD database are identified asbeing in dubious condition (damaged, destroyed,not found, displaced or inaccessible), it is expectedthat the current state of the vertical network is inmuch worse shape. Until 1996, GSD inspected3000 to 4000 kilometres of levelling lines annuallyas part of its vertical network maintenance pro-gram. These inspections found that 11 to 22 percent of the benchmarks inspected became unusableor were destroyed over an approximate 20-yearcycle, for an average rate of degradation of about

16 per cent over that period. In urban or near-urbansettings the rate can be much higher. A recent sys-tematic inspection of some 400 primary benchmarksestablished 25 years ago in the Greater VancouverRegional District reported 32 per cent of the bench-marks were either not found, inaccessible, ordestroyed. Consistent with these statistics, a studentsummer project recently carried out by the OntarioMinistry of Natural Resources (MNR) yielded alevel of destruction of 22 per cent based on theinspection of 110 benchmarks selected randomly inand around six cities. On the other hand, the Albertaprovincial geodetic agency indicated that between1988 and 2003 they inspected 12 per cent of thefederal benchmarks on their database since 1988and only 2.5 per cent were reported as destroyed orhaving an “anomalous” condition. At the other endof the spectrum, the Newfoundland provincial sur-vey agency estimates the destruction rate at about2.5 per cent per year (yielding a rate of about 40 percent over 20 years) based on the destruction rate oftheir own control monuments in the province.

Thus, we can probably estimate the degradationrate of the network across Canada to be in the rangeof 15 to 20 per cent per 20 years. In urban or near-urban areas the degradation rate could reach 35 percent for the same period. In the context of datummodernization, this implies that a significant portionof the existing monumented networks should remainintact and enable a transition period of a fewdecades. However, additional network maintenancemay be necessary in certain areas where damage tothe physical network occurs at a rate unacceptablefor a successful transition.

Subsidence or uplift of individual benchmarksdue to frost or other local instability is another weak-ness of the network, significantly affecting its accu-racy (or equivalently, confidence in that accuracy) ata local level. Occasional reports of such inconsisten-cies in the levelling network are expected to increaseas the length of time since the last maintenanceincreases. Further details about limitations of theCGVD28 can be found in [Véronneau et al. 2002].

3. The Geoid as a Vertical Datum

The alternative approach to spirit levelling forthe realization of a vertical datum is geoid model-ling. If the two approaches were errorless, theywould define the same datum. For the levellingtechnique, the datum is realized by monuments inthe Earth’s crust, which is an unstable surface due togeodynamic effects and local uplift and subsidence.A datum realized with spirit levelling only provides

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The alterna-tive approachto spirit level-ling for therealizationof a verticaldatum isgeoidmodelling.

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known heights at the benchmark locations. On theother hand, the geoid is realized in relation to anellipsoid (e.g., GRS80) and represents a continuoussurface known everywhere across the Canadian ter-ritory. Height modernization using a geoid model isa novel approach that is also being considered inNew Zealand [Hannah 2001].

The geoid is an equipotential surface, i.e., alevel surface where gravity (plumb line) is perpen-dicular at all points on the surface and water staysat rest. There are an infinite number of equipoten-tial surfaces. The geoid, by definition, correspondsto the surface that best approximates mean sea levelglobally (Gauss-Listing geoid). The differencebetween the geoid and actual mean sea level is thepermanent sea surface topography (SST), whichranges globally from –1.8 m to +1.2 m [LeGrand etal. 2003]. This SST is due to factors such as watertemperature, salinity, and surface ocean currents.

The separation between the ellipsoid and geoidis the geoid height or geoid undulation (N) and isdetermined from measurements of the Earth’s grav-ity field. The geoid height allows the relationbetween the ellipsoidal height (h), which can beobtained by GPS, and the orthometric height (H):

H = h – N. (1)

Thus, the application of the geoid model forheight determination is simply a subtraction, butone has to make sure that the ellipsoidal heightsand geoid heights are in the same reference frame(e.g., NAD83 (CSRS)). On the other hand, the real-ization of a geoid model is complex. It requires aglobal set of gravity measurements, which are cor-rected for a series of topographical effects beforesolving for Stokes’s integral [Heiskanen and Moritz1967]. The accuracy of the geoid model is as goodas the input data.

The development of an accurate geoid modelfor a country as vast as Canada is always a chal-lenge. The rough relief of the Western Cordillera,the large water surfaces such as the surroundingoceans, Hudson Bay and Great Lakes, the harshArctic conditions, and the natural motion of thetopography due to earthquake and postglacialrebound render the collection of land and marinegravity measurements difficult with the requiredaccuracy, density, and homogeneity. Furthermore,detailed digital elevations models are also requiredto refine the geoid model especially in the moun-tainous regions. However, the recently launchedsatellite missions of CHAMP and GRACE[Reigber et al. 2002; Reigber et al. 2005; Tapley etal. 2004], dedicated to the measurements of theEarth gravity field, contribute significantly in

improving the long wavelength components of thegeoid model for Canada.

The vertical datum for Canada does not neces-sarily have to be defined by the “global” geoid i.e.global mean sea level. Any equipotential surfacethat best serves Canada’s requirement can beselected as a datum. Therefore, in addition to theequipotential surface (W0), which represents globalmean sea level for which values can be obtainedfrom scientific papers (e.g., Burša et al. 2002;Burša et al. 2004; Mäkinen 2004), GSD is current-ly investigating an equipotential surface that wouldcorrespond to the mean water level at the tidegauge on the Saint-Lawrence River in Rimouski(Pointe-au-Père), Quebec. This gauge is one of thesix used to define mean sea level for CGVD28[Cannon 1929], but also the only gauge definingmean sea level for both NAVD88 [Zilkoski 1986;Zilkoski et al. 1992] and the International GreatLakes Datum of 1985 (IGLD85). This could beaccomplished by selecting the equipotential surfacesuch that the geoid height (N) in Rimouski is equalto the difference between the NAVD88 height (H)and ellipsoidal height (h) at that location:

NNAD83 = hNAD83 – HNAVD88. (2)

This definition would represent a difference ofapproximately 35 cm from the equipotential surfacerepresenting global mean sea level.

The advantage of using the geoid defining theglobal mean sea level is the direct compatibilitywith a global standard, facilitating the integrationof geoid undulations with ellipsoidal heightsobtained from space-based technology (e.g., GPS).It allows also the determination of the absolute seasurface topography from satellite radar altimetry.The advantage of using the mean water level atRimouski is the closeness with the datums current-ly used in Canada and the USA (CGVD28,NAVD88 and IGLD85) meaning that, at least, theheights in eastern Canada would remain approxi-mately the same as before. At this time, the selec-tion of the equipotential surface defining the verti-cal datum is still a subject of discussion.

Figure 2 shows the difference between the lev-elling-based datum CGVD28 and a preliminarygeoid-based datum aligned with mean water levelat Rimouski. Table 1 gives the change in heightsbetween CGVD28 and the two considered datumoptions for selected localities along an east-westcorridor in southern Canada. The values in Figure 2and Table 1 should not be considered as finalbecause they are based on the latest developmentalgeoid model CGG05 determined from earlydatasets processed from the CHAMP and GRACE

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The devel-opment of

an accurategeoid modelfor a country

as vast asCanada is

always achallenge.

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satellite gravity missions. Unfortunately, the newdatum would not coincide with NAVD88. There isan increasing systematic difference between thepreliminary geoid-based datum and NAVD88 thatreaches approximately 85 cm in Vancouver whenboth datums are defined at Rimouski. Early inves-tigations indicate that this discrepancy is related tosystematic errors in NAVD88.

This paper will not discuss the use of the ellip-soid as a vertical datum [Burkholder 2002].Although this proposition eliminates the use of ageoid model, it significantly impacts the absoluteheights. For example, using an ellipsoidal reference,the western coastline of Hudson Bay would have anelevation of approximately –50 m. Selecting thisoption would therefore require that topographical

maps in Canada be updated which would introduceadditional complexity in monitoring water flowsince ellipsoidal heights have no physical meaning(water can flow uphill). This alternative would onlyadd confusion to the process of moving to a mod-ernized height system that is already difficult formany users.

With the adoption of a new vertical datumcompatible with space-based positioning tech-niques, a drastic reduction in reliance on the densemonumented ground network is expected. Thisreduction would go hand-in-hand with the increas-ing adoption by the geomatics community of newtechnologies with their related improvements inaccuracy and efficiency.

4. Monumented HeightNetwork in the New Datum

Contingent on the recommendations of stake-holder consultations, it is likely that the futurenational monumented network for heights, at thehighest-level, will consist of the federal ActiveControl Points (ACP) and Canadian Base Network(CBN) points. It is anticipated that NRCan willcontinue the physical maintenance and monitoring atthese sites to ensure datum stability and observe thedata required to derive the mathematical transfor-mations required by users. Densification could beprovided by the Provincial High Precision Network(HPN) points, as required. NRCan will also contin-ue the mathematical maintenance of the existingprimary vertical network with respect to the newdatum through links to the CBN and ACPs. This willrequire an overall readjustment of the network, notonly to generate heights with respect to the newdatum, but also to remove much of the distortionresulting from the piece-meal construction of thenetwork. It should be noted however that the historiclevelling observations have their limitations and anew adjustment will not account for or correct formonuments that have moved over the years, nor forchanges in the Earth’s crust (uplift/subsidence) thataffect the accuracy of individual benchmarks.

The availability of heights referenced to thisnew datum for the existing network should greatlyfacilitate the transition to the new datum. To helpease the potential burden associated with movinginformation to a new datum, and as further incentive,NRCan will also provide and maintain transforma-tion and corresponding software tools to support theconversion of existing data sets from CGVD28 tothe new datum.

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Figure 2: Change between the levelling-based datum CGVD28 and a prelimi-nary geoid-based datum when using mean water level at Rimouski. (C.I.: 5 cm)

Locality Datum a Datum b Locality Datum a Datum b

(Global) (Rimouski) (Global) (Rimouski)

Halifax -64 -36 Regina -26 1

Montréal -37 -10 Edmonton -3 25

Toronto -34 -6 Banff 54 81

Winnipeg -32 -5 Vancouver 22 50

Table 1: Change of heights in centimetres in new proposed datums in relationto CGVD28 (preliminary results).

a: Datum is the equipotential surface representing global mean sea levelb: Datum is the equipotential surface at mean water level at the tide gaugein Rimouski.

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5. Summary

In terms of accuracy and accessibility, theCanadian Geodetic Vertical Datum of 1928(CGVD28) does not satisfy today’s users needs forprecise height determination. Furthermore, it pre-empts the use of modern and efficient space-basedtechnologies for both precise height determinationand maintenance of the vertical reference frame. Areadjustment of the levelling network similar to theNAVD88 project, albeit more accurate thanCGVD28, would only be a temporary solution. Itwould not solve the problem of limited coverageand cost of maintenance. More importantly, itwould not account for the effect of crustal motionand localized benchmark movements. The mostviable alternative for the realization of a long-termvertical datum for Canada is a geoid model. Itwould define the datum in relation to an ellipsoid,making it compatible with space-based positioningtechnologies (e.g., GPS and satellite radar altime-try). It would allow easy access to orthometricheights everywhere across the Canadian territory.The existing monumented first-order levelling net-work would be readjusted by constraining it toorthometric heights derived from ellipsoidal andgeoid heights at selected CBN stations acrossCanada. The new datum would change the heightsassigned to benchmarks within a range of onemetre across Canada. However, the height differ-ences locally would maintain the same relative pre-cision of a few millimetres. CGVD28 will continueto co-exist with the new datum for quite some time,but its use will progressively diminish as geomaticscommunities across the country acknowledge thebenefits and efficiency that should come with amodernized datum.

References

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Burša M., E. Groten, S. Kenyon, J. Kouba, K. Rad?j, V.Vatrt, and M. Vojtišková. 2002. Earth’s dimensionspecified by geopotential. Studia geophysica et geo-daetica, 46, 1-8.

Burša M., S. Kenyon, J. Kouba, Z. Šima, V. Vatrt, M.Vojtišková. 2004. A Global vertical reference frame

based on four regional vertical datums. Studia geo-physica et geodaetica, 48, 493-502.

Cannon J.B. 1929. Adjustments of the precise level net ofCanada 1928. Publication No. 28, Geodetic SurveyDivision, Earth Sciences Sector, Natural ResourcesCanada, Ottawa, Canada.

Gareau R. 1986. History of precise levelling in Canadaand the North American vertical datum readjust-ment. Graduate thesis, University of Calgary,Calgary, Alberta.

Hannah J. 2001. An assessment of New Zealand’s heightsystems and options for the future height datum.Report for the Surveyor General, Land InformationNew Zealand, PO Box 5501, Wellington.

Heiskanen W.A. and H. Moritz. 1967. Physical Geodesy,Freeman.

Huang J., M. Véronneau. 2006. In preparation.LeGrand P., E.J.O. Schrama and J. Tournadre. 2003. An

inverse estimate of the dynamic topography of theocean. Geophys. Res. Lett., 30(2), 1062,doi:10.1029/2002GL014917.

Mäkinen J. 2004. Some remarks and proposals on the re-definition of the EVRS and EVRF. Presented at themeeting of the IAG Subcommission for the EuropeanReference Frame (EUREF), Bratislava, June 1, 2004.

Reigber, Ch., G. Balmino, P. Schwintzer, R. Biancale, A.Bode, J.-M. Lemoine, R. Koenig, S. Loyer, H.Neumayer, J.-C. Marty, F. Barthelmes, F. Perosanzand S.Y. Zhu. 2002. A high quality global gravityfield model from CHAMP GPS tracking data andaccelerometry (EIGEN-1S) Geophysical ResearchLetters, 29(14), 10.1029/2002GL015064.

Reigber Ch., R. Schmidt, F. Flechtner, R. König, U.Meyer, K.-H. Neumayer, P. Schwintzer, S.Y. Zhu.2005. An Earth gravity field model complete todegree and order 150 from GRACE: EIGEN-GRACE02S, Journal of Geodynamics 39(1), pp. 1-10.

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Véronneau M., A. Mainville and M.R. Craymer. 2002.The GPS height transformation (v2.0): An ellip-soidal-CGVD28 height transformation for use withGPS in Canada. Report, Geodetic Survey Division,Earth Sciences Sector, Natural Resources Canada,Ottawa, Canada.

Véronneau M. 2002. The Canadian gravimetric geoidmodel of 2000 (CGG2000). Report, Geodetic SurveyDivision, Earth Sciences Sector, Natural ResourcesCanada, Ottawa, Canada.

Zilkoski D.B. 1986. The new adjustment of the NorthAmerican datum. ACSM Bulletin, April, pp. 35-36.

Zilkoski D.B., J.H. Richards and G.M. Young. 1992.Results of the general adjustment of the NorthAmerican vertical datum of 1988. AmericanCongress on Surveying and Mapping, Surveying andLand Information Systems, 52(3), pp. 133-149.

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Authors

Marc Véronneau is a Geomatics Specialist atNatural Resources Canada where he has focused ondifferent aspects of vertical positioning (levelling,geoid modeling, satellite altimetry) for the last 17years. He primarily works on the development andenhancement of geoid models in Canada and hasco-authored over twenty papers in scientific jour-nals. Currently, he is responsible for implementinga new vertical datum for Canada, which willreplace the levelling-based datum with a geoid-based datum. He graduated from Université Lavalwhere he received a BSc and MSc in Geodesy in1986 and 1988, respectively.

