IMPROVING RESILIENCE TO EMERGENCIES THROUGH ADVANCED CYBER TECHNOLOGIES Report on EGNSS Integration Deliverable ID D3.4 Work Package Reference WP3 Issue 1.0 Due Date of Deliverable 30/11/2017 Submission Date 15/11/2017 Dissemination Level 1 PU Lead Partner ISMB Contributors - Grant Agreement No 700256 Call ID H2020-DRS-1-2015 Funding Scheme Collaborative I-REACT is co-funded by the Horizon 2020 Framework Programme of the European Commission under grant agreement n. 700256 1 PU = Public, PP = Restricted to other programme participants (including the Commission Services), RE = Restricted to a group specified by the consortium (including the Commission Services), CO = Confidential, only for members of the consortium (including the Commission Services)
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IMPROVINGRESILIENCETOEMERGENCIESTHROUGH
ADVANCEDCYBERTECHNOLOGIES
ReportonEGNSSIntegration
Deliverable ID D3.4
Work Package Reference WP3
Issue 1.0
Due Date of Deliverable 30/11/2017
Submission Date 15/11/2017
Dissemination Level1 PU
Lead Partner ISMB
Contributors -
Grant Agreement No 700256
Call ID H2020-DRS-1-2015
Funding Scheme Collaborative
I-REACT is co-fundedby theHorizon2020FrameworkProgrammeof theEuropeanCommissionundergrantagreementn.700256
Figure5-1:Resultsofdatacollections for thealgorithmtuning. (a)and (b)showresidualssinglevalues,theiraverageandaquadraticfittotheaveragevaluesinthecaseofavegetatedroadonthehillsrespectivelytowardssatelliteelevationandC/N0.Augmentationalgorithmflowchart..........30
Figure5-3:A10kmlongurbantrackzoomedinthemostcriticalpoint.Greenarrowpointstothecomputedpositionwhile the redarrowpoints to the realposition.Realposition falls inside theellipse. Inthiscasethedatawerecollectedusingabikepassingmorethanoncealongthesamestreets:thisexplainsthehighellipsesdensity.................................................................................31
Fromthealgorithmicpointofview,mostof theeffortwason thestudyofanovelapproachtoestimate the position confidence, which is not only based on the information provided byaugmentationsystems,butalsoleveragesonthecharacterizationoftheenvironmentsurroundingtheantennathroughtheprocessingofthereceivedincomingGNSSsignals.ThisconceptisemerginginLocationBasedService(LBS)andisreferredas“localintegrity”,becauseittakesintoaccountsthelocalimpairments(i.e.:multipath,interferingsignals,attenuations,etc.)thatactuallyhavethemostsignificanteffectsonthepositionaccuracyand,inturn,onthereliability.
EGNOS European Geostationary Navigation Overlay Service
EMS Emergency Management Service
EO Earth Observation
ESA European Space Agency
EU European Union
EWAN EGNOS Wide Area Network
EWS Early Warning System
FOC Full Operational Capability
FTP File Transfer Protocol
GCC Galileo Control Centres
GEO Geostationary satellites
GIVE Grid Ionospheric Vertical Error
GLONASS Global'naja Navigacionnaja Sputnikovaja Sistema (Russian GNSS)
GNSS Global Navigation Satellite System
GPS Global Positioning System
GSA European GNSS Agency
GSS Galileo Sensor Stations
HPL Horizontal Protection Level
ICAO International Civil Aviation Organization
IOC Initial Operational Capability
JSON JavaScript Object Notation
LBS Location Based Services
MCC Mission Control Centres
MOPS Minimum Operational Performance Standard
NLES Navigation Land Earth Stations
Ntrip Networked Transport of RTCM via Internet Protocol
OS Open Service
PACF Performance Assessment and Checkout Facility
PL Protection Level
PRN Pseudo Random Number (code)
PRS Public Regulated Service
PVT Position Velocity and Time
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RAIM Receiver Autonomous Integrity Monitoring
REST REpresentational State Transfer
RIMS Ranging Integrity Monitoring Stations
RTCA Radio Technical Commission for Aeronautics
RTCM Radio Technical Commission for Maritime
