SESSION 5: GROUND-BASED GPS IONOSPHERIC · PDF filetion and a daily set of GPS satellite DCBs in the form of IONEX format files (Schaer et al., 1997). ... The HIRAC/SolarMax campaign

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IONO_WG STATUS REPORT AND OUTLOOK- POSITION PAPER -

J. FeltensEDS Deutschland GmbH, based at

Navigation Research Office,ESA, European Space Operations Centre,

Robert-Bosch-Str. 5, D-64293 Darmstadt, Germany

ABSTRACT

The IGS Ionosphere Working Group (Iono_WG) was established by the IGS Governing Board on28 May 1998 and commenced working in June 1998. The working group’s main activity is at themoment the routine provision of ionosphere Total Electron Content (TEC) maps with a 2-hours timeresolution and of daily sets of GPS satellite (and receiver) hardware differential code bias (DCB)values. The computation of these TEC maps and DCB sets is based on the routine evaluation ofGPS dual-frequency tracking data recorded with the global IGS tracking network. Currently final at-tempts are made to establish from the individual contributions a combined IGS Ionosphere Productand to commence with the routine delivery of that product. The implementation of near-real-timeavailability is then the next important task and, medium-termed, the development of more sophisti-cated ionosphere models. Also the inclusion of other than GPS-data might be an aspect. The finaltarget is the establishment of an independent IGS ionosphere model.

Currently five IGS Ionosphere Associate Analysis Centers (IAACs) contribute with their iono-sphere products to the Iono_WG activities. Once per week these ionosphere products are comparedwith a dedicated comparison algorithm. This comparison/combination algorithm was worked outand coded in 1998 from scratch. In the meantime the original comparison/combination algorithmwas upgraded with new weights computed from the results of external self-consistency validations.The weekly comparisons are done with this new approach since August 2001. Furthermore, theIAACs TEC maps are routinely validated with TOPEX altimeter data since July 2001.

During the recent IGS/IAACs Ionosphere Workshop, ESOC, Darmstadt, Germany, January17-18, 2002, a list of final actions was decided, which shall soon lead to the routine delivery of anofficial IGS Ionosphere Product. Based on the outcome of the Darmstadt Workshop and on the dis-cussions at Ottawa, five recommendations were formulated in this Position Paper, which will be thebasis for the Iono_WG members on how to progress - especially to come soon into a position to startwith the routine delivery of an official IGS Ionosphere Product.

It is the intent of this Position Paper to give a short history and the current status of the Iono_WGactivities. The recommendations stated at the end of this paper shall then be an orientation for theIAACs on how to progress, so that the Iono_WG can soon start with the routine delivery of a com-bined IGS Ionosphere Product to external users through the Crustal Dynamics Data InformationSystem (CDDIS).

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1 INTRODUCTION

This Position Paper will start with a project report providing an overview over the Iono_WG activi-ties since its establishment in 1998.

The next aspect treated will be an overview about the routine comparisons, which are done untilnow at the designated Ionosphere Associate Combination Center (IACC) at ESOC. Key statistics ofthe routine TOPEX validations will be presented.

Based on the outcome of the IGS/IAACs Ionosphere Workshop in Darmstadt, 17-18 January,2002, and on the discussions made at Ottawa, five recommendations are then formulated definingthe way on how to progress by the Iono_WG.

Finally the Position Paper will conclude with a résumé of the achievements so far reached.

2 WG-ACTIVITIES SINCE ITS ESTABLISHMENT IN MAY’98

The Working Group started its routine activities in June 1998: Several so called Ionosphere Associ-ate Analysis Centers (IAACs) provide per day twelve global TEC maps with a 2-hours time resolu-tion and a daily set of GPS satellite DCBs in the form of IONEX format files (Schaer et al., 1997).The routine provision of daily ground station DCBs is under preparation. Currently five IAACscontribute with ionosphere products:

• CODE, Center for Orbit Determination in Europe, Astronomical Institute, University of Berne,Switzerland.