Robert Duval has served as the ProgramManager for the Canadian Spatial ReferenceSystem since 2002. He joined the Geodetic SurveyDivision (GSD) of Natural Resources Canada(NRCan) following his graduation from UniversitéLaval in 1981 with a BSc specialized in Geodesy.Involved with conventional and satellite position-ing techniques for geodetic and geodynamic appli-cations, he lead GSD’s first GPS projects in 1984;

he was subsequently involved with operationaldevelopments related to GPS, NAD83 referenceframe computations and the implementation andoperation of the GPS-based Canadian ActiveControl System. In 1998, he was appointed Chiefof the Gravity and Geodetic Network Section withthe responsibility for the maintenance of these net-works, associated standards, as well as relatedresearch and development. Subsequently, he head-ed, GSD Planning and Project Management Officeuntil his nomination as Program Manager,Canadian Spatial Reference System.

Jianliang Huang has been a geodesist atGeodetic Survey Division, CCRS, NaturalResources Canada since 1999. His current respon-sibility is geoid development for improvingCanadian geodetic vertical datum. He obtained aBachelor of Engineering in Geodesy from WuhanTechnology University of Surveying and Mappingin China (Now becoming part of WuhanUniversity) in 1987, an MSc in EarthquakeGeodesy from Institute of Seismology, StateSeismological Bureau of China in 1990, and a PhDin Physical Geodesy in 2002 from University ofNew Brunswick (UNB). o

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CRUSTAL MOTION AND DEFORMATIONMONITORING OF THE CANADIAN LANDMASS

Joseph A. Henton, Michael R. Craymer and Rémi FerlandGeodetic Survey Division Natural Resources Canada, Ottawa, Ontario

Herb Dragert and Stéphane MazzottiGeological Survey of Canada, Natural Resources Canada, Sidney, British Columbia

Donald L. ForbesGeological Survey of Canada, Natural Resources Canada, Dartmouth, Nova Scotia

The science of geodesy and the corresponding reference systems it develops have increasingly beenapplied to measuring motions and slow deformations of the Earth’s crust driven by plate tectonics.Improvements to geodetic methodologies have therefore enabled better understanding of the Earth’s systems,including improved modelling and forecasting of changes that may affect society. These geophysical process-es also systematically affect the reference frames used as standards for geodetic surveys. Reference framestherefore must not only define the system of coordinate axes (including orientation, origin, and scale), butalso characterize the time-evolution of spatial coordinates on the Earth’s surface. When evaluating the effecton reference standards within a given area, it is also important to realize that geodynamic processes operateon various spatial scales. In this paper we summarize some of NRCan’s efforts to monitor contemporarycrustal dynamics across Canada. Progressing from continental to smaller regional scales, we outline therationale, techniques, and results. The observational data and interpretations presented are fundamentallydependent on the Canadian Spatial Reference System yet in turn also contribute to the incremental improve-ment of its definition and maintenance.

1. IntroductionToday the Earth Sciences Sector (ESS) continues torespond to the Order in Council that created theGeodetic Survey of Canada in 1909 tasking the thenMinister of Interior “to determine… the positions ofpoints throughout the country… which may form thebasis of surveys for all purposes, topographical, engi-neering or cadastral, and thereby assist in the surveywork carried on by other departments of the DominionGovernment, by Provincial Governments, and bymunicipalities, private persons or corporations.”

[passage from Canadian Spatial ReferenceSystem (CSRS) service logic model].

Geodynamic monitoring is demanding interms of observational accuracy and stability over awide range of spatial- and temporal-scales. Anappropriate reference frame is a requisite tool thatenables better understanding of geodynamic systemsthat in turn provides better constraints to predictedimpacts on society. However, the effects of geody-namic and geophysical crustal deformationprocesses can systematically bias the referenceframes used for geodetic surveys. Kinematic


La science de la géodésie et les systèmes de référence correspondants ont été de plus en plus utilisés pourmesurer les mouvements et les lentes déformations de la croûte terrestre causés par les plaques tectoniques.L’amélioration des méthodes géodésiques nous a donc permis de mieux comprendre les systèmes de la Terreen nous permettant, entres autres, de mieux modéliser et prévoir les changements qui risquent de nous touch-er. Ces processus géophysiques modifient aussi systématiquement les cadres de référence qui servent de normesaux levés géodésiques. Les cadres de référence doivent alors non seulement définir le système des axes descoordonnées (incluant l’orientation, l’origine et l’échelle), mais doivent aussi définir l’évolution temporelledes coordonnées spatiales sur la surface terrestre. Lorsqu’on évalue leur effet sur les normes de référence dansune zone donnée, il est aussi important de réaliser que les processus géodynamiques se produisent à plusieurséchelles spatiales. Dans cet article, nous résumerons certains des efforts de surveillance de la dynamiquecontemporaine de la croûte terrestre canadienne par Ressources naturelles Canada. De l’échelle continentaleà l’échelle régionale, nous présenterons un survol du besoin, des techniques et des résultats. Les données et lesinterprétations observationnelles présentées dépendent fondamentalement du Système canadien de référencespatiale tout en contribuant à l’amélioration de sa définition et à sa maintenance.

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processes affecting the Earth’s surface can systemat-ically affect geodetic measurements and ultimatelylead to inconsistencies between observations at dif-ferent epochs. As measurement and processingaccuracies and requirements have increased, theCanadian Spatial Reference System (CSRS) is nowin an era in which the time-variability of its outputsmust be evaluated.

For Canada the largest observed deformationrates are at the level of 1-2 cm/yr. Millimetre-per-yearresolution, and better, is required to investigatemany processes of the dynamic Earth and conse-quently to better quantify their potential impacts onsociety (e.g. natural hazard and climate changeeffects). Measuring these rates requires very high-precision geodetic techniques coupled with suitablylong observation windows. The frequency of obser-vations is largely dependent on the magnitude ofthe deformation process and the precision of themeasurement technique. Survey design and dataprocessing must also consider any quasi-periodicsignals (e.g. the annual signals resolved by GPS) thatcould bias estimates of geodynamic motions.

Within ESS of Natural Resources Canada(NRCan), regions of geophysical interest (particu-larly hazards) have warranted densified geodeticinfrastructure. For ESS, long-term topics of interestfor geodetic investigations include studies of post-glacial isostatic adjustment across the Canadianlandmass, the Saint Lawrence seismic zone in east-ern Canada, the active plate boundary above theCascadia subduction zone along Canada’s westcoast, and the active fault margin of the QueenCharlotte Islands. Additionally, investigations ofthe vulnerability of Canada’s coasts to climatechange processes have increasingly relied on precisegeodetic observations.

Targeted regional studies within ESS augmentthe CSRS and thus contribute to aiding and strength-ening international services to the geophysical andgeomatics communities. The international- andnational-scale geodynamic monitoring efforts, inturn, support regional deformation studies. Within aspecific region there are often different kinematicprocesses operating on various spatial scales. At thelargest scale, rigid tectonic plate motions may needto be considered. Then, measuring long-wavelengthsignals at a broader (e.g. national) scale provides theregional deformation “background signal” which isuseful when assessing more localized deformationsignals during regionally-targeted kinematic studies.

This paper outlines a number of efforts withinESS that contribute to monitoring crustal motion anddeformation at differing scales across the Canadianlandmass. It is not a complete review, but ratherintends to provide a well-rounded cross-section of

Sector studies. Progressing from larger to smallerspatial scales, the rationale, methodology, and results(for established studies) are summarized for thegeodynamic investigations. Although this articleemphasizes the work of ESS, a considerableamount of data, analyses, and interpretations havebeen dependent on the work and productive collab-orations of numerous other groups and agencies.Further information and details may be found in thepublications referenced in each section.

2. Global-Scale PlateKinematics and the ITRF

With the wide-spread use of space-based tech-niques for positioning in the scientific and engineer-ing communities, a unified and consistent globalspatial reference system has long been necessary.The International Terrestrial Reference System(ITRS) was proposed and implemented over twodecades ago. Its physical realization, theInternational Terrestrial Reference Frame (ITRF),provides a high accuracy and easily accessibleglobal coordinate system for the study of Earthdynamics at all scales [Altamimi et al. 2001].Several updates of the ITRF have been made avail-able with each one improving on its predecessor.Although originally developed for and by the sci-entific community, this system eventually becamethe most widely used reference system for posi-tioning. For example, some countries have adoptedversions of the ITRF as the national standard andeven WGS84, the native reference system for GPS,is now based on ITRF. Additionally these framesprovide velocity information and use kinematicconstraints in their realizations.

Any ITRF is composed of a set of station coor-dinates at a given epoch and their velocities in threedimensions with appropriate covariance informa-tion. The frame is currently defined via four tech-niques: (1) Very Long Baseline Interferometry(VLBI), (2) Satellite Laser Ranging (SLR), (3)Global Positioning System (GPS), and (4)Détermination d’Orbite et RadiopositionementIntégré par Satellite (DORIS) [Boucher et al. 2004].A combination of the techniques is necessarybecause none alone is sensitive to all the parame-ters necessary to define a reference frame. Thecombination takes advantage of the strength of eachtechnique; e.g. the SLR contribution is essential forthe determination of the origin while VLBI and SLR

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…a unifiedand consis-tent globalspatial refer-ence systemhas longbeennecessary.

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contribute to determine the scale. The combinationof the networks from each technique also requiresthat they be connected at as many collocated sites aspossible. Repeated local surveys are required toprecisely measure the three-dimensional offsetsbetween the markers for each collocated techniqueand detect any potential change. All of the tech-niques contribute to the velocity field, which is usedto solve for the time evolution and the no-net rota-tion condition of the system. For the general usercommunity, it is the GPS products (coordinates andsatellite orbits) that enable easy access to the ITRF.

Earth scientists rely on ITRF products formany investigations related to large-scale massredistribution, including characterization of theEarth’s interior and climate change studies. In par-ticular, space-geodetic techniques have revolution-ized the study of plate tectonics. At one time, platemotion models were primarily constrained by sea-floor magnetic anomalies and other geologicalinformation with rates averaged over millions ofyears. With GPS it is possible to directly measurecontemporary tectonic plate movement on a timescale of years. The motion of any rigid body on thesurface of a sphere can then be represented by arate of rotation about an axis (i.e. Euler pole) andexpressed as a rotation vector. The North AmericanPlate, for example, undergoes a counter-clockwiserigid body rotation around an Euler pole locatednear equatorial, northwestern South America (referto Figure 1). The associated tectonic plate motionsfor Canadian sites are on the order of 2 cm/yr.

When accurately defining the rotation vectorof a plate, it is important to ignore stations biasedby local site effects and those in known deformingzones unless reliable models of such deformationsare available. Ideally, only the sites that best repre-sent the rigid parts of the tectonic plate should beused. High-precision GPS has allowed scientists todiscriminate subtle differences in velocities withinwhat had been considered individual tectonic units,and in the process identify new plates or sub-platesand quantify their individual rotation vectors. Theimproved resolution of plate rotation vectors is alsovery useful for investigations that utilize kinematicinformation (e.g. fault and other crustal deforma-tion studies). However, the comparison of preciseGPS-determined results is often complicated byinvestigators using different techniques to producea plate-fixed reference frame in which to expresstheir results. For North America the SNARF initia-tive (discussed in the next section) will provide aconsistent reference system in which scientific andgeomatics results (e.g. positions in tectonicallyactive areas) can be produced and inter-compared.

3. Continental & NationalScale Monitoring Efforts

3.1 NAREF Densification of ITRF inNorth America

Since the beginning of 2000, ESS has beenplaying a leading role in the North AmericanReference Frame (NAREF) Working Group of theIAG Sub-Commission 1.3c (Regional ReferenceFrames for North America) [Craymer andPiraszewski 2001]. One of the primary objectivesof this working group is the densification of theITRF in North America using continuously operat-ing GPS (CGPS) stations. Following the distributedprocessing approach advocated by the InternationalGNSS Service (IGS) [Blewitt 1997], the NAREFworking group members have been independentlycomputing weekly regional coordinate solutions forCGPS sites throughout North America.

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Figure 1: North American plate rotation pole and predicted veloc-ities. The blue vectors represent the motion for those sites predict-ed from North American rotation about its pole. The arrow lengthsare proportional to the rates at their origin point and are given forCanadian continuous GPS sites present in the IGS cumulativesolution (GPS week 1345). The North American rotation pole isfrom ITRF2000 [Altamimi et al. 2002].

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NRCan has been producing three such weeklyregional solutions for Canada. Two are being gener-ated by NRCan’s Geodetic Survey Division (GSD)on a regular basis for redundancy and quality con-trol. One of these solutions incorporates additionalstations just outside Canadian borders for furtherredundancy and continuity with American solutions.In addition, NRCan’s Geological Survey ofCanada—Pacific Division has also been contribut-ing weekly solutions for their Western CanadaDeformation Array (see later section) on the westcoast. Other solutions include a preliminary versionof the Plate Boundary Observatory from the ScrippsInstitution of Oceanography, and the entire USCORS (Continuously Operating Reference Stations)network from the US National Geodetic Survey thatincludes nearly 700 stations. More recently, effortsare underway to include stations from provincialGPS networks, including those of British Columbia,Quebec and, soon, New Brunswick.

Following internationally accepted densifica-tion methodologies [Ferland et al. 2003], these dif-ferent regional solutions are being combined into asingle NAREF weekly solution. NAREF combina-tion solutions beginning the first week of 2001have been submitted to the IGS data archives andare usually available with about a four-week latency.There are presently over 800 stations in the com-bined NAREF network (refer to Figure 2). Manystations are included in more than one solution toprovide redundancy checks and to allow for thecorrect weighting of the different solutions relativeto each other and to the global solutions. The goal isto have all stations included in at least two differentsolutions. All regional solutions agree with eachother and with the global ITRF and IGS solutions atthe level of a few millimetres.

The weekly NAREF solutions have recentlybeen combined into a single so-called cumulative(multi-epoch) solution to provide estimates of bothstation coordinates and their velocities with respectto a consistent reference frame throughout NorthAmerica. Velocity estimates with accuracies of lessthan 1 mm/yr are expected to be achievable in thenear future with the accumulation of several years ofdata. This relatively long time series is required inorder to estimate and remove systematic annual andsemi-annual signals due to seasonal effects and dis-continuities caused by changes in equipment or soft-ware (refer to Figure 3). Nevertheless, even today,these solutions can be used to support geodynamicsstudies of both large and small scale crustal motions,including the direct measurement of the motion ofthe North American tectonic plate and intra-platedeformations such as post-glacial rebound as dis-cussed later in this paper. A preliminary version of

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Figure 2: NAREF network of continuous operating GPS receivers. The redcircles represent stations in the IGS global network while the blue circles rep-resent densification stations.

Figure 3: Time series for station INVK in Inuvik, NWT illustrating relativelylarge seasonal fluctuations primarily due the effects of snow on the antennaphase centre. Note also the step (discontinuity) in the time series just after2003.5 due to the addition of an antenna dome.