RTK Real Time Kinematic
SAR Search And Rescue service
SARPS Standards and Recommended Practices
SBAS Satellite Based Augmentation System
SL (EDAS) Service Layer
SIS Signal in Space
SISNet Signal in Space over Network
SL Service Level
SoL Safety of Life
TOW Time Of Week
TTA Time To Alarm
UDRE User Differential Range Error
UTC Universal Time Coordinated
VANET Vehicular Ad hoc NETwork
VPL Vertical Protection Level
WAAS Wide Area Augmentation System
1.4 REFERENCEANDAPPLICABLEDOCUMENTS
ID Title Revision Date
[RD01] European GNSS Service Centre https://www.gsc-europa.eu/
Online: accessed Nov.2017
2017
[RD02] EGNOS Portal https://www.egnos-portal.eu/
Online: accessed Nov.2017
2017
[RD03] European Space Agency: Galileo http://www.esa.int/Our_Activities/Navigation/Galileo/What_is_Galileo
Online: accessed Nov.2017
2017
[RD04] International Standards and Recommended Practices – Annex 10 to the Convention on International Civil Aviation – Aeronautical Telecommunications – Volume 1 Radio Navigation Aids
6th ed. July 2006
[RD05] DO-229D Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System Airborne Equipment
D 13/12/2006
[RD06]
Don Jewell, Protect, Toughen, Augment: Words to the Wise from GPS Founder http://gpsworld.com/protect-toughen-augment-words-to-the-wise-from-gps-founder/
Online: accessed Nov.2017
15/04/2014
[RD07]
CEN/CENELEC, Space - Use of GNSS-based positioning for road Intelligent Transport Systems (ITS) - Part 1: Definitions and system engineering procedures for the establishment and assessment of performances
DRAFT EN 16803-1
October 2014
[RD08] E.D. Kaplan and C.J. Hegarty. Understanding GPS: Principles and Applications. Artech House. 2nd ed. 2006
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[RD09] EGNOS Data Access Service https://www.gsa.europa.eu/egnos/edas
[RD12] GSA. Galileo increases the accuracy of location based services https://www.gsa.europa.eu/news/results-are-galileo-increases-accuracy-location-based-services
Online: accessed Sep.2017
2017
[RD13]
The Local Integrity Approach for Urban Contexts: Definition and Vehicular Experimental Assessment. Margaria D., Falletti E. Sensors (Basel); 16(2):154. doi: 10.3390/s16020154.
- 26/01/2016
[RD14]
Cosmen-Schortmann, J.; Azaola-Saenz, M.; Martinez-Olague, M.A.; Toledo-Lopez, M. Integrity in Urban and Road Environments and its Use in Liability Critical Applications. In Proceedings of the IEEE/ION Position, Location and Navigation Symposium, Monterey, CA, USA,; pp. 972–983.
8 May 2008
[RD15]
Pullen, S.;Walter, T.; Enge, P. SBAS and GBAS Integrity for Non-Aviation Users: Moving Away from “Specific Risk”. In Proceedings of the 2011 International Technical Meeting of The Institute of Navigation, San Diego, CA, USA; pp. 533–545.
26 January 2011
[RD16] K. Borre. Gps easy suite ii - RAIM. Inside GNSS, 4(4):48–51 2009
[RD17] D. Margaria and E. Falletti. A novel local integrity concept for GNSS receivers in urban vehicular contexts. In Position, Location and Navigation Symposium, Monterey, pages 413–425. IEEE/ION
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2 EGNSS
This Chapter recalls the fundamentals of the European Global Navigation Satellite Systems(EGNSS)[RD01] and their related services. It also includes a terse description of the standardintegrityconceptusedintheaviationdomain.Suchaconceptisthestartingpointforallthenewalgorithms (developed by researchers for LBSs working in other domains) that propose thecomputationofaconfidenceintervalontopoftheestimatedGNSSpositions.
TheacronymEGNSSisusedtoaddresstwodifferentEuropeansystems:EGNOS[RD02]andGalileo[RD03].Whilethefirstisaregionalaugmentationsystemconceivedtoimprovetheexploitationofthe American Navstar Global Positioning System (simply known as GPS), the second is anindependentcivilGNSSsetupbytheEuropeanUnion(EU)andtheEuropeanSpaceAgency(ESA).