• ESOC, European Space Operations Centre of ESA, Darmstadt, Germany.

• JPL, Jet Propulsion Laboratory, Pasadena, California, U.S.A.

• NRCan, Natural Resources Canada, Ottawa, Ontario, Canada.

• UPC, Polytechnical University of Catalonia, Barcelona, Spain.

The mathematical approaches used by the distinct IAACs to establish their TEC maps are quite dif-ferent. Details about the individual IAACs modeling can be found in e.g. (Schaer 1999; Feltens,1998; Mannucci et al., 1998; Gao et al.; Hernandez-Pajares M. et al., 1999).

The IGS standards defining the form in which the ionosphere products must be delivered to theCrustal Dynamics Data Information System (CDDIS), are declared in the recommendations of theDarmstadt 1998 IGS Workshop Position Paper (Feltens and Schaer, 1998). In short summary themost important are: 1) TEC maps and GPS satellite DCBs must be delivered in form of dailyIONEX files (Schaer et al., 1997). 2) The TEC maps must have a time resolution of 2 hours, theymust be arranged in a fixed global grid and refer to a shell height of 450 km. 3) Ionosphere productsmust be made available not later than the IGS Final Orbits, i.e. 11 days after the last observations.

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Once per week the IACC performs the comparisons of the ionosphere products of all 7 days ofthe GPS week recently delivered to CDDIS. The comparison products and a weekly report are madeavailable at ESOC’s FTP account: ftp [email protected]. A short summary is e-mailedthrough the IONO-WG list to the Iono_WG.

Apart from the routine activities the Iono_WG organized so far two dedicated high-rate trackingcampaigns with the global IGS network during events which are of special relevance for the iono-sphere:

1) The Solar Eclipse campaign on 11 August 1999: About 60 IGS sites, being located along theeclipse path from the east coast of North America over Europe and the Near - and MiddleEast, recorded on that day dual-frequency GPS-data with 1- and 3-second sampling rates. Thehigh rate data are archived at the CDDIS and is open to research groups to study the iono-sphere’s reaction on the solar eclipse (anonymous ftp at cddisa.gsfc.nasa.gov in directory/gps/99eclipse).

2) The HIRAC/SolarMax campaign from 23 - 29 April 2001: About 100 IGS sites, being locatedin the northern and southern polar regions and in the low latitudes including the crest regionsat both sides of the geomagnetic equator, recorded over 7 days dual-frequency GPS-data with1- and 3-second sampling rates. This IGS/Iono_WG activity was coordinated with other iono-spheric observation programs or measurement campaigns using ionosondes, EISCAT, highresolution magnetometers, etc. to obtain a comprehensive view of the geomagnetic and iono-spheric state. The high rate GPS and GLONASS data are archived at the CDDIS and is opento research groups to study the ionosphere’s behavior under solar maximum conditions (anon-ymous ftp at cddisa.gsfc.nasa.gov in directory /gps/01solarmax).

The Iono_WG is open to organize further campaigns of this type.

3 RECENT IMPROVEMENTS

3.1 Upgraded Comparison/Combination Approach

In short, the old comparison/combination approach (☞ see Appendix B attached) was based on un-weighted and weighted mean TEC maps, which could be considered as something like “combined”TEC maps, and the individual IAACs TEC maps were compared with respect to the weighted meanTEC maps. The comparison of DCBs was done basically in the same way. However, it was wellknown from the beginning, that the different IAACs models are based on very different mathemati-cal approaches and the weights obtained with the old approach did obviously not represent the truequality of the input IAACs TEC maps.

The Iono_WG thus decided to upgrade the comparison/combination algorithm with a newweighting scheme, whereby the individual IAACs-weights are derived from external validationswith self-consistency tests (☞ see Appendix A attached). The weekly comparisons are done withthis new approach since August 2001. The external validations needed for this method are made rou-tinely by the Ionosphere Associate Validation Centers (IAVCs) UPC and NRCan prior to the weeklycomparisons at the IACC at ESOC.