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this NAREF velocity solution has also been used tohelp define a North American plate-fixed referenceframe for studies of intra-plate crustal motions (seenext section).

3.2 A Stable North AmericanReference Frame (SNARF)

As discussed earlier, regional reference framesfixed to the stable part of a tectonic plate are oftenrequired to facilitate geophysical interpretation andinter-comparison of geodetic solutions of crustalmotions. In 2003, the Stable North AmericanReference Frame (SNARF) Working Group wasestablished under the auspices of UNAVCO, Inc.and IAG Regional Sub-Commission 1.3c for NorthAmerica especially to address needs for the US-ledEarthScope project. The goal is to define a regionalreference frame that is consistent and stable at thesub-mm/yr level throughout North America.

The SNARF Working Group identified andaddressed several issues that must be dealt with toproperly define such regional frames, including (1)the selection of “frame sites” based on geologic andengineering criteria for stability, (2) the selection ofa subset of “datum sites” which represent the stablepart of the plate and will be used to define a no-netrotation condition, (3) the modelling of any signifi-cant intra-plate motions using a relatively dense GPSvelocity field, and (4) the generation and distributionof reference and deformation products for generaluse [Blewitt et al. 2005]. The SNARF vertical datumis consistent with ITRF2000 in that the centre ofmass of the whole Earth system is taken to be theorigin while the horizontal datum differs by a rota-tion rate that brings the rotation of the stable part ofNorth America to rest.

The first release of SNARF provides a rotationrate vector that transforms ITRF2000 velocitiesinto the SNARF frame, and an initial referenceframe is defined via a list of selected sites, epochcoordinates and velocities in a Cartesian system.This rotation effectively defines the SNARF refer-ence frame in relation to the ITRF. It was comput-ed using only stable sites from a combination ofvelocity solutions from GSD for the NAREF net-work and the Canadian Base Network (see nextsection), and a velocity solution from PurdueUniversity for a selection of US CORS stations.Horizontal intra-plate velocities in the SNARFframe exhibit a pattern that is more consistent withexpected deformations from post-glacial reboundthan velocities obtained from other estimates; e.g.,ITRF2000 (refer to Figures 4a & 4b). In addition,these velocity solutions were also used to determine

a semi-empirical model of post-glacial reboundbased upon a novel assimilation technique thatcombines GPS velocities with a geophysical model[Blewitt et al. 2005] (see Figure 5). Over the nextfew years SNARF will be incrementally improvedthrough further research and as more accuratevelocity solutions become available.

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Figure 4a: Residual horizontal intra-plate velocities of CBN network usingNorth American plate motion estimates (rotations) from ITRF2000. The platemotion estimate absorbs much of the horizontal motion from post-glacialrebound that is expected to radiate outward from the areas of maximumuplift. Little outward horizontal motion is seen in the intra-plate velocities.

Figure 4b: Residual horizontal intra-plate velocities of CBN network usingNorth American plate motion estimates (rotations) from SNARF 1.0. In thisreference frame defined by the SNARF plate motion estimate, the outwardpattern of intra-plate velocities is more clearly visible. The SNARF platemotion estimate is affected less by post-glacial rebound.

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3.3 Geodynamic Observations fromthe CBN

Initiated in 1994, the Canadian Base Network(CBN) is a national network of high-stability pillarmonuments with forced-centering mounts for GlobalPositioning System (GPS) receiver antennas. Theinitial role of the CBN was to complement theActive Control System (ACS) of the CSRS by pro-viding easily accessible 3D reference coordinatesites, with a reasonable distribution across Canada.In order to maintain the accuracy of the CSRS ref-erence frame realization, CBN sites have been re-occupied to confirm initial positions and to detectcrustal movements. By combining nearly 10 years ofrepeated multi-epoch (episodic) GPS measurements,GSD has begun to estimate velocities at the CBNsites in order to provide an increased spatial sam-pling of crustal deformation throughout Canada. Todetermine individual station velocities, regionalCBN solutions for each measurement epoch aresystematically combined into a single Canada-wide,multi-epoch cumulative solution (i.e. similar to theprevious discussions for NAREF and SNARF). Inorder to generate time series of consistent, high-accuracy coordinates for velocity estimation, it isnecessary to ensure consistency of the integrationinto the reference frame. This is accomplished byaligning each of the individual CBN solutions to asubset of stations from a recent cumulative solutionfor the IGS global network in ITRF. Fortunately,there are many IGS stations in Canada and mostwere included in each regional CBN solution tostrengthen the connection to reference frame and

ensure consistency between epochs. Consistent andrealistic weighting of the individual CBN solutionsis improved through the estimation of separate vari-ance components relative to the IGS global solu-tion. After the individual CBN solutions are alignedand weighted, they are combined together in asimultaneous cumulative solution to producevelocities at each site [Henton et al. 2004].

On the national scale, glacial isostatic adjust-ment is the most significant geodynamic processdriving vertical deformation [e.g., Tushingham andPeltier 1991; Peltier 1994]. Preliminary resultsfrom the combination of CBN regional solutions inCanada exhibit a spatially coherent pattern of upliftconsistent with the expected post-glacial rebound(PGR) signal (refer to Figure 6). Regions of high-est uplift rates are generally consistent with areas ofgreatest ice accumulation during the last period ofcontinental glaciation [e.g., Dyke 2004; Peltier1994]. Horizontal velocities associated with PGRare also spatially coherent (typically directed radi-ally outward from regions of highest uplift) buthave smaller rates. These modest horizontal veloc-ities can provide an important additional constraintto PGR models. However, the horizontal vectorsmay be significantly biased by differing referencesystem rotation-rate vectors (e.g., Figures 4a & 4b).The definition of SNARF should minimize thisissue. Eventually the deformation maps, coupledwith PGR model predictions, may allow NRCan tointerpolate or estimate coordinates at differentepochs for regions that display spatial coherence inthe velocity field.

3.4 Using Absolute Gravity toMonitor Uplift at CBN Sites

The integration of geodetic techniques is desir-able when monitoring geodynamic processes (andsimplifies any connections between correspondingreference standards). Absolute gravimetry (AG),which is independent of GPS, has demonstrated thatit plays a complimentary role to GPS especiallywhile measuring vertical crustal motions [e.g.,Lambert et al. 2001]. In addition, repeated GPS andAG observations at common sites provide insightinto the geophysical processes that drive theobserved deformation since AG is sensitive tointernal mass changes and not just deformationalone. Therefore, issues such as mass redistributionor changes in density contrasts within the Earth maybe addressed by monitoring positional changes (i.e.,primarily height changes) and integrating theseobservations with gravitational variations. However,the observed rates of gravity change resulting from

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Figure 5: Post-glacial rebound rates from the SNARF model (courtesy of JimDavis, Harvard-Smithsonian Center for Astrophysics [after Blewitt et al. 2005]).This empirical model of post-glacial rebound employs a novel technique thatcombines observed GPS velocities with a geophysical model.

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post-glacial rebound in Canada are no greater thanapproximately -2 µGal/yr (one µGal is approxi-mately one part-per-billion of the Earth’s surfacegravity). Observing such a small signal requires asuitable combination of high-precision and obser-vation length. The principle of most modernabsolute gravimeters is to observe (using laserinterferometry) the acceleration of a free-fallingtest mass in a vacuum. This technique thereforerequires very precise measurements of length andtime over repeated “drops.” Despite the simple fun-damental principles, high-accuracy absolutegravimeters involve a great deal of instrumental andelectronic sophistication. Properly operated and aftercareful processing, AG can provide the value of theEarth’s gravity at a point with an accuracy of onepart-per-billion, and the instruments can be main-tained nearly drift-free. Also the AG instruments ofthe CSRS are tied to international standards throughcomparisons at the Bureau International des Poids etMesures (BIPM). Continued improvements inabsolute gravimetry have made these instrumentsmore compact, robust, and efficient.

In order to monitor the temporal variations ingravity resulting from regional glacial isostaticadjustment, a set of absolute gravity measurementsites has been established in northern Quebec, col-located near pillars of the CBN. The NouveauQuebec-Labrador region of eastern Canada was thesite of one of the major ice domes of the LaurentideIce Sheet [e.g., Dyke 2004; Peltier 1994] and is cur-rently experiencing post-glacial rebound. For east-ern Canada the highest uplift rates are in the vicin-ity of James Bay through to southwestern Labrador.Rates decrease to the south and become negativetowards the coastal Atlantic margins. For post gla-cial rebound, surface gravity measurements sensethe effect of increasing distance from the centre ofmass of the earth (i.e. gravity decreases) coupledwith the redistribution of mass due to viscous flowat great depth (increases gravity). The resultingregional gravity decrease associated with uplift dueto post-glacial uplift is approximately -0.15µGal/mm [Lambert et al. 2001]. Preliminaryabsolute-gravity trends for this region showdecreasing gravity with time. Values range from

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Figure 6: Contour map of observed CBN vertical rates. Preliminary results from the combination of CBN regional solutions in Canadaexhibit a spatially coherent pattern of uplift consistent with the expected post-glacial rebound (PGR) signal. Black dots represent thelocations of CBN sites.

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about -2 µGal/yr (i.e. ~13 mm/yr for regional PGR)near Kuujjuarapik and Radisson and decrease south-ward to approximately -0.5 µGal/yr (~3 mm/yr)near Val d’Or. These preliminary results exhibitgeneral agreement between the rates for GPS upliftvelocities and gravity trends. Additionally, theobserved (GPS & AG) rates are generally consistentwith predictions of vertical crustal motion frompost-glacial rebound models. To better contribute tothe definition of the vertical component of a highlyaccurate, multi-purpose, active and integratedCSRS, efforts are now underway to create a sparsearray of absolute gravity observation sites collocatedwith geometric reference (i.e. VLBI, continuous andepisodic GPS) stations across Canada.

4. Monitoring Relative Sea Levelfor Coastal Impact Studies

4.1 Introduction

As potential impacts of climate change andsea-level rise on coastlines depend in large measureon the localized rates of relative sea-level heightchange, it is important to quantify vertical defor-mation velocities in vulnerable coastal regions. Themagnitude of the rate of relative sea-level changecan be used to better understand future impacts (e.g.flooding, storm-surge levels). Sea level heights havetraditionally been determined by tide gauges whichmeasure the relative sea level height with respect toland. The long-term changes in relative sea levelobserved at tide gauges reflect both the verticalcomponent of regional crustal kinematics and thechange in the regional (and/or global) sea level sur-face. Through this linkage with relative sea levelobservations, understanding the kinematics of post-glacial rebound in vulnerable coastal areas is animportant adjunct to relative sea level studies.

4.2 Eastern Canada

While most of the Canadian landmass is current-ly experiencing uplift associated with post-glacialisostatic adjustment, the Maritimes are experiencingsubsidence. This is primarily due to collapse of theperipheral bulge following the deglaciation of theLaurentide Ice Sheet (refer to Figure 7), in additionto the effects of rising post-glacial sea level loadingthe continental shelf. The subsidence rates are notparticularly large, typically on the order of -1 to -2mm/yr. However they are of the opposite sign tosea level height change (on the order of +2 mm/yr)and consequently relative sea-level rise (withrespect to land) is regionally more rapid. Sea-levelrise can produce significant impacts in the coastalzone, particularly for low-lying parts of theMaritime Provinces [e.g., Forbes et al. 2004b].These include storm impacts on the coast (waves,surges, and flooding), sediment movement and ero-sion hazards, impacts on ecological systems (e.g.coastal wetlands and fisheries), and damage to pri-vate or commercial property and public infrastruc-ture [e.g., O’Reilly et al. 2005]. Work is underwaywithin ESS to better quantify these hazards throughthe installation of additional geodetic infrastructure(e.g. new continuous GPS sites at select regionaltide gauges).

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Figure 7: Generalized process of post-glacial rebound. The mantle acts like anextremely viscous fluid; it continues to flow back to regions where heavy icesheets had forced out portions of the mantle 18,000 years ago. These regions,like northern Quebec, experience “post-glacial rebound.” Around the edges ofthe ice sheet the crust flexes upward creating a peripheral fore-bulge. Afterdeglaciation, the fore-bulge slowly collapses resulting in subsidence forregions such as the Maritimes.

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4.3 Great Lakes

The Great Lakes is an important economic cor-ridor where lower water levels can have significantimpacts such as the need for expensive dredging ofports and harbours, and reduction in hydro-electricpower generation. Additionally, elevated erosionrates have been evident in regions of the Great Lakesduring previous higher water levels. These potentialimpacts ultimately depend in large measure on therate of change of relative water-level heights withrespect to land. Again, although the rate of crustaltilting driven by PGR is not particularly large, itsteadily accumulates over time and can be clearlyobserved in water-level gauge records [e.g.,Mainville and Craymer 2005]. This regional verticaldeformation must be considered when evaluatinglong-term impacts on datums, water management,infrastructure, and basin ecology.

The regions north of the Great Lakes are risingfaster than the regions to the south. In fact, much ofthe southern areas are part of the collapsing glacial

“forebulge” and are experiencing subsidence. Thison-going process of crustal “tilting” (see Figure 8)results in a pattern of slowly declining lake levels onnorthern shores with a commensurate rise in lakelevels on southern shores. These relative changes ofwater level for a single lake can be resolved quiteprecisely from a long history of water gauge obser-vations [e.g., Mainville and Craymer 2005].However, it is difficult to accurately relate levels onone lake with those of another.

GPS stations have recently been established atGreat Lakes water gauge sites in Canada by GSD incollaboration with the Ohio State University, and inthe US by the US National Geodetic Survey.Combining these with other GPS measurements (e.g.the CBN) is enabling the determination of an accu-rate and spatially coherent pattern of absolute crustalvelocities that is consistent with the expected rates ofglacial isostatic adjustment. These results will enablelakes to be linked to each other as well as to sea levelin support of bathymetry, hydraulic operations, andhydrological studies in the Great Lakes Basin.

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Figure 8: Glacial isostatic effects along lake shorelines. In addition to the effect of “tilting” generalized in this illustration, on-goingchanges in average water level may also contribute to the impact on vulnerable coastal regions.

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4.4 Arctic Canada

Observed climate warming in the Arctic maylead to increases in Arctic Ocean sea level[Proshutinsky et al. 2001]. Sea-level rise increasesthe flooding risk and is a clear concern for coastalcommunities such as Tuktoyaktuk [Manson et al.2005]. Furthermore the coastal area subject to risingrelative sea level is expected to expand [Forbes etal., 2004a]. With recent infrastructure developmentassociated with hydrocarbon production in theMackenzie River delta region, this work has becomemore timely. As the impacts on coastlines ultimatelydepend in large measure on the localized rate of rel-ative sea level height change, it is important to betterquantify crustal deformation velocities throughoutcoastal northern Canada. Inherent to this study is theneed to better map the observed pattern of PGRthroughout northern Canada in order to more tightlyconstrain post-glacial isostatic adjustment models.