2.1 EGNOS
TheEuropeanGeostationaryNavigationOverlayServicewasbornastheEuropeanversionoftheAmericanWideAreaAugmentationSystem(WAAS).Boththesesystemsareconceivedspecificallytosupporttheautomaticnavigationofaircraftsprovidingsupplementalinformationviasatellites:forthisreasontheyarecalledSatelliteBasedAugmentationSystems.WAASprovidessupportforNorthAmericawhileEGNOSforEurope;bothofthemarecompliantwithtwosetsofInternationalStandards that enable their use by Civil Aviation Authorities: the Standards and RecommendedPractices(SARPS)StandardforSBASsystems[RD04]andtheMinimumOperationalPerformanceStandard (MOPS) DO229 [RD05]. The SARPS has been established and controlled by theInternational Civil Aviation Organization (ICAO). It provides standards regarding the type andcontentofdata,whichmustbegeneratedandtransmittedbyanSBASsystem.Ingeneral,theSBASprovidershallbroadcastaSBASSignalinSpace(SIS)complianttothisstandardintermsofradio-frequency characteristics, and data content and format. The MOPS has been established andcontrolledbytheUSRadioTechnicalCommissionforAeronautics(RTCA)anditprovidesstandardsforSBASreceiverequipment.
Integrity is the most stringent requirement for SoL systems, because it relates to the systemreliability.Aninformal,butveryeffective,definitionwasprovidedbyDr.BradParkinson[RD06]:
Inthesecondcase,foreachsatelliteoftheconstellation,theSBASprovidesinformationtocorrectsatelliteclockparameters,positionofthesatellite,ionosphereeffects,whiletheuseofamodelisforeseen to compensate for the effect of the troposphere. At the same time, supplemental
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• σ2GIVEisthevarianceofaNormaldistributionassociatedwiththeresidualGridIonosphericVerticalError(GIVE)anditisassociatedwithaspecificpoint(belongingtoadefinedgrid).Itis provided for all the grid points for the specific service area (for EGNOS it roughlycorrespondstoEurope)usedforpositioning(throughaspecificinterpolationmethod).
Forthecomputationoftheconfidenceintervalontopoftheestimatedposition(i.e.:thatintheaviation domain is called Protection Level), four parameters, for each satellite i included in thepositionsolution,areneeded.Theyare:
Intheotherterrestrialapplications,itisstillpossibletoexploittheinformationmadeavailablebyEGNOS, because it is related to the satellite clocks and orbits and signal disturbances due toionosphericandtroposphericpropagation.Thisisindependentfromthespecificenvironment.
However,thelocalenvironmenthasimportanteffectsonthesignalpropagationand,inturn,onthefinalaccuracyandreliabilityofthecomputedposition.Usuallyoneofthemostdetrimentalsourceof error is theaforementionedmultipath,which refers to the receptionof the samenavigationsignals after one ormore reflections fromnearby objects: in the case of an aircraft the effectsproducedbymultipatharetakenintoaccountbymodelsspecificallydevelopedforaviation.Ifotherapplicationsareconsidered,aviationspecificmodelsareunfitforthelastpartofthesignalpath,dealingwithcomplexenvironmentslikeurbanoneorwherevegetationispresent.Thismeansthatproper strategies able to evaluate the degradations due to the local environment shall beimplemented. This is an open frontier of research, especially for safety-related terrestrialapplicationsthatestimatetheuser’spositionwithGNSS.
ThesameconceptisexploitedinI-REACT.Althoughtheapplicationcannotbeconsideredsafety-critical, the knowledge of a confidence interval on top of the estimated position is consideredvaluableforthewholeservice.ThiswillbematterofChapter5onLocalIntegrity.
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The main reasons behind the development of this service were the EGNOS signal receptionproblemsduetothegeostationaryorbitofthetransmittingsatellites.Infact,whileGNSSnavigationsatellitesarealmostevenlydistributedinthesky,sothatasubsetofthemisusuallyvisible,it’snotunusualtohavealltheEGNOSsatellitesobstructedbyabuilding(oranyothersurroundingobject),inparticularathighlatitudes.