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Feltens (2002a) presents results obtained with the old and with the new comparison scheme:1) The new comparison/combination approach favors the higher quality TEC maps more than theold approach did. 2) Currently discrete weights are assigned to defined geographic areas, which cancause “chessboard-like” patterns in the IGS TEC RMS maps and might in extreme cases also be-come visible in the IGS TEC maps. At Ottawa it was thus decided to compute from these regionalweights corresponding global weights, which shall then be introduced into the comparisons/combi-nations. 3) The satellite DCBs series provided by most of the IAACs are quite constant, oscillatingbetween 0.2 and 0.4 nanoseconds around their mean values.

3.2 TOPEX Validations

Since July 2001 JPL provides VTEC data derived from TOPEX altimeter observables to the work-ing group to enable validations. Due to its orbital geometry TOPEX scans every day only a limitedband of the ionosphere. Additionally, the TOPEX data may be biased by +2-5 TECU. These two as-pects must be kept in mind when interpreting the validations with TOPEX VTEC data. The TOPEXvalidations are attached to the weekly comparisons.

Principally these TOPEX validations work as follows: JPL provides per day a so called TOPEXfile containing VTEC values derived from TOPEX altimeter data in dependency of time, latitudeand longitude. In the different IAACs IONEX files VTEC values for the same times/latitudes/longi-tudes are interpolated, and the corresponding TOPEX VTEC values are then subtracted. TheVTEC-differences thus obtained are used to establish different kind of statistics, like mean daily off-sets & related RMS values for each IAAC.

3.2.1 Results

Figure 1 below condenses the basic statistics that were obtained from the TOPEX validations since19 August 2001. The numbers plotted are:

• mean ... mean IAAC VTEC offset with respect to the TOPEX VTEC values, i.e. the mean valueover n differences d = tecval(IAAC) - TOPEXtec:

,

• rms-diff ... RMS of differences: ,

• rms ... RMS of residuals with respect to the mean, set v = tecval(IAAC) - mean:

.

From GPS week 1158 on, the following two statistics parameters are included too (not in Figure 1):

• sf/rms ... estimate of the scale factor of the RMS-values obtained from the TOPEX validationin relation to the corresponding IAAC RMS values, should be close to one forIAAC = IGS, i.e. for the combined TEC maps:

,

• wrms ... corresponds to a “mean” RMS and might be an indicator for a TEC map’s quality:

.

mean d∑ n⁄=

rmsdiff d2∑ n⁄=

rms v2∑ n 1–( )⁄=

sf r⁄ ms d tecrms IAAC( )⁄{ } 2∑ n⁄=

wrms d tecrms IAAC( )⁄{ } 2∑ 1 t⁄ ecrms IAAC( ){ } 2∑⁄=

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The TOPEX validations are done globally for all latitudes (“+90..-90”) and separately also formedium and high northern latitudes (“+90..+30”), equatorial latitudes (“+30..-30”) and mediumand high southern latitudes (“-30..-90”). Beyond the IAACs TEC and the IGS TEC, also TECcomputed with the GPS broadcast model (“gps”) and TEC computed with CODE’s Klobuchar-Style Ionosphere Model (“ckm”) enter into the daily TOPEX validations. The latter two are provid-ed by CODE.

When inspecting the curves in Figure 1 for the different latitude bands one recognizes immedi-ately that the best agreement of the distinct ionosphere models with the TOPEX data is achieved atmedium and high northern latitudes, while the worst agreement is in the equatorial region. Theagreement in the southern medium and high latitudes is more worse than in the northern ones, but asfar as not as worse as in the equatorial latitude band.

The other thing that can be seen from Figure 1 is that the IAACs TEC and the IGS TEC values,which are derived from GPS dual-frequency data, are considerably closer to the TOPEX TEC thanthe Klobuchar and especially the GPS broadcast model - and what is essential for the delivery of acombined IGS Ionosphere Product: The routine validations with TOPEX since July 2001 show anagreement of the "combined" IGS TEC maps with the TOPEX data on the same order as the bestIAACs TEC maps.