To provide a framework in which to monitorthese changes, a consistent velocity field will bedetermined from GPS observations throughoutNorth America, including the Canadian ArcticArchipelago and the Mackenzie Delta region. Todetermine the contribution of vertical motion tosea-level rise under climate warming in theCanadian Arctic, ESS in collaboration with theCanadian Hydrographic Service has establishedcollocated tide gauges and continuous GPS at anumber of sites across the Canadian Arctic, fromAlert and Qikiktarjuaq in the east to Ulukhaktok(Holman) and Tuktoyaktuk in the west. The contin-uous GPS sites have been augmented with multi-epoch (episodic) sites in places such as Kugluktuk(Coppermine) and Mercy Bay (northern BanksIsland). This expanded network will furtherenhance regional geophysical studies including thediscrimination of crustal motion, other componentsof coastal subsidence, and sea-level rise.

5. Campaign GPS Constraintson Regional Tectonicsand Seismicity

5.1 Introduction

GPS and other geodetic measurements ofcrustal deformation provide an important new toolfor estimating earthquake hazard. In addition to thestandard method of earthquake hazard based on pastearthquake statistics, it is now possible to relatecurrent deformation rates to the frequency of large

earthquakes. Both continuous and campaign GPSstations have been established for earthquake hazardin four areas, the western Canada subduction zone,the Queen Charlotte transform fault zone, the Yukoncrustal deformation region, and the eastern Canadaregion of high seismicity.

As a complement to a network of continuousGPS stations, campaign GPS surveys provide aneconomical way to obtain a denser map of crustaldeformation. Major limitations associated withcampaign GPS measurements are: (1) lack of reso-lution of short-term (less than one year) deformationepisodes, and (2) less accurate estimates of long-term velocities. Relative velocities across tectonical-ly active regions are typically of the order of 5 - 50mm/yr, which can generally be resolved with cam-paign GPS data acquired every year over a three tofive year period. In the following sub-sections, twoexamples of campaign surveys in western Canada(Queen Charlotte Margin and Northwestern Canada,investigated in collaboration with University ofVictoria) are described, where the relative motionsare associated with plate boundary interactions andstrain transfer within the Canadian Cordillera.

In contrast, relative velocities across intraplate(tectonically stable) regions are typically less than 1mm/yr. Such relative motions are at the current limitof resolution of GPS measurements. An example ofa GPS survey (in collaboration with UniversitéLaval) of intraplate deformation at the eastern edgeof the Canadian Shield along the lower SaintLawrence Valley is presented. In all of the exampleswithin this section, typical surveys consisted ofmeasurements of two to three full days at each site.The data, along with data from nearby continuousstations, were processed at the Pacific GeoscienceCentre (Geological Survey of Canada—PacificDivision, Sidney, BC) with the Bernese GPSSoftware package. Processing details can be found inthe publications referenced in each section.

5.2 The Queen Charlotte Margin

Since mid Eocene (ca. 42 Ma), the QueenCharlotte (Q.C.) margin has been primarily under astrike-slip tectonic regime associated with thePacific-North America relative motion [Hyndmanand Hamilton 1993]. At about 5 Ma, a small changein the Pacific-America relative motion resulted inthe current oblique convergence along Q.C. margin.A wide range of geophysical data indicates a com-ponent of convergence and possibly under-thrustingalong the southern Q.C. margin (e.g. high offshoreheat flow, seismic structure studies) [Smith et al.2003]. The seismicity shows primarily strike-slipfaulting (e.g. the great M=8.1 earthquake of 1949),

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although there are some thrust mechanisms near thesouthern end of the islands.

In 1998, NRCan established a small local networkof five campaign GPS sites in the central and north-eastern Q.C. Islands. These sites were re-surveyedin 1998 and 2001 [Mazzotti et al. 2003a]. In 2002,three new sites along the west coast of the islands andone on the mainland at Prince Rupert were then added[Bustin et al. 2004]. The full network (nine sites) hasbeen surveyed every year from 2002 to 2005. Thesecampaign GPS network sites were augmented by thecontinuous station at Williams Lake, to provide astable tie to the North America plate, and campaignmeasurements from the Canadian Base Network sitesin northern British Columbia, to provide some con-straints for the kinematics of the central Cordillera. TheGPS velocity field, with respect to North America,indicates oblique convergence along the Q.C. margin,with relative motion of 5 - 10 mm/yr directed mostlynorthward (refer to Figure 9). The landward gradient inleft lateral shear (i.e. decrease in margin-parallelmotion) shows that the Q.C. fault is currently lockedand is building up strain until the next large earthquakerupture [Mazzotti et al. 2003a]. The results also showthat a small component of the Pacific-North Americaoblique convergence (~10 per cent) is probablyaccommodated by distributed shear across the Q.C.margin and possibly in mainland [Mazzotti et al.,2003b]. A possible interpretation is that this obliqueconvergence is accommodated mainly by partitioninginto strike-slip earthquakes on the Q.C. fault andinfrequent large under-thrusting earthquakes beneaththe margin [Bustin et al., 2004]. Thus, the earth-quake hazard along the Q.C. margin may be higherthan current estimates based solely on an activetranscurrent Q.C. fault.

5.3 The Yukon & Northwestern Canada

Northwestern Canada is known for the intensetectonism during the Mesozoic (ca. 250 to 65 Ma)associated with the accretion and deformation of thedifferent terranes that form the Canadian Cordillera.No sign of significant tectonic activity has beenrecorded along the major faults and deformationzones in most of northwestern Canada for the last~50 Myr. In contrast, the southwestern YukonTerritory and adjacent Alaska Panhandle region isthe locus of intense deformation due to the collisionof the Yakutat block, a composite oceanic-continen-tal terrane that is currently being accreted in the cor-ner of the Gulf of Alaska. This collision produces thehighest mountain ranges in Canada (e.g. St. Eliasrange and Mount Logan). Earthquake activity is veryhigh in this collision zone including several M~8events, but surprisingly substantial seismicity also

occurs in the far-field Mackenzie and RichardsonMountains, around 800 kilometres northeast of thecollision zone [Mazzotti and Hyndman 2002;Hyndman et al. 2005]. The Mackenzie Mountainsappear to be undergoing NE-SW shortening, whileN-S right-lateral strike-slip deformation is occurringin the Richardson Mountains.

In order to understand the details of the wholenorthwestern Canada tectonics and seismicity,NRCan started a campaign GPS survey in 1999across the Yukon and Northwest Territories. The cur-rent network comprises 20 campaign GPS sites, sup-plemented by a larger network of 28 continuous GPSstations from British Columbia to Alaska [Leonardet al. 2005]. The campaign sites were surveyed atfour or five epochs since 1999. The interpretation ofthese results have been complicated by the occur-rence of the very large (magnitude M~8) Denaliearthquake in December 2002. Coseismic and post-seismic displacements related to this earthquake at

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Figure 9: GPS velocities along the Queen Charlotte margin. Redand orange vectors show velocities with respect to stable NorthAmerica for campaign sites with five years of data or more andthree years of data, respectively. Uncertainty ellipses show the 95per cent confidence region. The large black vector shows themotion (from the rotation vector of DeMets and Dixon [1999]) ofthe Pacific plate relative to North America.

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our GPS sites reach a few centimetres and create anoffset that needs to be removed in order to estimatethe long-term velocities of our sites. Overall, thecontinuous and campaign GPS velocities are con-sistent with the transfer of compressive strain (~5mm/yr) across the northern Canadian Cordillera tothe foreland fold-and-thrust belt with little inter-vening deformation [Mazzotti and Hyndman 2002;Leonard et al. 2005] (see Figure 10). These resultsindicate that the whole northern Cordillera is cur-rently part of the large-scale Pacific-North Americaplate boundary zone and accommodates about 10per cent of the full relative plate motion.

5.4 The Lower Saint Lawrence Valley

The Saint Lawrence Valley, Quebec, presentsone of the largest concentrations of earthquakes ineastern North America. Background seismicityextends over 900 kilometres from the Gulf of SaintLawrence to Montreal following the PaleozoicIapetan Rift system [e.g., Adams and Basham 1991;Lamontage et al. 2003]. Two main seismic zonesoccur along this trend: Charlevoix, the most activein eastern Canada and the locus of at least five M6+earthquakes in the last 350 years; and lower Saint

Lawrence, where the largest known earthquakes areabout M5. Integration of earthquake moment statis-tics in both zones indicates that the equivalent seis-mic deformation rates are 1.0 ± 0.5 mm/yr and 0.2± 0.3 mm/yr, respectively [Mazzotti and Adams2005; Mazzotti et al. 2005].

Unlike plate margins where the seismic activityis directly correlated with plate interactions, easternCanada earthquakes lie in the “stable” interior of theNorth American Plate. The driving mechanismsbehind of these earthquakes are thus more difficult todetermine. Additionally, the hazard posed by poten-tially devastating earthquakes for this region is notwell constrained due to limitations with the proba-bilistic seismic hazard estimation method (e.g. theinexact nature of extrapolating the rate of occurrenceof frequent small events to the occurrence of infre-quent larger events, relatively short instrumental andhistorical records, and limited paleoseismic evidencefor past large events).

In 2003, NRCan started a new program to studythe geodynamic aspects of earthquake hazard ineastern Canada. The study [Mazzotti et al. 2005]used the existing network and data for the CanadianBase Network and added an additional survey to anetwork of 16 stations surrounding the SaintLawrence Valley. These high-precision campaigndata provide relative velocities and strain ratesacross both the Charlevoix and lower SaintLawrence seismic zones based on three to fourcampaigns over the last seven to nine years. On aregional scale, horizontal strain rates are 0.5-2nanostrain per year of roughly NNW-SSE shorten-ing (refer to Figure 11). This strain pattern agreeswell with earthquake focal mechanisms. Horizontalvelocity vectors on both sides of the Saint LawrenceRiver suggest that this shortening corresponds to amaximum convergence of 0.5 ± 0.5 mm/yr betweenthe north and south shores, in general agreementwith the rate from earthquake statistics. Assumingthat seismicity in Charlevoix follows typicalGutenberg-Richter statistics, the GPS results con-strain the return of a potentially very damagingmagnitude M~7 earthquake to ~170 years.

6. Monitoring Crustal Motionsat an Active Plate Boundarywith Continuous GPS

6.1 Introduction

The coastal region of southwestern BritishColumbia comprises the northern portion of the

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Figure 10: GPS velocities in northwestern Canada. Red and blue vectorsshow velocities with respect to stable North America for continuous and cam-paign GPS sites, respectively. The green vector shows the Yakutat site GPSvelocity. Uncertainty ellipses show the 95 per cent confidence region. Thelarge orange vector shows the Pacific/North America relative motion fromDeMets and Dixon [1999].

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Cascadia Subduction Zone (CSZ) that forms theconvergent plate boundary between the Juan deFuca and North America plates. This boundary ismarked by extensive earthquake activity as well asa belt of active arc volcanoes stretching from north-ern California to southwest British Columbia.Special aspects of this subduction zone are the rel-ative youth of the subducting oceanic crust, whichranges from six to 10 Ma at the oceanic trench, anda modest convergence rate of under 40 mm/yr.Seismic, thermal, and geodetic studies have deter-mined that the shallower interface of the CSZ islocked, and that stress is accumulating for the nextgreat subduction thrust earthquake. From paleo-seismic evidence, such great events occur every500 to 600 years, the last having occurred in 1700.

The monitoring of crustal motions along thecoastal margin of southwestern BC using continuousGPS stations was initiated on Vancouver Island in1992 with sites established in Victoria (ALBH) andHolberg (HOLB) [Dragert and Hyndman 1995].These sites, along with a reference site nearPenticton (DRAO), form the basis of the WesternCanada Deformation Array (WCDA) which nowconsists of 16 stations in southwestern BC. Usinglow-multipath antennas mounted on stable geodeticmonuments, the relative position of these stationshundreds of kilometres apart can be monitored on adaily basis to an accuracy of a few millimetres. Thislevel of accuracy allows the resolution of long-termcrustal movement associated with major platemotions as well as the relative squeezing and buck-ling of the plate margins. More recently, GPS datafrom a number of WCDA stations have also revealedtransient crustal motions that are related to repeatedslow slip on the deeper subducting plate interface.

6.2 Long-term Deformation

Analyses of GPS data from WCDA sites haveconfirmed that long-term elastic deformationoccurs along the northern Cascadia Margin due tothe locking of converging plates across a portion ofthe subduction interface between the Juan de Fuca(JDF) plate and the overlying North America (NA)plate [cf. Flueck et al. 1997; Mazzotti et al. 2003c].The motion vectors shown in Figure 12, the largestexceeding 1 cm/yr, are based on the linear trends inthe time series of changes in horizontal positions ofGPS sites with respect to the reference site DRAO,located south of Penticton, British Columbia, andassumed fixed on the NA plate. The pattern of theregional crustal velocity field is a key constraint indetermining the location and extent of the lockedfault zone - i.e. that portion of the fault that willultimately rupture in a great subduction-thrust

earthquake [cf. Wang et al. 2003]. For the northernCascadia margin, the fully locked portion of thesubducting plate interface lies offshore beneath thecontinental slope and the “transition zone,” markedby a transition from fully-locked to free-slippingplates, terminates directly beneath the coastline ofVancouver Island and the western edge of theOlympic Peninsula. The landward extent of theincipient rupture zone for the next megathrustearthquake is a key parameter for estimating strongmotions that can be expected in the densely popu-lated areas of Vancouver and Seattle from such anearthquake.

6.3 Episodic Tremor and Slip

Improvements in the accuracy of IGS preciseorbits and the regional densification of continuousGPS coverage were two key factors leading to thediscovery of a “silent” slip event that occurredalong the Cascadia Subduction Zone in August1999 [Dragert et al. 2001]. Analysis of GPS datafrom 1994 to 2005 has revealed that the motions ofcontinuous GPS sites in northern Washington Stateand southern Vancouver Island are marked bynumerous, brief, episodic reversals. This is bestillustrated by the east-component time series at the

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Figure 11: GPS horizontal strain-rates for the Saint Lawrence seismic zones.Dark cyan arrows show the average horizontal principal strain rates for thenetwork (spatial extent given by dotted grey polygon). Red arrows show thehorizontal principal strain rates for sub-networks around the Charlevoix andlower Saint Lawrence seismic zones (spatial extent given by dotted red poly-gons). Light grey circles indicate the pattern of regional seismicity [afterMazzotti et al. 2005].

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Victoria GPS site (ALBH) where the motion relativeto DRAO is clearly characterized by a sloped saw-tooth function: For periods of 13 to 16 months, thereis eastward motion that is more rapid than the long-term rate, followed by a one to three week period ofreversed motion (refer to Figure 13). These transientreversals of motion are surprisingly regular events,having a recurrence interval of 445 ± 35 days. Thecumulative displacements over the two-week peri-ods of these transient events is generally less than sixmillimetres, in a direction opposite to the longer-term deformation motion. For the longer-lastingevents, station displacements were not simultaneousat all coastal margin sites but appear to migrate alongstrike of the subduction zone at speeds ranging from5 to 15 km/day. These transient events are accompa-nied by distinct, low-frequency tremors [e.g., Rogersand Dragert 2003], similar to those reported in theforearc region of southern Japan [Obara, 2002]. Theone-to-one correspondence of slip events with pro-nounced tremor activity prompted the naming of thisphenomenon as “Episodic Tremor and Slip” (ETS)[Rogers and Dragert 2003].