• SISNeT serviceprovides access to the EGNOSGEO satellitesmessages over the InternetthroughtheSISNeTprotocol(definedbyESAin2002).
• Ntrip service provides data from the EGNOS network through the Ntrip protocol whichrepresentthestandardfordifferentialcorrectiondistribution.
2.4 GALILEO
Galileo is the EuropeanGlobalNavigation Satellite System. It has been conceived for twomainpurposes:theimprovementoftheperformanceswithrespecttoexistingGNSSandmakingEuropeindependentfromothernon-civiliansystems.
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TheprogrammeisdesignedtobecompatiblewithallexistingandplannedGNSSandinteroperablewith GPS and GLONASS. In this sense, Galileo is positioned to enhance the coverage currentlyavailable–providingamoreseamlessandaccurateexperienceformulti-constellationusersaroundtheworld.
Satellitepositioninghasbecomeanessentialservicethatweoftentakeforgranted,butthisisnottrueforotherGNSS,whileGalileoissettoguaranteeavailabilityoftheserviceunderallbutthemost extreme circumstances. This is fundamental in a world where the use of satellite-basednavigation systems continues to expandand consequently the implicationsof a potential signalfailurebecomeevengreater. TheadditionofGalileo to the globalGNSS constellationsnotonlyminimisestheserisks,butalsoensuresbetterperformanceandaccuracyfortheend-user.
• Commercial Service (CS): a service complementing the OS by providing an additionalnavigationsignalandadded-valueservicesinadifferentfrequencyband.TheCSsignalcanbeencryptedinordertocontroltheaccesstotheGalileoCSservices.
• Public Regulated Service (PRS): service restricted to government-authorised users, forsensitiveapplicationsthatrequireahighlevelofservicecontinuity.
• SearchandRescueService(SAR):Europe’scontributiontoCOSPAS-SARSAT,aninternationalsatellite-based searchand rescuedistressalertdetection system.Satellitesare thereforeequippedwitha transponder,which isable to transfer thedistresssignals fromtheusertransmitters to regional rescue co-ordination centres,whichwill then initiate the rescueoperation.Atthesametime,thesystemwillsendaresponsesignaltotheuser,informinghimthathissituationhasbeendetectedandthathelpisontheway.Thislatterfeatureisnewandisconsideredamajorupgradecomparedtotheexistingsystem,whichdoesnotprovideuserfeedback.
ThefullydeployedGalileosystemwillconsistof24operationalsatellitesandupto6activespares,positioned in threecircularMediumEarthOrbitplanes.Eachorbithasanominalaverage semi-majoraxisof29600km,andaninclinationof56degreeswithreferencetotheequatorialplane.
AnInitialOperationalCapability(IOC)phaseisforeseentobebasedon18satellites.Atthisstage,theOpen Service, Search and Rescue and Public Regulated Servicewill be availablewith initialperformances.Thenastheconstellation isbuilt-upbeyondthat,newserviceswillbetestedandmadeavailabletoreachFullOperationalCapability(FOC).
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aglobalnetworkofGalileoSensorStations(GSSs)aresenttotheGalileoControlCentresthrougharedundantcommunicationsnetwork.TheGCCsusethedatafromtheSensorStationstocomputetheintegrityinformationandtosynchronisethetimesignalofallsatelliteswiththegroundstationclocks. The exchange of the data between the Control Centres and the satellites is performedthroughup-linkstations.
Galileowillprovide severaladvantages inparticular through theexploitationof theCommercialServicethatwillenablePrecisePointPositioning,howeverbeingthedeploymentofsuchserviceforeseenafter2018,theI-REACTprojectwillpointontheexploitationGalileoandGPSaugmentedbyEGNOSandEDAS,plusGLONASS,thatevennotaugmentedcanhelpin improvingpositioningavailability(seeparagraph4.3)
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Through the I-REACTORbackend, theAM receives rawGNSSmeasurements fromtheusersandprovidestheusers’PVT,correctedwithEDASdata.Italsoprovidesaconfidenceinterval,basedonthe“localintegrity”concept.