Figure 1: The basic TOPEX validation statistics mean, rms-diff and rms.

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Figure 1 (cont.): The basic TOPEX validation statistics mean, rms-diff and rms.

4 OUTCOME FROM THE WORKSHOPS IN DARMSTADT AND IN OTTAWA - RECOMMENDATIONS

On 17-18 January 2002 an IGS/IAACs Ionosphere Workshop was held at ESOC, Darmstadt,Germany. The major target of this workshop was (for the complete list see Feltens, 2002b): To talkabout actions still needed to be undertaken before the routine delivery of a combined IGS Iono-sphere Product can be started. Apart from that, discussions were made about new research activitiesto be considered by the Iono_WG, discussions of points which are of vital interest for the Iono_WGwithin the IGS, implementation of near-real-time availability of Iono_WG products, guarantee of re-liability of Iono_WG products.

Based on the conclusions of the Darmstadt workshop (Feltens, 2002b) and on the discussions atOttawa the following five recommendations were formulated, which shall serve as orientation for

cod: red, emr: green, esa: blue, jpl: black, upc: orange, igs: pink, gps: dark red, ckm: dark green.

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the Iono_WG on how to progress - as stated above, the major target is the start of the routine deliv-ery of a combined IGS Ionosphere Product.

Recommendations:

(1) Start with the delivery of a combined IGS Ionosphere Product, as soon as the last required up-grades in the comparison/combination program are made in summer 2002.

(2) Combined IGS Total Electron Content (TEC) and RMS maps should be produced for the evenhour numbers, i.e. 0h, 2h, 4h, 6h, ... , 24h. In this way the 24h maps of the previous day cor-respond to the 0h maps of the current day.

(3) Global IGS Ionosphere Associate Analysis Centers (IAACs) TEC/RMS maps should coverall parts of the world.

(4) Explore the use of ENVISAT and JASON satellites for validation of IGS Ionosphere Products.

(5) In view of Near Real Time Monitoring of the Ionosphere the distribution of ground stations aswell as the data flow (latency) has to be improved.

5 CONCLUSIONS AND OUTLOOK

The Iono_WG started working in June 1998 with the routine provision of daily IONEX files con-taining global TEC and RMS maps with a time resolution of 2 hours and a daily set of GPS satelliteDCB values. Currently five IAACs contribute with their ionosphere products.

For the weekly comparison of IAACs ionosphere products a dedicated algorithm was worked outand coded from scratch at the IACC at ESOC. This “old” comparison algorithm was based on theconcept of unweighted and weighted means and provided, so to say as by-product, also somethinglike a “combination” of the IAACs individual ionosphere products. However, the IAACs use verydifferent mathematical approaches and estimation schemes in their ionosphere processing, and thiscircumstance strongly reflected in the comparison results. The Iono_WG thus decided to upgradethis “old” comparison algorithm with a new weighting scheme using the results of external self-con-sistency test validations as input. The “new” comparison algorithm is now in operational use sinceAugust 2001. An analysis of the results obtained so far shows, that, apart from some minor weak-nesses, the new approach seems to meet the demands for the computation of a combined IGS Iono-sphere Product.

Additionally, since July 2001, routine validations of the IAACs TEC maps plus the “combined”IGS TEC maps with VTEC values derived from TOPEX altimeter data are attached to the weeklycomparisons. The results of these validations show an agreement of the "combined" IGS TEC mapswith the TOPEX data on the same order as the best IAACs TEC maps.

Based on the conclusions made at the IGS/IAACs Ionosphere Workshop in Darmstadt, 17 - 18January, 2002, and on the discussions at Ottawa, five recommendations were formulated on how to

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do away with remaining minor problems and to bring the Iono_WG soon into a position to start withthe routine delivery of a combined IGS Ionosphere Product.