The transient surface displacements can bereplicated by simple slip-dislocation models if oneadopts the subduction interface geometry fromFlueck et al. [1997] and assumes that slip occurs atthe plate interface and consists of updip motion ofthe overlying crustal block. Figure 14 shows mod-elling results for four of these events. The goodagreement between observed and modeled dis-placements shows that the transient motions can berepresented by simple slip on the subducting plateinterface between depths of 25 and 45 km, with theslip region parallel to the strike of the subductingplate. The downdip boundary appears to be sharperwhereas the updip boundary is more diffuse, requir-ing a gradual tapering of the slip amplitude.Although the maximum slip is only a few centime-tres per event, the large area of slip generates anequivalent moment magnitude for these “slowearthquakes” ranging from 6.5 to 6.8.

Although the physical processes involved arenot well understood, Figure 15 outlines the concep-tual kinematic model for ETS on the northern CSZ.Both offshore and at greater depths (>50 km), thetwo plates converge steadily at ~4 cm/yr, the geo-logical average rate. Across the shallower interface,the plates are locked for centuries, moving cata-strophically past each other only at times of greatthrust earthquakes. At depths of 25 to 45 km, platesresist motion temporarily for ~14 months accumu-lating some stress, and then slip the equivalent of afew centimetres over periods of one to two weeksreleasing that small stress accumulation. Thisrelease is accompanied by unique seismic tremors

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Figure 12: Long-term velocities of regional continuous GPS sites. Three to 10-year linear trends in the horizontal position with respect to Penticton (DRAO)for some of the sites of the Western Canada Deformation Array (WCDA: greensquares) and the Pacific Northwest Geodetic Array (PANGA: yellow squares)are plotted by green arrows with 95 per cent error ellipses. The position of thestrongly coupled zone determined from slip-dislocation models is indicated bythe locked and transition zones. The convergence vector of the Juan de Fucaplate is with respect to the North America (NA) plate, and the GPS referencestation DRAO is assumed fixed on the NA plate [from Dragert et al. 2004].

Figure 13: Record of slip and tremor activity observed for the Victoria area.Blue circles show day-by-day change in the east component of the GPS siteALBH (Victoria) with respect to DRAO (Penticton) which is assumed fixed onthe North America plate. Continuous green line shows the long-term (interseis-mic) eastward motion of the site. Red line segments show the mean elevatedeastward trends between the slip events which are marked by the reversals ofmotion every 13 to 16 months. Bottom graph shows the total number of hoursof tremor activity observed for southern Vancouver Island within a sliding 10-day period. Ten days corresponds to the nominal duration of a slip event.Pronounced tremor activity coincides precisely with slip events.

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on and above the region of slip. In the context ofseismic hazard, the recognition of this ETS slipzone has two significant implications. First, sincethe repeated slip events define a region where littleor no shear stress accumulates over longer periodsof time, the updip edge of the slip zone can serve asa proxy for the maximum downdip (i.e. landward)extent for subduction thrust rupture. This can beused to improve estimates of strong motion in thedensely populated regions of southwest BC.Secondly, it can be shown that the occurrence ofslip on this deeper part of the plate interface addsstress to the shallower portion of the locked plateinterface, driving it closer to rupture in discreteincrements. Consequently, during the time of slip,the weekly cumulative probability of a megathrustearthquake is significantly greater than at timesbetween slip events [Mazzotti and Adams 2004].This provides, for the first time, a potential basisfor time-dependent seismic hazard estimates.

7. Summary

Geophysical processes systematically affect thespatial reference frames used for geodetic surveys.Geodynamic rates in most of Canada (i.e. away fromactive plate margins) are generally rather modest(typically 1-2 cm/yr). However, when highest accu-racy is required, the measurable effects of geody-namic processes must be considered. When evaluat-ing the effect on reference frames within a given

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Figure 14: Models for slow slip events. The geometry of the subducting plateinterface established from structural studies [Flueck et al., 1997] is adopted,and simple linear slip distribution directed up-dip is assumed. The four panelsillustrate the results of elastic dislocation modelling [Okada, 1985] of four recentslip events. Blue arrows show observed horizontal surface displacements with95 per cent error ellipses that were used to constrain the models; green arrowsshow surface displacements obtained from separate analyses and not used inmodel constraint; yellow vectors show model displacements. Dark shading indi-cates fault areas with full slip whose magnitudes are shown in panel headers;light shading indicates fault areas where slip is tapered linearly from full to zeroat the up-dip end. Also shown are equivalent moment-magnitudes assuming arigidity of 40 Gpa [from Dragert et al. 2004].

Figure 15: Cross-section of a conceptual model for CSZ plate-interface kinematics. The blue region represents the subducting Juande Fuca plate and the green and brown regions, the overlying margin of the North America plate. Seaward of the trench (to the leftof the diagram) and landward of Victoria (to the right), the plates converge continuously. Beneath the continental slope, mostly com-prised of the accretionary prism, the plates are fully locked thereby causing an accumulation of stress that will be released in thenext megathrust earthquake. At depths of 25 to 45 km, plates resist motion temporarily for ~14 months and then slip a few cen-timetres over periods of one to two weeks, accompanied by distinct seismic tremors (yellow stars). The transition zone marks theregion on the plate interface where a transition from “locked” to “stick-slip” behaviour occurs.

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area it is important to note how kinematic processesoperate on various spatial scales.

Within regions of particular interest (e.g. earth-quake hazards), NRCan has complemented theCSRS and installed additional geodetic infrastruc-ture to monitor specific geodynamic processes. Anobservation strategy for geodynamics based mainlyon GPS should optimize both the accuracy of resultsand the use of complementary data sets from inde-pendent techniques where tenable (e.g. absolutegravity, tide gauges). All geodynamic investigationsrequire a long-term commitment to systematicmonitoring in order to determine the relatively smallkinematic rates and to subsequently develop usefulquantitative models. Many years of observations aretherefore invaluable and often required to betterunderstand the particular regional geodynamicprocesses. Furthermore, the data and results can beused to improve the realization and maintenance ofnational NAD83(CSRS) reference frame for thegeneral georeferencing community.

For much of the Canadian landmass the mostsignificant crustal movement is vertical motion, asdemonstrated by current models and analyses ofpost-glacial rebound. Post-glacial isostatic adjust-ment, whose maximum uplift rates occur generallynear Hudson Bay, is a process that is evidentthroughout much of Canada. Crustal tilt associatedwith post-glacial rebound can, albeit slowly, affectwater resources and alter risks associated withflooding. Of specific practical importance arechanges in relative sea level for coastal areas includ-ing regions that may be particularly vulnerable toclimate change such as arctic regions. It is thereforehighly useful to monitor and confirm coastal heightvariations at geodetic reference stations over time.

Seismic activity in intraplate zones tends to berelated to the regional stress fields, with earth-quakes concentrated in regions of crustal weakness.Horizontal strain rates can be directly related to thefrequency of large earthquakes. Vertical motionsmay provide additional insight into the regionalseismic process. Thus the occurrence of large earth-quakes in active seismic regions of eastern Canada(e.g. lower Saint Lawrence Valley, Charlevoix, andOttawa Valley) can be better characterized throughlong-term precise geodetic monitoring.

Detailed regional surveys continue to map thecrustal deformation field associated with the geo-physical processes in Canada noted above. Forregions with active seismicity and/or faults, meas-urements of crustal deformation supply direct infor-mation on crustal-strain accumulation which isessential for studies of tectonic and seismogenicprocesses and is increasingly used in seismic hazardassessments. Particularly exciting was discovery of

the “silent-slip” phenomenon along the Cascadiasubduction-zone interface. This was first observedwithin the time-series of precise, continuous GPSmeasurements on Canada’s west coast and itsimplications continue to be explored.

Acknowledgments

The authors first wish to acknowledge andexpress our appreciation to the many dedicated indi-viduals of NRCan for their roles in providing theGPS and AG data sets. From site selection and mon-umentation to acquisition, validation and data pro-cessing, the calibre of results from continuous andepisodic sites reflects a collective effort to attain thehighest quality solutions possible. Universities andother government agencies have been tremendouspartners in these efforts. We would like to thank themultitude of national and international sources ofsupport that have made possible or enhanced manyof the studies presented in this paper. Detailedacknowledgments of support agencies and collabo-rative partners can be found in the publications ref-erenced within each section. Work reported here onArctic sea levels and coastal stability is supported byPERD and ArcticNet (Network of Centres ofExcellence Canada). Numerous graduate studentsworking under the academic guidance of GeorgeSpence (University of Victoria) have contributed tothe studies of crustal deformation along the westernmargin of Canada. Mike Schmidt (NRCan/GSC) isgratefully acknowledged for his long-standing com-mitment to the operation of the WCDA and techni-cal support for western Canada GPS surveys. Wealso would like to acknowledge Earl Lapelle andMike Piraszewski (NRCan/GSD) for their dedica-tion to GPS data processing and network adjust-ments and combinations. Many of the maps in thisdocument were drafted using GMT [Wessel andSmith 1998], an open source collection of ~60 toolsfor manipulating geographic and Cartesian datasets. We would like to thank Pierre Héroux(NRCan/GSD) for his role leading the CSRS-relatedcontributions collected in this special issue ofGEOMATICA. Finally, the authors wish to especial-ly thank Robert Duval, Calvin Klatt, Yves Mireault(NRCan/GSD), and Roy Hyndman (NRCan/GSC)for their constructive reviews of the manuscript.

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Mazzotti S., H. Dragert, J. Henton, M. Schmidt, R.D.Hyndman, T.S. James, Y. Lu and M. Craymer. 2003c.Current tectonics of northern Cascadia from a decadeof GPS measurements, J. Geoph. Res., vol. 108,2554, doi:10.1029/2003JB002653.

Mazzotti, S., T.S. James, J. Henton and J. Adams. 2005.GPS crustal strain, postglacial rebound, and seismichazard in eastern North America: The SaintLawrence valley example, J. Geophys. Res., vol. 110,B11301, doi:10.1029/2004JB003590.

Obara, K. 2002. Non-volcanic deep tremor associated withsubduction in southwest Japan, Science, vol. 292, pp.1679-1681.

Okada, Y. 1985. Surface deformation due to shear and ten-sile faults in a half space, Bull. Seismol. Soc. Am.,vol. 75, pp. 1135-1154.

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Wang, K., R. Wells, S. Mazzotti, R.D. Hyndman and T.Sagiya. 2003. A revised dislocation model of inter-seismic deformation on the Cascadia subductionzone, J. Geophys. Res., vol. 108, pp. 2009-2022.

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sion of the Generic Mapping Tools released, EosTrans. Amer. Geophys. U., 79(47), pp. 329.

Authors

Joseph Henton has served as a Space Geodetic& Gravity Systems Research Geophysicist with theGeodetic Survey Division of Natural ResourcesCanada (NRCan) since 2002. While pursuing hisDoctoral degree at the University of Victoria (UVic),he studied and collaborated with researchers at theGeological Survey of Canada (GSC) in Sidney,British Columbia. He obtained his PhD from theSchool of Earth and Ocean Sciences of UVic in2000. He holds B.S. and M.S. degrees in solid-earthgeophysics from the New Mexico Institute ofMining & Technology. His interests currently lie inthe application of geodetic techniques (precise GPSand absolute gravimetry) to geoscience and geomat-ics issues. Within NRCan his work primarily sup-ports the Canadian Spatial Reference System, but hisresearch interests also contribute to a number of ini-tiatives and projects, including studies of Canada'svulnerability to climate change, groundwater, andearthquake hazards.

Mike Craymer has been with the GeodeticSurvey Division of Natural Resources Canadasince 1991 and is presently head of GeodeticNetworks & Standards. He received his PhD ingeodesy from the University of Toronto and hasauthored many papers on a variety of geodetic top-ics. He is an associate member of the InternationalGGNS Service and is presently co-Chair of IAGSub-Commission 1.3c on Regional ReferenceFrames for North America. He is also co-editor ofthe GPS Toolbox column for the journal "GPSSolutions," editor of the Literature Review columnfor the journal "Surveying and Land InformationScience," and publisher of the popular Tables ofContents in Geodesy <www.geodetic.org/tcg>.

Dr. Herb Dragert is a research scientist with theGeological Survey of Canada (GSC) at the PacificGeoscience Centre (PGC) in Sidney, BritishColumbia. He obtained his BSc degree inMathematics & Physics at the University of Torontoin 1968, and his MSc and PhD degrees inGeophysics at the University of British Columbia in1970 and 1973 respectively. He next held a NATOPost doctoral Fellowship at the Institute forGeophysics, Goettingen, Germany, and returned toCanada to teach at UBC in the fall of 1974. Hejoined the Gravity and Geodynamics Division of the

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Earth Physics Branch in Ottawa in 1976, and movedto PGC at the end of 1978. He currently also holdsan adjunct professor position with the School ofEarth & Ocean Sciences at the University ofVictoria. His principal area of research has been thestudy of crustal deformation within active seismicareas on the west coast of Canada. In 1992, he ledthe establishment of the Western CanadaDeformation Array, the first continuous GPS net-work in Canada for the express purpose of monitor-ing crustal motions. It was data from this networkthat led to the discovery of "Episodic Tremor andSlip" in the Cascadia Subduction Zone.

Dr. Stéphane Mazzotti obtained his BSc andMSc at the University of Paris-Orsay (France), andafterwards he worked as a visiting scientist at theUniversity of Tokyo (Japan) in 1996 to 1997. Hethen returned to France and obtained his PhD in geo-dynamics at Ecole Normale Superieure, Paris, in1999. Dr. Mazzotti has been working in Canadasince, first as a post-doctoral fellow at the Universityof Victoria, now as a research scientist with theGeological Survey of Canada since 2003. Dr.Mazzotti specializes in the study of relationshipsbetween seismicity and lithospheric deformation.His main research focuses on the integration of GPS,earthquake, and other geophysical data to studycrustal dynamics, seismicity, and seismic hazards ofplate boundary and intraplate deformation regions.Currently, his principal areas of interest are westernCanada (Cascadia, Queen Charlotte Islands, Yukon)and the St. Lawrence and Ottawa valleys.

Rémi Ferland has been involved in variousGPS research and development activities sincemid-1980. With the Geodetic Survey Division of

Natural Resources Canada, he has been involved inthe early 1990s with the estimation of precise GPSorbit, station and Earth related products. Since1999, he has been actively involved as the coordi-nator of the IGS Reference Frame Working Group.His responsibilities include the weekly combina-tion of the IGS station position and velocity prod-ucts, and Earth rotation parameters along with theselection and maintenance of the stations whosepositions and velocities are included in the IGSrealization of the International TerrestrialReference Frame (ITRF). Rémi has also beeninvolved with the IERS combination activities andis a member of the NAREF Reference FrameTransformation Working Group, responsible fordetermining the relationship between regional,national, and international reference frames.