An AMwith basic functionalities was already developed by ISMB in the frame of the FLOODISproject.TheI-REACTprojectreviewed,completedandupdatedsuchoriginalversion.Inparticularanew algorithm for the computation of the “Local Integrity” has been added to the componentdevotedtothepositioncomputation(i.e.:ComputePVTandlocalintegritymodule).
3.1 MODULEOVERVIEW
TheAMcanbeseenascomposedbytwomainparts the firstmanagesthe interfacewithEDASincludingEDASdatastorage,thesecondcontainthecorealgorithmsforthepositioncomputationandaugmentationandmanagestheinterfacewiththeI-REACTORbackend.
The positioning augmentation process starts with a usermaking a request to the RESTful webservicesexposedbytheAM.GNSSreceiversparameters(TOWandrawdatacomingfromtheGNSSreceiver)arereceivedthroughthewebservices(JSONformatisused).Aftervalidatingtherequest,theAMretrievestherequireddata(originatingfromEDAS)fromthecloudstorageandrunstheaugmentationalgorithms. It returns theaugmentedposition including integrity information.TheoverallprocessofacquiringthedataandprovidingtheserviceissketchedinFigure3-1.
• Basedonthemessagetype,identifiestheEGNOSdatacontained.Thelengthofbitsshouldbe takenvariesbasedon themessage type.Afterdecoding, theextracted information isstoredintointernaldatastructure.HerearesomeSL2messagetypes:
o Message1004.ExtendedL1&L2GPSRTKobservables.o Message1005.StationaryRTKreferencestationARP.o Message1007.Antennadescriptor.o Message1010.ExtendedL1-onlyGLONASSRTKobservables.
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o Message1013.Systemparameters.o Message1019.GPSephemerides.o Message1020.GLONASSephemerides.o Message4085whichcanbeofdifferentsubtypes:
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Figure3-2:EDASstreamdecodingandstorage
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3.3 AUGMENTEDPVTANDINTEGRITYCOMPUTATION
Theaugmentationalgorithmruns inAzurecloudasacontinuouswebjob. It isusedtocomputeuserspositionsusingleastsquarealgorithmorKalmanfilteranditisabletoaugmentpositionusingEDAScorrectiondata.Itisalsousedtodeterminetheconfidenceintervalsaroundthecomputedpositions.
it is possible to determine a slant correction to be applied on each rangemeasurement to compensate for thedelayexperiencedby the signal as itpassesthroughtheionosphere;
• application of the corrections on the pseudoranges and satellite position for each GPSsatellite;
• PVT computationusing correctedpseudoranges andother data selected from theAzureTable(ephemeris,clockparameters).ThecomputationcanbedoneusingKalmanfilterorleastsquaremethod;
• computationofsatelliteselevations:thesedataisusedtogetherwiththeCarriertoNoiseratio(C/N0)estimatedbytheGNSSreceiveranddeliveredalongtherawdata,areusedtoestimate the signal reliability used to compute PLs.This step is fundamental, because itprovidestheinputforimplementingthelocalintegrityalgorithm(seethenextstep);
• computationofthelocalintegrity.Thissteprepresentstheaddedvaluewithrespecttothestandard implementation (i.e. aviation integrity). The output are confidence levels, here
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namedHPLandVPLalongtheaviationnomenclature.Incasetheusermotionisrelevant,instead of the circle defined by the HPL, an ellipse with its axes oriented along andperpendicularlytotheuserpathcanbedefined.Theseaxesarethecrosstrackandalongtrack PLs and are computed taking into account user motion direction and satellitesgeometryandreliability;
After the project has been conceived, several smartphone producers started to delivermodelsprovidinginoutputGNSSrawdata[RD10].ThisfurtherwidenedtheexploitabilityoftheI-REACTAM.InfacttherawdataavailabilityenablestheapplicationofEGNOScorrections,henceabetterpositioningaccuracy.
In any case the wearable device still holds its role within the project from the positioningperspective: an accurate selection of the GNSS chipset and the antenna guarantees betterperformanceswithrespecttotheonesachievablewithsmartphones.