Beyond the realization of the combined IGS Ionosphere Product, goals and next steps are: en-hancement of the IGS TEC maps time resolution, implementation of rapid products up to near-real-time availability, further validations, e.g. with ENVISAT altimeter data, and inclusion of higher or-der terms into ionospheric delay corrections modeling.

REFERENCES

Feltens, J., 1998, Chapman Profile Approach for 3-d Global TEC Representation, IGS Presentation,in Proceedings of the 1998 IGS Analysis Centers Workshop, ESOC, Darmstadt, Germany,February 9-11, 1998, pp 285-297.

Feltens, J. and S. Schaer, 1998, IGS Products for the Ionosphere, IGS Position Paper, in Proceedingsof the 1998 IGS Analysis Centers Workshop, ESOC, Darmstadt, Germany, February 9-11,1998, pp. 225-232.

Feltens, J., 2002a, Résumé of Three Years of Iono_WG Activities and Status of Comparisons & Val-idations, IGS/IAACs Presentation, in Collection of Presentations of the 2002 IGS/IAACsIonosphere Workshop, ESOC, Darmstadt, Germany, January 17-18, 2002, in preparation.

Feltens, J., 2002b, IGS/Ionosphere Workshop, 17 and 18 January - Conclusions, IGSMAIL-3711,submitted on 25 January 2002, also in Collection of Presentations of the 2002 IGS/IAACsIonosphere Workshop, ESOC, Darmstadt, Germany, January 17-18, 2002, in preparation.

Gao, Y., P. Heroux and J. Kouba, Estimation of GPS Receiver and Satellite L1/L2 Signal DelayBiases using Data from CACS, 10 pages.

Hernandez-Pajares M., J.M. Juan and J. Sanz (1999): ‘New approaches in global ionospheric deter-mination using ground GPS data’, Journal of Atmospheric and Solar Terrestrial Physics,Vol 61, pp 1237-1247, 1999.

Mannucci, A. J., B. D. Wilson, D. N. Yuan, C. H. Ho, U. J. Lindqwister and T. F. Runge, 1998,A global mapping technique for GPS-derived ionospheric total electron content measurements,in Radio Science, Vol. 33, pp. 565-582, 1998.

Schaer, S., W. Gurtner and J. Feltens, 1997, IONEX: The IONosphere Map EXchange FormatVersion 1, February 25, 1998, in Proceedings of the 1998 IGS Analysis Centers Workshop,ESOC, Darmstadt, Germany, February 9-11, 1998, pp. 233-247.

Schaer, S. (1999): ‘Mapping and Predicting the Earth’s Ionosphere Using the Global PositioningSystem’, Dissertation, Astronomical Institute, University of Berne, Berne, Switzerland,25 March 1999.

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CURRENT STATUS OF ESOC IONOSPHERE MODELINGAND

PLANNED IMPROVEMENTS

J. FeltensEDS Deutschland GmbH, based at

Navigation Research Office,ESA, European Space Operations Centre,

Robert-Bosch-Str. 5, D-64293 Darmstadt, Germany

SUMMARY

The ESOC Ionosphere Monitoring Facility (IONMON) software is in operational use since the be-ginning of the year 1998 for routine IGS ionosphere processing. It employs a 3-dimensional iono-sphere model, based on a Chapman Profile approach. However, three years of routine applicationshow certain weaknesses and limitations of this algorithm:

• The ionosphere is a rapidly changing medium. ➙ The current 24 hours time resolution must beenhanced with a sequential estimate processor: Establishment of normal equation systems with acertain time resolution, say 1 hour, and estimation of the ionosphere model parameters on onehand and of the Differential Code Biases (DCBs) on the other hand from this basic set of normalequations in different ways

• The current mathematical model describes the vertical electron density distribution as only onelayer. ➙ The ionosphere must mathematically be described as superimposion of different layers.