Dr. Donald L. Forbes is a Research Scientistwith the Geological Survey of Canada at the BedfordInstitute of Oceanography and an Adjunct Professorat Memorial University of Newfoundland. He holdsdegrees from Carleton University, the University ofToronto, and the University of British Columbia. Hisresearch interests range from seabed mapping andremote sensing for coastal change detection to sea-level change, vertical motion, coastal hazards, andclimate-change adaptation. In the mid 1990s, he wasseconded to the South Pacific Applied GeoscienceCommission working on coastal issues in Fiji, CookIslands, Niue, and Kiribati. Over the past decade, hehas played lead roles in projects on coastal impactsof climate change in the Canadian Arctic andAtlantic provinces. He has wide experience inapplied science studies of coastal systems for appli-cation to public policy, integrated management,hazard assessment and adaptation planning. o

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GLOBAL GEODETIC OBSERVING SYSTEM—CONSIDERATIONS FOR THE GEODETICNETWORK INFRASTRUCTURE

M. Pearlman, Harvard-Smithsonian Center for Astrophysics (CFA), Cambridge MAZ. Altamimi, Institut Géographique National, Marne-La-Vallée, FranceN. Beck, Geodetic Survey Division-Natural Resources Canada, Ottawa, OntarioR. Forsberg, Danish National Space Center, Copenhagen, DenmarkW. Gurtner, Astronomical Institute University of Bern, Bern, SwitzerlandS. Kenyon, National Geospatial-Intelligence Agency, Arnold, MOD. Behrend, F.G. Lemoine, C. Ma, C.E. Noll, E.C. PavlisNASA Goddard Space Flight Center, Greenbelt, MDZ. Malkin, Institute of Applied Astronomy, St. Petersburg, RussiaA.W. Moore, F.H. Webb, R.E. Neilan, Jet Propulsion LaboratoryCalifornia Institute of Technology, Pasadena, CAJ.C. Ries, Center for Space Research, The University of Texas, Austin, TXM. Rothacher, GeoForschnungsZentrum Potsdam, Potsdam, GermanyP. Willis, Institut Géographique National, Saint Mande, France

Properly designed and structured ground-based geodetic networks materialize the reference systems tosupport sub-millimetre global change measurements over space, time, and evolving technologies. The GroundNetworks and Communications Working Group (GN&C WG) of the International Association of Geodesy’sGlobal Geodetic Observing System (GGOS) has been working with the IAG measurement services (the IGS,ILRS, IVS, IDS and IGFS) to develop a strategy for building, integrating, and maintaining the fundamentalnetwork of instruments and supporting infrastructure in a sustainable way to satisfy the long-term (10 to 20years) requirements identified by the GGOS Science Council.

Activities of this Working Group include the investigation of the status quo and the development of aplan for full network integration to support improvements in terrestrial reference frame establishment andmaintenance, Earth orientation and gravity field monitoring, precision orbit determination, and other geo-detic and gravimetric applications required for the long-term observation of global change. This integrationprocess includes the development of a network of fundamental stations with as many co-located techniquesas possible, with precisely determined intersystem vectors. This network would exploit the strengths of eachtechnique and minimize the weaknesses where possible. This paper discusses the organization of the workinggroup, the work done to date, and future tasks.


Des réseaux géodésiques terrestres bien conçus et structurés permettent de matérialiser les systèmes deréférence afin de prendre en compte les changements mondiaux dans l’espace, le temps et les nouvelles tech-nologies à un niveau inframillimétrique. Le groupe de travail sur les communications et les réseaux terrestres(Ground Networks and Communications Working Group (GN&C WG)) du Système global d’observationgéodésique (GGOS) de l’Association internationale de géodésie (AIG) a travaillé avec les services de prisesde mesures de l’AIG (l’IGS, l’ILRS, le SIR, l’IDS et l’IGFS) afin d’élaborer une stratégie pour édifier, inté-grer et maintenir le réseau essentiel d’instruments et d’infrastructures de façon durable afin de répondre auxbesoins à long terme (10 à 20 ans) cernés par le Conseil des sciences du GGOS.

Le Groupe de travail se prête notamment à l’évaluation du statu quo et à l’élaboration d’un plan pourl’intégration complète du réseau afin de comprendre les améliorations à l’élaboration et au maintien du cadrede référence terrestre, la surveillance de l’orientation et du champ gravitationnel terrestres, la déterminationprécise de l’orbite et d’autres applications géodésiques et gravimétrique nécessaires à l’observation deschangements mondiaux à long terme. Ce processus d’intégration comprend l’élaboration d’un réseau destations principales intégrant autant de techniques conjointes que possible et de vecteurs, déterminés avecprécision, entre les systèmes. Ce réseau exploiterait les forces de chacune des techniques et minimiserait leursfaiblesses. Cet article présente l’organisation du groupe de travail, le travail accompli à ce jour ainsi queses prochaines tâches.

This is an extension of a paper originally published in: 2005 IAG/IAPSO/IABO Joint Assembly. Cairns Australia.August 22-26, 2005. Series: IAG Symposia, Vol. 130, in press.

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1. IntroductionThe Ground Networks and Communications

Working Group (GN&C WG) of the GlobalGeodetic Observing System (GGOS) is chargedwith developing a strategy to design, integrate, andmaintain the fundamental space geodetic network. Inthis report, we review the significance of geodeticnetworks and the GGOS project. We also summarizethe present state of, as well as future improvementsto, and requirements on space geodetic networks,services, and products. The approach of the GN&CWG and preliminary conclusions follow.

1.1 Significance of the TerrestrialReference Frame

Space geodesy provides precise position,velocity, and gravity on Earth, with resolution fromlocal to global scales. The terrestrial reference sys-tem defines the terrestrial reference frame (TRF) inwhich positions, velocities, and gravity are reported.The reference surface for height reckoning, thegeoid, is defined through the adopted gravity model,which is referenced to the TRF. The TRF is thereforea space geodesy product that links each of theseobservable quantities to other geophysical parame-ters on Earth. Its position, orientation, and evolutionin space and time are the basis through which weconnect and compare such measurements overspace, time, and evolving technologies. It is themeans by which we verify that observed temporalchanges are geophysical signals rather than artefactsof the measurement system. It provides the founda-tion for much of the space-based and ground-basedobservations in Earth science and global change,including remote monitoring of sea level, sea sur-face, and ice surface topography, crustal deforma-tion, temporal gravity variations, atmospheric cir-culation, and direct measurement of solid Earthdynamics. A precise TRF is also essential for inter-planetary navigation, astronomy, and astrodynamics.

The realization of the TRF for its mostdemanding applications requires a mix of technolo-gies, strategies and models. Different observationalmethods have different sensitivities, strengths, andsources of error. The task is complicated by thedynamic character of Earth’s surface, whichdeforms on time scales of seconds to millennia andon spatial scales from local to global.

1.2 The Role of GGOSIn early 2004 under its new organization, the

International Association of Geodesy (IAG) estab-lished the GGOS project (www.ggos.org) to coordi-nate geodetic research in support of scientific appli-

cations and disciplines [Rummel et al. 2002]. GGOSis intended to integrate different geodetic techniques,models, and approaches to provide better consisten-cy, long-term reliability, and understanding of geo-detic, geodynamic, and global change processes.Through the IAG’s measurement services(International GNSS Service, formerly theInternational GPS Service (IGS), International LaserRanging Service (ILRS), International VLBI Servicefor Geodesy and Astrometry (IVS), InternationalDORIS Service (IDS), International Gravity FieldService (IGFS), and future International AltimeterService (IAS)), GGOS will ensure the robustness ofthe three aspects of geodesy: geometry and kinemat-ics, Earth orientation, and static and time-varyinggravity field. It will identify geodetic products andestablish requirements on accuracy, time resolution,and consistency. The project will work to coordinatean integrated global geodetic network and imple-ment compatible standards, models, and parameters.

A fundamental aspect of GGOS is the establish-ment of a global network of stations with co-locatedtechniques, to provide the strongest referenceframes. GGOS will provide the scientific and infra-structural basis for all global change research andprovide an interface to geodesy for the scientificcommunity and to society in general. GGOS willstrive to ensure the stability and ready access to thegeometric and gravimetric reference frames byestablishing uninterrupted time series of state-of-the-art global observations.

As shown in Figure 1, GGOS is organized intoworking groups headed by a Project Board and guid-ed by a Science Council that helps define the scien-tific requirements to which GGOS will respond.

1.3 Role of the Ground Networks andCommunications Working Group

The ground network of GGOS is fundamentalsince all GGOS data and products emanate fromthis infrastructure.

The Charter of the Ground Networks andCommunications Working Group (GN&C) withinGGOS is to develop a strategy to design, integrate,and maintain the fundamental geodetic network ofinstruments and supporting infrastructure in a sus-tainable way to satisfy the long-term (10 to 20 years)requirements identified by the GGOS ScienceCouncil. At the base of GGOS are the sensors andobservatories situated around the world providingthe timely, precise, and fundamental data essentialfor creating the GGOS products. Primary emphasismust be on sustaining the infrastructure needed tomaintain evolving global reference frames while atthe same time ensuring support to the scientific

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A fundamen-tal aspect ofGGOS is theestablishmentof a globalnetwork ofstations withco-locatedtechniques,to providethe strongestreferenceframes.

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applications’ requirements. Opportunities to betterintegrate or co-locate with the infrastructure andcommunications networks of the many other EarthObservation disciplines now organizing under theGlobal Earth Observation System of Systems(GEOSS) should be sought and taken into account[Group on Earth Observations 2005].

Recognizing that the infrastructure and opera-tions collectively contributing to the Services of theIAG are possible solely due to the voluntary contri-butions of the globally distributed collaboratingagencies and their interest in maximized systemperformance and sustainable long-term efficientoperations, the Working Group is made up of rep-resentatives of the measurement services plus otherentities that are critical to guiding the activities ofthe Working Group:

• IGS: Angelyn Moore, Norman Beck • ILRS: Mike Pearlman, Werner Gurtner• IVS: Chopo Ma, Zinovy Malkin• IDS: Pascal Willis• IGFS: Rene Forsberg, Steve Kenyon• ITRF and Local Survey: Zuheir Altamimi,

Jinling Li• IERS Technique Combination Research

Centers: Marcus Rothacher• IAS (future): Wolfgang Bosch • Data Centers: Carey Noll• Data Analysis: Erricos Pavlis, Frank Lemoine,

Frank Webb, John Ries, Dirk Behrend

2. Global GeodeticNetwork Infrastructure

All infrastructure, and resulting analysis andproducts of GGOS and its constituent services aremade possible through the goodwill voluntary con-tributions of national agencies and institutions andare coordinated by the IAG governance mechanisms.

The ground network of GGOS includes all thesites that have instruments of the IAG measurementservices either permanently in place or regularlyoccupied by portable instruments. Some sites havemore than one space geodesy technique co-located,and knowledge of the precise vectors between suchco-located instruments (known as “local ties”) isessential to full and accurate use of these co-locations.

Analysis centres use the ground networks’ datafor various purposes including positioning, Earthorientation parameters (EOP), the TRF, and thegravity field. The ground stations of the satellitetechniques provide data for precise orbit determi-nation (POD). The individual sites’ reference points

of the contributing space geodesy networks are thefiducial points of the TRF.

2.1 IAG Measurement Services

Each service coordinates its own network,including field stations and supporting infrastruc-ture. Here we will review the current status of eachmeasurement service.

2.1.1 IGS The foundation of the International GNSS

Service (IGS, formerly the International GPSService) is a global network of more than 350 per-manent, continuously operating, geodetic-qualityGPS and GPS/GLONASS sites. The station dataare archived at four global data centres and sixregional data centres. Ten analysis centres regular-ly process the data and contribute products to theanalysis centre coordinator, who produces the offi-cial IGS combined orbit and clock products.Timescale, ionospheric, tropospheric, and referenceframe products are analogously formed by special-ized coordinators for each. More than 200 institutesand organizations in more than 80 countries con-tribute voluntarily to the IGS, a service formallybegun in 1994. The IGS intends to integrate futureGNSS signals (such as Galileo) into its activities, asdemonstrated by the successful integration ofGLONASS. [Kouba et al. 1998; Beutler et al.1999; Dow 2003].

2.1.2 ILRS The International Laser Ranging Service (ILRS),

created in 1998, currently tracks 28 retroreflector-195

Figure 1. GGOS Organization.

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equipped satellites for geodynamics, remote sensing(altimeter, SAR, etc.), gravity field determination,general relativity, verification of GNSS orbits, andengineering tests [Pearlman et al. 2002]. Satellitealtitudes range from a few hundreds of kilometresto GPS altitude (20,000 kilometres) and the Moon.The network includes forty laser ranging stations,two of which routinely range to four targets on theMoon. Satellites are added and deleted from theILRS tracking roster as new programs are initiatedand old programs are completed. The collected dataare archived and disseminated via two centres, andseveral analysis centres voluntarily and routinelydeliver products for TRF, EOP, POD, and gravitymodelling and development.

2.1.3 IVS

The International VLBI Service for Geodesyand Astrometry (IVS) was established in 1999 andcurrently consists of 74 permanent components:coordinating centre, operation centres, network sta-tions, correlators, analysis centres, and technologydevelopment centres. The IVS observing networkincludes about 30 regularly-observing IVS stationsand 20 to 30 collaborating stations participating inselected IVS programs on an irregular basis[Behrend and Baver 2005]. Twenty-four-hour ses-sions twice per week as well as other less frequentsessions are used to determine the complete set ofEOP (polar motion, celestial pole coordinates,UT1-UTC), station coordinates and velocities, andthe positions of the radio sources. VLBI is the onlyspace technique capable of Universal Time (UT1)monitoring. IVS uses daily one-hour single base-line sessions with low latency for this purpose[Schlueter et al. 2002].

2.1.4 IDS

The International DORIS Service (IDS) wascreated in 2003 [Tavernier et al. 2005]. The currentground tracking network is composed of 55 stationsallowing an almost continuous tracking of the cur-rent five satellites (SPOT-2, -3 and -4 used forremote sensing applications, Jason-1, and Envisatused for satellite altimetry). The main applications ofthe DORIS system are precise orbit determination,geodesy and geophysics [Willis et al. 2005]. Usingimproved gravity Earth models derived from theGRACE mission [Tapley et al. 2004], DORIS week-ly station positions can now be regularly obtained atthe 10mm level [Willis et al. 2004]. DORIS data areavailable at the two IDS Data Centers since 1990(SPOT-2). In 1999 a DORIS Pilot Experiment wascreated by the IAG [Tavernier et al. 2002] leading

gradually to the IDS. The French space agency(CNES) has the leading role in the IDS.

2.1.5 IGFSThe International Gravity Field Service (IGFS)

was created in 2003 to provide coordination andstandardization for gravity field modelling. It sup-ports the IAG scientific and outreach goals andtherefore GGOS, through activities such as collect-ing data for fundamental gravity field observationnetworks (e.g., a global absolute reference network,co-located with satellite stations and other geodeticobservation techniques), data collection and releaseof marine, surface, and airborne gravity data forimproved global model development (e.g., EGM96[Lemoine et al. 1998]), and advocating consistentstandards for gravity field models across the IAGservices. Establishing new methodology and sci-ence applications, particularly in the integrationand validation of data from a variety of sources, isanother focus of the service. The IGFS is composedof a variety of primary service entities: BureauGravimétrique International (BGI), InternationalGeoid Service (IGeS), International Center forEarth Tides (ICET), and International Center forGlobal Earth Models (ICGEM), with the NationalGeospatial-Intelligence Agency (NGA) participatingas an IGFS Technical Center.