Firstly,thecomputationofpositions,exploitingtheEGNOSaugmentationdataprovidedviaEDAS,cannotbeperformeddirectlybyaGNSSreceiver(orbetter:nomassmarketdeviceonthemarkethasthisfeature).Inordertodothis,itisnecessarytohaveaccesstothereceiverrawdata.Secondly,data are also needed to provide an estimation of the position reliability, i.e. for the integritycomputation.Inthiscase,asitwillbedetailedinChapter5,somedatahavebeenexploitedduringthealgorithmdevelopmentphase,whileothersareneededtorunit.
Themaindrivertakenintoconsiderationhasbeentheprovisionofapositioninformationwhichisbetterbyfarwithrespecttotheoneprovidedbyagenericsmartphone.ApartfromthespecificI-REACT implementation, the greatest difference can be done by the antenna used by theGNSSreceiver:usuallyhand-helddevicesnotspecificallydesignedasnavigatorsareequippedwithalinearantenna(i.e.withlinearpolarization)havinganomnidirectionalpatternandasmallgroundplane,whileatypicalGNSSpatchantennaisrighthandcircularlypolarizedwithanhemisphericalpatternwhichprovideasensibleadvantage;furthermore,inthecaseofanexternalpatchantenna,abiggergroundplanecanbeeasilyadded,theonlylimitationgivenbyitsoverallsize.
AsitwillbeclearinChapter5,thealgorithmdevelopmenthasbeenbasedonthedatacollectedwithau-bloxreceiverabletodeliveralsoinformationaboutpseudorangeresiduals,whileforthedevice operation theparameters usually providedby theGNSS receivers (carrier to noise ratio,azimuthandelevation)areenough.
4.3 MULTICONSTELLATIONGNSSRECEIVERS
ThelastdecadesawthegrowthofGNSSalternativeorbettercomplementarytoGPS.TheRussianGNSS,GLONASShasbeenbroughtbacktoitsfullpotential,Galileo,theEuropeanGNSS,isinthemiddle of its deployment [RD11], but it presents new interesting features, while the Chineseconstellation,namedBeidou,itisalmostcomplete.
GNSSreceiversproducersareexploitingthepossibilitiesprovidedbythecontemporarypresenceofthese systems. Consumer grade GNSS receiver manufactures found a cost benefits balance in
1 Pseudorange residual: the difference between the expectedmeasurement and the observedmeasurement. Theexpectedmeasurementisthedistancebetweenthesatelliteandthecomputedposition.Seeparagraph5.2forfurthersdetails.
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5 LOCALINTEGRITY
ThepresentchapterdescribesthedevelopmentsperformedwithinI-REACTforthelocalintegritycomputation. The idea at the basis of the development is to the joint use EGNOS information(throughEDAS)with local informationextractedfromtheprocessingof thereceivedsignal.Thisallows for the computation of a confidence interval of the computed position. Following thegeomatics fundamentals (i.e.:dataandmeasurementsassociatedtoageo-localizedpointof theenvironment), the confidence interval introduces an estimate of the level of reliability of geo-localizedmeasurements.
The work performed in the frame of I-REACT has been based on the local integrity conceptintroducedbytheauthorsin[RD13].Initsturn,thisstudy([RD13])wastriggeredbypreviousworks([RD14],[RD15]) about the limits of applicability of the aviation-born integrity to othertransportationfieldsandtheneedofadeepreconsiderationoftheapproachtoeffectivelyexploititinnon-aviationoperations.Thisnovelcooperativeintegritymonitoringconceptwasconceivedtobe suitable to automotive applications in urban scenarios. The idea is to take advantage of acollaborative Vehicular Ad hoc NETwork (VANET) architecture in order to perform aspatial/temporal characterization of possible degradations ofGlobalNavigation Satellite System(GNSS)signals.Suchcharacterizationenables thecomputationof theso-called"LocalProtectionLevels", taking into account local impairments to the received signals. Starting from theoreticalconcepts,thispaperdescribestheexperimentalvalidationbymeansofanintensemeasurementcampaign and the real-time implementation of the algorithm on a vehicular prototype. A livedemonstration in a real scenario was carried out successfully, highlighting effectiveness andperformanceoftheproposedapproach.