• From pure Total Electron Content (TEC) observables it is difficult to estimate profile shapeparameters. ➙ Electron density profiles derived from Champ occultation data will be introducedas additional observables to allow for a better spatial resolution.

To improve performance, modifications are currently ongoing into the following directions:

• Enhancement of the time resolution for ionosphere fits.

• Modified TEC/DCBs estimation scheme plus computation of TEC RMS maps.

• Software tool to predict the ionosphere’s state.

• Inclusion of other observation types than TEC data, namely Champ occultation profiles.

• Improvement of mathematical modeling into several directions (composition of several layers,alternative profile functions, α-layer handling, correction for the plasmasphere, height-dependentScale Height).

• Availability of the improved ionosphere models through an upgraded external user interface.

• Inclusion of higher order terms (in the medium-term).

At the current stage of work the new algorithms are completely worked out, coded and compiled. Inthe next step they must be unit-tested and validated and then be implemented into the operationalIONMON software. It is hoped that these different kinds of modification will lead to an improvedroutine ionosphere processing at ESOC.

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Brief summary of UPC ionospheric activities

M. Hernandez-Pajares, J.M. Juan, J. Sanz, M. Garcıa and R. OrusgAGE/UPC, Barcelona, Spain (contact e-mail: [email protected])

The authors report several ionospheric activities with GPS data, including those of IGS sta-tions: the daily generation of global TEC maps, the development of new algorithms for real-timeionospheric corrections and for electron density retrieval. Let’s describe them briefly:

The generation of TEC Global Ionospheric Maps (GIM’s), from IGS data as an IGS As-sociate Analysis Center, are being done and delivered to the IGS community on a daily basis fromJune 1st 1998 (some details of the technique are described in Hernandez-Pajares M., J.M. Juanand J. Sanz, New approaches in global ionospheric determination using ground GPS data, Journalof Atmospheric and Solar-Terrestrial Physics, Vol 61, 1237-1247, 1999). Moreover, since 2001,weights for the 5 different IAAC’s involved in GIM’s computation are being generated on a weeklybasis as a function of the RMS for STEC predictions over a certain subset of IGS stations. Thisis done as one IGS Associate Validation Center (IAVC).

In order to improve the capability of computing accurate ionospheric corrections in real-time, we have developed a new technique combining a tomographic modeling of the Ionospherewith an accurate geodetic computation, which allows to improve the performance of both iono-spheric and navigation software (Hernandez-Pajares M., J.M. Juan, J. Sanz, O.L. Colombo, Ap-plication of ionospheric tomography to real-time GPS carrier-phase ambiguities resolution at scalesof 400-1000 km and with high geomagnetic activity, Geophysical Research Letters, Vol 27, No 13,2009-2012, 2000). The performance of this approach for fixed GPS sites separated several thou-sands of kilometers has been tested over four consecutive weeks in hard ionospheric conditions andover a wide range of latitudes crossing the equator (Hernandez-Pajares M., J.M. Juan, J. Sanzand O. Colombo, Improving the real-time ionospheric determination from GPS sites at Very LongDistances over the Equator, Journal of Geophysical Research - Space Physics, In Press , 2002).

The last but not the least, in the context of the existing and incoming GPS receivers onboard Low Earth Orbiters with antennas pointing to the Earth limb (CHAMP, SAC-C,...), theauthors have developed algorithms to improve the electron density estimations. Theyare based on modeling the horizontal gradients of the electron content with a TEC model, thatcan be computed from IGS fixed sites. The new technique also takes into account the topsideelectron content, specially in very low satellites such as CHAMP (Hernandez-Pajares M., J.M.Juan, J. Sanz, Improving the Abel inversion by adding ground data LEO radio occultations inthe ionospheric sounding. Geophysical Research Letters, 27, 2743-2746, 2000). Moreover, wehave applied new schemes combining complementary kind of data in the common frameworkof a tomographic voxel model, such as ground GPS and ionosonde which can provide a similarperformance as the use of LEO GPS occultation data.

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