2.2 Communications

Transmission of data from the network instru-ments to data centres and processing or analysiscentres is a function critical to all the techniques.For the satellite services, data transmission is nor-mally via primarily the Internet through terrestrialor satellite communications networks. Due to thevolume of data (terabytes per station per 24 hours),VLBI data are currently shipped on recordedmedia, but transmission of data via high-speedfibre is a future goal. Gravity data are currentlyexchanged via Internet or massive storage media onan “as needed” basis. Control and coordinationinformation is also routinely and primarily sent viathe Internet. Sites are often situated where suitableaccess to communications networks, and ideallyInternet, exists. In some cases, however, connectiv-ity must be installed at existing sites.Communications costs are borne by the operatingagencies, which in remote areas is often at consid-erable expense. The GN&C WG will improve effi-ciency through coordinated implementation ofmodern methods and additional sharing of commu-nications facilities and infrastructure.

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3. Synergy of theObserving Techniques

At the dawn of space age about half a centuryago, the individual national classical systems thatwere then dominating geodesy started slowly to bereplaced by initially crude global equivalents (e.g.,the SAO Standard Earth models), and later on,when the first satellite navigation constellationslike TRANSIT became available, by more sophisti-cated “World Geodetic Systems” (e.g., the USDoD-developed WGS60, 66, 72, and WGS84). Asspace techniques proliferated throughout the world,it soon became apparent that the optimal approachwould be to make use of all available systems, andto share the burden of the development throughinternational coordination and cooperation. Thissection reviews the synergistic contributions ofspace geodetic techniques to various products.

3.1 The Terrestrial Reference Frame

The dramatic improvement of space geodesytechniques in the eighties, thanks to NASA’s CrustalDynamics Project and Europe’s WEGENER Project,has dramatically increased the accuracy of TRFdetermination [Smith and Turcotte 1993]. However,none of the space geodesy techniques alone are ableto provide all the necessary parameters for the TRFdatum definition (origin, scale, and orientation).While satellite techniques are sensitive to Earth’scentre of mass, VLBI is not. The scale is dependenton the modelling of some physical parameters, andthe absolute TRF orientation (unobservable by anytechnique) is arbitrary or conventionally definedthrough specific constraints. Once the conventionsare established, VLBI, unlike the other space tech-niques, can observe the progression of ITRF orien-tation in space. The utility of multi-technique com-binations is therefore recognized for the TRFimplementation, and in particular for accuratedatum realization.

Since the creation of the International EarthRotation and Reference Systems Service (IERS),the current implementation of the InternationalTerrestrial Reference Frame (ITRF) has been basedon suitably weighted multi-technique combination,incorporating individual TRF solutions derivedfrom space geodesy techniques as well as local tiesof co-location sites. The IERS has recently initiateda new effort to improve the quality of ties at existingco-location sites, crucial for ITRF development[Richter et al. 2005].

The particular strengths of each observingmethod can compensate for weaknesses in others.

SLR defines the ITRF2000 geocentric origin,which is stable to a few mm/decade, and SLR andVLBI define the absolute scale stability to around0.5 ppb/decade (equivalent to a shift of approxi-mately 3mm in station heights) [Altamimi et al.2002]. Measurement of geocentre motion is underrefinement by the analysis centres of all satellitetechniques. The density of the IGS network pro-vides easy and rigorous TRF access world-wide,using precise IGS products and facilitates theimplementation of the rotational time evolution ofthe TRF in order to satisfy the No-Net-Rotation con-dition over tectonic motions of Earth’s crust. DORIScontributes a geographically well-distributed net-work, the long-term permanency of its stations, andits early decision to co-locate with other trackingsystems. We recognize that we will need to consid-er non-linear motions in future reference framesolutions. A first step towards this goal is the use oftime series analysis rather than just position andvelocity products.

The TRF is heavily dependent on the quality ofeach network and suffers with any network degra-dation over time. The current distribution and quan-tity of co-location sites as depicted in Figure 2 (inparticular sites with three and four techniques) issub-optimal.

3.2 Earth Orientation ParametersEarth orientation parameters measure the ori-

entation of Earth with respect to inertial space(which is required for satellite orbit determinationand spacecraft navigation) and to the TRF, which isa precondition for long-term monitoring. Polarmotion and UT1 track changes in angular momen-tum in the fluid and solid components of the Earthsystem driven by phenomena like weather patterns,

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Figure 2. Distribution of space geodesy co-location sites since 1999.

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ocean tides and circulation, post-glacial rebound,and great earthquakes. The celestial pole position, onthe other hand, is dependent on the deep structure ofEarth. Only VLBI measures celestial pole positionand UT1, and VLBI also defines the ICRF(International Celestial Reference Frame) [Ma et al.1998], whose fiducial objects (mostly quasars) haveno detectable physical motion across the sky becauseof their great distance. The two-decade VLBI dataset contributes a long-time series of polar motion,UT1 and celestial pole position. Satellite techniques(GPS, SLR, and DORIS) measure polar motion andlength of day relative to the orbital planes of thesatellites tracked. In practice, recent polar motiontime series are derived from GPS with a high degreeof automation, and predictions of UT1 rely on GPSlength of day and atmospheric excitation functions.

3.3 Gravity, Geoid, and Vertical Datum

Gravity is important to many scientific andengineering disciplines, as well as to society in gen-eral. It describes how the “vertical” directionchanges from one location to another, and similar-ly, it defines at each point the equipotential surface;therefore, it describes the direction that “waterflows.” Global scale models of terrestrial gravityand geoid [Lemoine et al. 1998] are now routinelydelivered on a monthly basis by missions likeGRACE, with a resolution of 200 kilometres or so,and high accuracy [Tapley et al. 2004]. The addi-tion of surface gravity observations can extend theresolution of these models down to tens of kilome-tres in areas of dense networks. Worldwide data-bases of absolute and relative gravity, airborne andmarine gravity are collected and maintained byIGFS. Astronomically-driven temporal variationsof gravity (Earth, ocean, and atmospheric tides) arealso a product of this and other IAG services. Thecombination of all this information is crucial in pre-cisely determining instantaneous position on Earthor in orbit, the direction of the vertical and theheight of any point on or around Earth, and thecomputation of precise orbits for near-Earth as wellas interplanetary spacecraft. Similarly, the verticaldatum is the common reference for science, engi-neering, mapping, and navigation problems.Achieving a globally consistent vertical datum ofvery high accuracy has been a prime geodetic prob-lem for decades, and only recently (thanks to satel-lite altimetry and the latest gravity missions likeCHAMP and GRACE) is a successful result inreach. Strengthening and maintaining a close linkbetween the “geometric” and “gravimetric” refer-ence frames is of paramount importance to thegoals of GGOS.

3.4 Precise Orbit Determination

Precise orbit determination is one of the prin-cipal applications of the satellite techniques (GPS,SLR, DORIS), and has direct application to manydifferent scientific disciplines such as ocean topog-raphy mapping, measurement of sea level change,determination of ice sheet height change, precisegeo-referencing of imaging and remote sensingdata, and measurement of site deformation usingsynthetic aperture radar (SAR) or GPS. The tech-niques have evolved from metre-level orbit deter-mination of satellites such as LAGEOS in the early1980’s to cm-level today. The computation of pre-cise orbits allows these satellite tracking data to beused for gravity field determination (both static andtime-variable) and the estimation of other geophys-ical parameters such as post glacial rebound, oceantidal parameters, precise coordinates of trackingsites, or the measurement of geocentre motion.

Precise orbit determination, which requiresprecise UT1 and gravity models, underpins theanalysis that in parallel has resulted in improvedstation coordinate estimation, and therebyimproved realizations of the TRF (e.g., ITRF2000).There is close synergy between POD and TRF real-ization. The density of data available from GPS (andin the future from other GNSS including Galileo)allows the estimation of reduced-dynamic or kine-matic orbits with radial accuracy of a few centime-tres even on low-altitude satellites such as CHAMPand GRACE. Only a few satellites carry multipletracking systems, but space-based co-location isinvaluable. The detailed inter-comparison of orbitscomputed independently from SLR, DORIS, andGPS data confirms that Jason-1 orbits have a one-centimetre radial accuracy [Luthcke et al. 2003].These techniques are complementary; the precisebut intermittent SLR tracking of altimeter satellites,such as Envisat or TOPEX/Poseidon, is comple-mented by the dense tracking available from theDORIS network. SLR tracking of the GPS,GLONASS or future Galileo satellites is and willbe vital to calibrating GNSS satellite biases andassuring the realization of a high quality TRF.

4. Future RequirementsThe measurement requirements for GGOS will

be set by the GGOS Project Board with guidancefrom the Science Council [Rummel et al. 2002].Until these requirements are formally specified, wejudge the practical useful target for the TRF andspace geodetic measurement accuracy to be roughlya factor of five to 15 below today’s levels. Given thatthe TRF and global geodesy are now accurate to the

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order of 1cm (or five to 15 millimetres for differentquantities) and 2mm/yr, we foresee near-term utilityin global measurements with absolute accuracies ator below 1mm and 0.2mm/yr. Corresponding levelsof improvement are required for Earth orientationand gravity.

5. Evolution of the TechniquesEach of the GGOS Services techniques envi-

sions technological and operational advances thatwill enhance measurement capability. Someadvances are currently being implemented whileothers are in the process of design or development.In addition, each technique-related service is seekingto improve not only data quality and precision, butalso reliability of data and product delivery, per-formance, continuity, station stability, data latency(which in the case of GNSS includes real-time) anddata handling techniques and modelling. Whilemaking these improvements, contributors seekoperational efficiencies in order to minimize costs.

5.1 GNSS

Geodetic GNSS has already evolved fromGPS-only operations to inclusion of GLONASS,and upgrades to next-generation receivers willallow full benefit from modernized GPS signalstructures, Galileo signals, and GLONASS signals.Studies leading to improved handling of calibrationissues such as local signal effects (e.g., multipath)and antenna phase patterns are underway, as are ini-tiatives to fill remaining network gaps, particularlyin the southern hemisphere. Elsewhere, station den-sity is less problematic and the focus has shifted toconsolidation of supplementary instrumentationsuch as strain metres and meteorological sensors.

5.2 Laser Ranging

Newly designed and implemented laser rangingsystems operate semi-autonomously and autonomous-ly at kilohertz frequencies, providing faster satelliteacquisition, improved data yield, and extended rangecapability, at substantially reduced cost. Improvedcontrol systems permit much more efficient pass inter-leaving and new higher resolution event-timers deliverpicosecond timing. The higher resolution will maketwo-wavelength operation for atmospheric refractiondelay recovery more practical and applicable formodel validation. The current laser ranging networksuffers from weak geographic distribution, particu-larly in Africa and the southern hemisphere. Thecomprehensive fundamental network shouldinclude additional co-located sites to fill in this gap.

Improved satellite retroreflector array designswill reduce uncertainties in centre-of-mass correc-tions, and optical transponders currently underdevelopment offer opportunities for extraterrestrialmeasurements.

5.3 VLBI

The VLBI component of the future fundamentalnetwork will be the next-generation system nowundergoing conceptual development. Critical ele-ments include fast slewing; high efficiency 10-12mdiameter antennas; ultra wide bandwidth front endswith continuous radio frequency (RF) coverage; dig-itized back ends with selectable frequency segmentscovering a substantial portion of the RF bandwidth;data rate improvements by a factor of two to16; amixture of disk-based recording and high speed net-work data transfer, near real-time correlation amongnetworks of processors, and rapid automated gen-eration of products. Better geographic distribution,especially in the southern hemisphere, is required.

5.4 DORISThe DORIS tracking network is being modern-

ized using third-generation antennae and improve-ments to beacon monumentation [Tavernier et al.2005]. Efforts are underway to expand the networkto fill in gaps in existing coverage. DORIS beaconsare also being deployed to support altimeter cali-bration, co-location with other geodetic techniques,or specific short-term experiments. A specific IDSworking group is selecting sites and occupations forsuch campaigns, using additional DORIS beaconsprovided by CNES to the IDS.

5.5 GravityGravity observations are most sensitive to height

changes; they therefore provide an obvious way todefine and control the vertical datum. A uniformly-distributed network of regularly cross-calibratedabsolute gravimeters supported by a well-designedrelative measurement network that will be repeatedlyobserved at regular intervals, and a sub-network ofcontinuously operating superconducting tidalgravimeters are expected in a fundamental networkof co-located techniques. These permanent networksshould be augmented with targeted airborne and shipcampaigns to collect data over large areas that aredevoid of gravimetric observations. A well-distrib-uted global data set of surface data is necessary tocalibrate and validate products of the recent(CHAMP and GRACE) and upcoming (GOCE)high-accuracy and -resolution missions. Eventually,gravimetry will need to devise a method analogous

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to InSAR, to continuously “map” changes in thefield with resolution many orders of magnitude high-er than currently achievable from any geopotentialmapping mission. The organization of a gravity fieldservice is underway and the integration of its activi-ties should emerge shortly.

6. Approaches to NetworkDesign

The final design of the GGOS network musttake into consideration all of the applicationsincluding the geometric and gravimetric referenceframes, EOP, POD, geophysics, oceanography, etc.We will first consider the TRF, since its accuracyinfluences all other GGOS products. Early steps inthe process are:

1. Define the critical contributions that each tech-nique provides to the TRF, POD, EOP, etc.

2. Characterize the improvements that could beanticipated over the next 10 years with eachtechnique.

3. Examine the effect in the TRF and Earth orien-tation resulting from the loss of a significant partof the current network or observation program.

4. Using simulation techniques, quantify theimprovement in the TRF, Earth orientation,and other key products as stations are addedand station capability (co-location, data quan-tity, and quality) is improved. We will alsoexplore the benefit of adding new SLR targets.

6.1 Impact of Network Degradationon the TRF

Preliminary results [Govind 2005] indicate theorigin drift caused by removal of one station,Yarragadee (Australia), from SLR analysis. Thedrift is about 0.6, 1, and 1mm/yr over the origincomponents around the three axes X, Y, Z, respec-tively. This drift is at least three times larger thanrequirements for high-precision Earth scienceapplications such as sea level change and othergeophysical processes.

6.2 Effect of System and NetworkDegradation on Other GGOSProducts

The TRF is a primary space geodesy product,but it is also the basis on which every other productis referenced. As such, degradation in its definition

and maintenance influences the quality of these otherproducts and services, such as EOP, geocentremotion, temporal global gravity variations, and POD.

The degradation can originate in two ways:geometric changes (as those shown by the exampleof section 6.1) and changes in the type, amount andspatiotemporal distribution of the observations. Inpractice what happens is a combination of both. Toquantify the resultant errors is not an easy taskbecause there are infinitely many possible variationsin the network of TRF stations, supporting tech-niques, and selection of data. Examination of partic-ular station deletions that either happened in practiceor had been proposed indicates [Pavlis and Kuzmicz-Cieslak 2005] that even moderate degradationsimpact results significantly more than their quotedaccuracies. This confirms the present ILRS networkis not robust to any contraction; the smallest per-turbation of the system yields large uncontrolledchanges in the products.

The closing of the Arequipa Peru andHaleakala Hawaii SLR sites for example, degradedorigin, orientation and scale by 3 to 4 times thestandard deviation of the relevant parameters.Impact on geocentre motion was almost two timesworse. Temporal variations of the gravity fieldcoefficients are less sensitive due to their nature asproxies of global scale changes, but were stilldegraded by several standard deviations. On thepositive side, for a modest improvement from anold TRF (ca. 1995) to the current one (ITRF2000),POD-based products (such as altimeter derivedMean Sea Level) improved by 30 per cent.