5.2 PROPOSEDALGORITHM
Thiswork[RD13]hasbeenusedasastartingpointforthedevelopmentofanewalgorithmbasedon the characterization of the behaviour and performances of GNSS receivers in differentenvironmentalconditions,whilethefirsthasbeenbasedonthecharacterizationoftheoperationalenvironment. Inotherwords, theobjectiveof theoriginalalgorithmwasthedeterminationofareliability index (σ2i) for themeasurements obtained by any receiver for any specific time andlocation.Thiswaspossiblebytheexploitationofaverypopulateddatabase,composedofthedatacollectedbyseveralGNSSreceiversmountedon-boardthevehiclestravellingaround.Intheframeofanemergencyscenario,thisapproachtothelocalintegritymaybenotapplicablebecausetheoperationstheatresortheareasofinterestcanbealmosteverywhereandanextensivepreventive
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mapping (and creation of a corresponding data base) of the signals quality is unfeasible. Theapproachfollowedforthedevelopmentofthisgeneralizedversionofthelocalintegrityalgorithmforesawfourmainphases:
Thedevelopmentof thealgorithmstarted fromtheanalysisofsomedatasets taken indifferentenvironments(opensky,country,hillmostlycoveredwithvegetationandurban)withaconsumergradeGNSSreceiver(au-bloxLEO8U).Amongthemeasurementsmadeavailablebysuchareceivertheanalysiswasaddressedtothesocalledpseudorangeresiduals.
Inprincipleitispossibletocomputesuchvaluesoncethereceiverhascomputedaposition.ItisworthtorecallthatthepseudorangesaretherawmeasurementperformedbythereceiveroneachoftheincomingGNSSsignals.Thesearerangingmeasurements,buttheyarecalledpseudorangesbecauseofacommontermaffectingallthembythesameamount;thisbiasisduethemisalignmentofthereceiverclockwithrespecttotheGPStime(tiedtotheUniversalTimeCoordinated).AnyGNSSreceivercancomputeapositionusingtheknownpseudorangesfromatleastfoursatellitesandsolvinganon-linearsystemoffourequations,inordertodetermineitscoordinatesanditstimebias (unknowns). In casemoremeasurements are introduced, the achieved redundancy can beexploitedtoimprovethepositionestimation;thisisobtainedminimizingtherootmeansquareofthedifferencesbetweenthepseudoranges(oncethetimebiashasbeenremoved)andthedistancebetweentheestimatedpositionandtherelatedsatellites.Suchdifferencesarethepseudorangeresiduals which are related to the errors affecting the measurements [RD16]. Ideally, all theresidualsshouldbeequaltozero,howeverthedifferenterrorsourcesintroducesomebiasestothemeasurements,whichaffectthepositioncomputation.Therefore,itisimportanttohighlightthatthetruepseudorangeerrordoesn’tequaltheresidual:infactthefirstisthedifferencebetweenthereal satellite-receiverdistanceand themeasurement,while the second,beingestimatedby thereceiver,derivesfromthecomputedpositionwhichisaffectedbythepseudorangeerrors.Inany
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Theideaatthebasisofthealgorithmusestheinformationaboutresidualstoestimatetheerrorvariance of the user position. For each epoch, the set of residual variances is then combinedaccording to the satellitesgeometry inorder toprovideanestimateof thepositionqualityandconsequentlytheprotectionlevelsalongthedifferentdirections(alongtrackandcrosstrack).
InsteadofresortingtogenericanalyticalmodelsforaUserEquivalentRangeError(UERE)(providingσUERE,ivaluesforeachi-thsatelliteasithappensinthecaseofSBAS),withouttakingintoaccounttherealeffectsofnoiseandmultipathinaspecificconditions,itispossibletodefinean“effectiveUERE” parameter (σUERE,eff) as anensemble average of several “instantaneous” estimates of thecovarianceoftheresiduals.Asdetailedin[RD17],thevectorwcontainingthepseudorangeresidualscanbeputinrelationwiththe(non-observable)pseudorangeerrorsεthroughaprojectionmatrixS:
Startingfrom(1),confidenceintervalscanbecomputedtakingintoaccountthesatellitegeometryandthemotiondirectionoftheuserasdescribedin[RD17].TheyaretheAlongTrackProtectionLevel(ATPL)thatboundtheerrorinthedirectionofmovement,theCrossTrackProtectionLevel(CTPL)andtheVerticalProtectionLevel.Thesevaluescanbeusedtodefineanellipsecanteredatthecomputeduserpositionandcontainingthetrueuserpositionwithacertainprobability. It isorientedaccordingtothedirectionofmotion.