Much more work is required to assess theeffects of such changes in the tracking networks ofall space geodesy techniques, and their combinedeffect on the final products. The sizes of these sep-arate networks and the infinite possible variationsin their design, overlap and operation, and the qual-ity of their data and the targets used for collectingtheir observations complicate this task, but a fewwell-thought-through scenarios will be tested withfuture simulations.

6.3 Improvements in the TRF andOther Key Products

Expected advances in instrumentation, asdescribed in section 5, will cause improvements inthe TRF and the various products, but the accuracyneeded for future science applications will requireoptimization of the ground network. Simulationcapabilities will be developed that will allow forevaluation and optimization of the locations ofpotential sites.

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In addition, the benefit of introducing a few newSLR targets needs to be evaluated. Target interactionwith the current large LAGEOS satellites is one ofthe principal limitations in mm-level SLR, and small-er targets would support the necessary accuracy. Newlower-altitude targets would allow more observationopportunities per day, increased probability of track-ing from lower-power systems (particularly duringdaylight) and a more accurate determination of theEarth’s mass centre, critical for both controlling thedrift in the origin of the TRF as well as observingthe seasonal geocenter motions associated withlarge-scale mass transport within the Earth system.

Simultaneously, enhanced performance of eachof the individual techniques should result as eachtechnique’s data and analysis outputs are furthercombined and compared and eventually integrated.

7. Sustaining the GroundNetwork Over the Long Term

The measurement techniques services haveeach maintained their own networks and supportinginfrastructure, routinely producing data, but sufferfrom severe budget constraints of the voluntarilycontributing agencies that prevent appropriate main-tenance and development of physical and computa-tional assets. This degradation of the observing net-work capability coincides with the deployment ofhigh-value science investigations and missions, suchas sea level studies from ocean and ice-sheetaltimetry missions, eroding their scientific returnand limiting their ability to meet the mission goals.

Many of the elements of the current networksare funded from year to year and depend on specificactivities. Stations are often financed for capital andmaintenance and operations costs through researchbudgets, which may not constitute a long-term com-mitment. Sudden changes in funding as prioritiesand organizations change have resulted in devastat-ing impacts on station and network performance. Onthe other hand, missions and long-term projects haveassumed that the networks will be in place at no costto them, fully functioning when their requirementsneed fulfillment. GGOS will be proactive in helpingto persuade funding sources that the networks areinterdependent infrastructure that needs long-term,stable support. The GGOS community must securelong-term commitments from sponsoring and con-tributing agencies for its evolution and operationsin order to support its users with high-quality prod-ucts. Since the present networks must support cur-rent as well as future requirements, the GGOS net-work must evolve without interruption of data and

data products. In view of the difficulties in securinglong-lasting and stable financial support by theinterested parties, new financial models for the net-works must be developed. This Working Group willwork with the Strategy and Funding Working Groupto develop an approach.

Since the present networks must support currentas well as future requirements, the GGOS networkmust evolve without interruption of data and dataproducts. In particular, the TRF relies on a long con-tinuous history of data for its stability and robust-ness. New and upgraded systems, changes in stationslocations, and changes in the way products areformed must be planned and phased so that theimpacts are well documented and well understood.

The analysis and simulation procedures beingundertaken by the Working Group will identify net-work voids and shortcomings. The GroundNetworks and Communications Working Group, inconcert with the other GGOS entities, will workwith agencies and international organizationstoward filling in these gaps.

8. SummaryA permanent geodetic network of complemen-

tary yet interdependent space geodetic techniquesis critical for geodetic and geophysical applicationsand underpins the Global Earth ObservationSystem of Systems. Thanks to the generous andvoluntary contributions of many national agenciesand institutions around the world, the IAG has beenable to coordinate global collaborations for geodet-ic technique-based services from which all benefit.There is a strong need for coordination of the plan-ning, funding and operation of future integratedgeodetic networks to maximize performance inmeeting evolving requirements while taking intoaccount the need for sustainable infrastructure andefficient operations. The GGOS Ground Networks& Communications Working Group has initiatedstudies, which will guide the services in infrastruc-ture planning for optimal benefit to Earth scienceand associated engineering and societal concerns.

AcknowledgmentsThe authors would like to acknowledge the

support of IAG services (IGS, ILRS, IVS, IDS,IGFS, and IERS) and their participating organiza-tions. Part of this work was carried out at the JetPropulsion Laboratory, California Institute ofTechnology, under a contract with the NationalAeronautics and Space Administration.

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AuthorsMichael Pearlman is a program manager at the

Harvard-Smithsonian Center for Astrophysics inCambridge, MA. He has a PhD in physics fromTufts University and a S.M. in management fromthe Sloan School at M.I.T. Dr. Pearlman joined theSmithsonian Astrophysical Observatory (SAO) in1968 to work on the early development and deploy-ment of the satellites ranging systems for the SAOnetwork. In 1971to 1972 he was a visiting scientistat the Office of Geodetic Satellites at NASAHeadquarters. From 1972 to1983 he was the man-ager of the SAO satellite tracking program thatincluded the SLR and Baker-Nunn Camera net-work and supporting infrastructure. In 1984 Dr.Pearlman became a special consultant to the NASACrustal Dynamics Program for operation, engineer-ing, and overseas program development. He wasthe NASA representative to the InternationalWEGENER Program and the Asian Pacific SpaceGeodynamics Program. Dr. Pearlman is now theDirector of the Central Bureau of the InternationalLaser Ranging Service (ILRS) and a member of itsGoverning Board. He is a member of the GGOSSteering Committee and Executive Committee. Heis also the coordinator for the GGOS WorkingGroup on Ground Networks and Communication.

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Zuheir Altamimi received his PhD in SpaceGeodesy from the Paris Observatory in 1990. Hisresearch interest is theory and realization of terrestri-al reference systems. He has nearly 20 years of expe-rience in analysis of global terrestrial referenceframes within the ITRF activity. Dr. Altamimi is headof the ITRS/ITRF Product Center and a member ofthe IERS Directing Board. He serves as president ofIAG sub-commission on Regional Reference Framesand Chair of the EUREF Technical Working Group.

Norman Beck is an Ontario Land Surveyor(Geodesy) and has been working with GPS for over26 years. After graduating from the University ofToronto, he joined Nortech where he developed GPSpositioning and integrated navigation software for thefirst available commercial receivers. In the mid1980’s, he joined the Geodetic Survey Division ofNRCan where he has had a variety of duties andassignments including: GPS software development,GPS data processing and analysis, managing fieldsurveys, heading Client Services Section and morerecently the Active Control Systems Section. He iscurrently chief of the Division.

Rene Forsberg is currently working at theDepartment of Geodesy of the Danish NationalSpace Centre. Previously he was the State Geodesistat the National Survey and Cadastre of Denmark andHead of the Geodynamics Department where he ledresearch and development in the broad field of geo-dynamics, physical geodesy, and seismology. Heholds PhDs completed in Denmark and Norway andhis personal research interests focus on activities inphysical geodesy, GPS/INS, and static and kinematicgravimetry. He has authored or co-authored approxi-mately 190 contributions and papers since 1978, ofwhich 55 in reviewed proceedings or journals.

W. Gurtner received his diploma in surveyingengineering in 1973 and a PhD in technical sciencesin 1978, both from the Federal Institute ofTechnology (ETH) in Zurich. From 1979 to1986 hewas a research assistant at the Astronomical Instituteof the University of Bern. In 1987 he became thehead of the institute's observatory in Zimmerwald.He was awarded the title of Professor in 1999. Hedeveloped several versions of the RINEX GPSexchange format, was project manager for theupgrade of the Zimmerwald SLR/CCD system anddeveloped a major part of its control software. He iscurrently chairman of the Governing Board of theInternational Laser Ranging Service.

Mr. Steve Kenyon is currently a Senior Scientistin the National Geospatial-Intelligence Agency(NGA) concentrating on International Programs andTechnical Initiatives in Geodesy. He has led majorgeodetic projects including the development of

EGM96 and the Arctic Gravity Project. He is cur-rently leading efforts for a new Earth GravitationalModel, which is planned to be released in late 2006.Mr. Kenyon is a Fellow of the IAG, an OfficialRepresentative of the International Gravity FieldService, and was the 2000 Heiskanen Award winnerat The Ohio State University.

Dr. Dirk Behrend is a scientist with NVI,Inc./NASA Goddard Space Flight Center. He is theCoordinating Center Director of the InternationalVLBI Service for Geodesy and Astrometry. Heholds a diploma degree in Geodesy from theUniversity of Hannover, Germany, and a doctor'sdegree in Geodesy from the same university.

Frank Lemoine is a geodesist at the NASAGoddard Space Flight Center in the PlanetaryGeodynamics Branch. His specialty is precision orbitdetermination, precision orbit modelling, gravityfield, and geodetic parameter estimation. He hasworked on the Clementine, and Mars GlobalSurveyor missions where he developed precise orbitsfor use with the laser altimeters. He has developedgravity models for the Moon, Earth, and Mars, is aco-investigator on the Mars Reconnaissance OrbiterGravity Science Team, and on the GRACE scienceteam. He currently serves as the analysis coordinatorfor the International DORIS Service.

Dr. Chopo Ma is a specialist in VLBI in thePlanetary Geodynamics Laboratory of NASA'sGoddard Space Flight Center. He has a BA fromHarvard and a PhD from University of Maryland.His current research activities include determina-tion of the terrestrial and celestial reference frames.He is chair of the International Earth Rotation andReference Systems Service and sits on the boardsof the Global Geodetic Observing System, theInternational VLBI Service for Geodesy andAstrometry, and the National Earth OrientationService. He leads the work to update theInternational Celestial Reference Frame.

Carey Noll is the manager of the CrustalDynamics Data Information System, the CDDIS,located at NASA’s Goddard Space Flight Center.This system is internationally recognized as NASA’sarchive of space geodesy data, particularly, GNSS,laser ranging, VLBI, and DORIS data sets. Ms. Noll,who holds a BA degree in Mathematics from WesternMaryland College, has over 25 years of experience atGoddard, in the development of data systems, database administration, applications programming, com-puter system management, and web development.She is a member of the International GNSS Service,International Laser Ranging Service, andInternational DORIS Service Governing Boards andserves as secretary of the ILRS Central Bureau.

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Erricos C. Pavlis is a Research AssociateProfessor, Physics, at the Univ. of Maryland BaltimoreCounty, doing research for NASA and the NGA atUMBC’s Joint Center for Earth Systems Technology.He participated in the LAGEOS, LAGEOS-2, theCrustal Dynamics Project, the TOPEX/PoseidonMission, and the EGM96 development. Currently he isa science team member of GRACE and OST missions,and a co-PI of the LARES mission, vice-chairman ofCOSPAR’s Panel on Satellite Dynamics, member ofILRS’s Central Bureau and Analysis and ModelingWG, chairman of the ILRS Refraction Study group,IAG’s Subcommission 1.4 Working Group 1.4.4, andassociate editor for Celestial Mechanics andDynamical Astronomy.

Zinovy Malkin is Dr. Sci, specialist in analysis ofthe Space Geodesy observations. For the last 10 yearshe has served as the head of the IAA EOP Service,and has been involved in computation and analysis ofEOP and station coordinates time series from VLBI,SLR and GPS, and algorithm and software develop-ment. He is a member of the IVS Directing Board, anAssociate Member of the IERS, a member of IAUCommission 19 Organizing Committee, and severalinternational working groups.]

Angelyn Moore is the Network Coordinatorand Deputy Director of the International GPSService's Central Bureau at the Jet PropulsionLaboratory, California Institute of Technology.Prior positions at JPL include the GPS Networksand Ionospheric Systems Development group(1995 to 1998) and Time and Frequency StandardsResearch and Development group (1988 to 1995).Angelyn's PhD dissertation from the University ofCalifornia, Riverside (1995) is entitled "Dynamicsof Laser-Cooled Ions Confined in Radio-FrequencyTraps." She also holds an MS in Physics fromCalifornia State University, Los Angeles and a BSin Physics from Harvey Mudd College.

Dr. Webb is a Principal Member of theTechnical Staff at the Jet Propulsion Laboratory,California Institute of Technology. From 2000 to2006, he was the Group Supervisor for the SatelliteGeodesy and Geodynamics Systems Group at JPL.Responsibilities within his group included the JPLIGS Analysis Center products, the development ofthe GIPSY software, and NASA’s Global GPSNetwork. Prior to this, Dr. Webb was the ProgramElement Manager for the Southern CaliforniaIntegrated GPS Network (SCIGN). In 2005, he andhis colleagues were inducted into the SpaceFoundation’s Space Technology Hall of Fame fortheir contributions to high precision GPS position-ing. In addition to his duties at JPL, he is a memberof the Board of Directors of UNAVCO Inc.

Ms. Ruth E. Neilan is Director of the CentralBureau of the International GNSS Service (IGS,formerly the International GPS Service from 1994to 2005, a recognized service of the InternationalAssociation of Geodesy (IAG). The IGS CentralBureau is located at NASA's Jet PropulsionLaboratory of the California Institute ofTechnology (http://igscb.jpl.nasa.gov) in Pasadena,CA. Ms. Neilan has been involved with the civilGPS networks and applications since 1984. From1992 to 1998 she was the GPS operations managerfor NASA/JPL’s GPS Global Network and projectmanager for scientific support of regional experi-ments. She received her MSc in Civil andEnvironmental Engineering from the University ofWisconsin-Madison in 1986.

Dr. John Ries is a senior research scientist at theCenter for Space Research at The University ofTexas in Austin. His research activities includeorbital mechanics, geodesy, relativity, and the appli-cation of computers and computational techniques tothe solution of problems in those areas. His currentefforts are toward improving precision orbit deter-mination for the Jason-1 altimeter mission and grav-ity model determination for the Gravity Recoveryand Climate Experiment (GRACE). He is a Fellowof the IAG and a member of the InternationalDORIS Service Governing Board.

Markus Rothacher, born in Thun, Switzerland,studied astronomy, physics and mathematics at theAstronomical Institute of the University Berne.There, he became Head of the GPS Group. His doc-toral thesis is entitled “Satellite Orbits in SpaceGeodesy” (1991), and his professorial dissertation“Recent Contributions of GPS to Earth Rotationand Reference Frames” (1998). In 1999 he becameProfessor for Space Geodesy at the TechnicalUniversity Munich. He is currently Professor forSatellite Geodesy and Earth Studies at theTechnical University Berlin and Director of theDepartment “Geodesy & Remote Sensing” atGeoForschungsZentrum Potsdam (since 2005). Heis a member of various international boards (IAG,IERS, IGS) and Chair of GGOS.

Pascal Willis received his PhD in Geodesy at theParis Observatory, France, in 1989. He has been work-ing at the Institut Geographique National, France since1983 on satellite geodesy, using GPS, GLONASS orthe DORIS system. Since 2001, he has been workingat the Jet Propulsion Laboratory on precise orbit deter-mination. He is the current vice-President ofCommission 4 (Positioning and Applications) of theInternational Association of Geodesy and a previousEditor of the Journal of Geodesy. o

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