Atthispointitisworthtohighlighttwofactsthatdrovethealgorithmpracticalimplementation.The first is that the information about residuals is seldomprovided byGNSS receivers (at leastconsumergradeones), fromherethenecessitytoexploitother informationaboutsatellitesandsignalsmoreeasilyavailable.For this reasononeof themainactivitiesperformedtosetup thealgorithmwasaddressedtocharacterizedifferentenvironmentsusingasinputstheelevationofthesatellitesabovethehorizonandthesignalstrength,namelytheC/N0andmakingresidualensembleaveragesforasetofelevationandC/N0valuesintervals.Forexamplealltheresidualvaluescollectedforallthesatellitesinaspecificenvironmentandwithanelevationof10°areaveragedasdescribedby (11), note that satellite elevation resolutionusuallyprovidedbyGNSS receivers is 1°.During
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Figure 5-1 shows how two different data collections yielding averaged values of residuals fordifferentsatelliteelevations(a)andC/N0values(b).
Thesecondfactthatdrovethealgorithmpracticalimplementationislessmanifestbeingrelatedtothepossibilityofsystemdisruptionsthatoriginfromproblemsatsystemlevel(global):theseeventseventhoughextremelyrarecanhaveabigimpactonpositionintegrity.Indeed,SBASareconceivedtoprovidetimelywarning incaseoftheseeventsthatcan involveoneormoresatellites: this isachievedthroughtheconstantreal-timemonitoringoftheGNSSconstellations.Thedevelopmentof the algorithm was addressed to the estimation of signal degradations at local level. Thesedegradationsareindependentfromsystemlevelproblemsthat,ifpresent,arefilteredout.Infact,thecharacterizationofsignalreceptioninthedifferentenvironmentalconditionswasbasedontheaveragingof the residualsvariancesoverextendeddatacollections.For these reasons localandglobaleffectsare taken intoaccountsimultaneouslyas it isdone in theclassical integrity,usingEGNOSinformation(throughEDAS),butreplacingthevariableswhichtakesintoaccountthelocaleffects(σ2air)withthenewones(σ2local).
5.3 VALIDATIONOFTHEALGORITHM
Inordertovalidatethealgorithm,fourtrialswereperformedalongfourpathrepresentativeofthedifferent environments: urban, hillwith vegetation, country and open sky. They lasted from30minutesto1houreach.ThreeGNSSreceiverswereusedforthevalidation:
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Thevalidationhasbeenperformedusingareferencepath(groundtruth)computedwithaNovatelSPAN-CPT receiver [RD18], which integrates GNSS measurements with an inertial sensor andprovide a sub-decimetric accuracy and using Google Earth orthophotographs to have a visualinformation.
Figure5-2:A25km long track zoomed in themost criticalpoint (i.e.worstoff-track).Greenarrowpoints to thecomputedpositionwhiletheredarrowpointstotherealposition.Realpositionfallscorrectlyinsidetheellipse.
Theprocessingofthedataacquiredduringthevalidationtestdidn’tshowanycriticalpoint.Fromtheanalysisofdataitispossibletoseethatinthecaseofthetestalongthevegetatedtrack(wood)theerrorwasboundedbytheconfidencelevelbyonly41cm(seeFigure5-2),whileintheothercases themargin ismore consistent,meaning that there is still some possibility to reduce theconfidencelevelsvalues.
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6 CONCLUSIONS
TheworkcarriedoutintheframeofTask3.6oftheI-REACTprojectrepresentsanapplicationofageneralizedversionof theLocal integrityalgorithmdescribed in [RD13]. Itproved toprovideaneffectivemeasureofapositioningsystemreliabilityinmostcircumstances,whichcanbeassociatedalso toemergencyevents . The jointexploitationof the local integrity conceptwith theEGNOSintegrityallowstoprovideaguaranteealsoinparticularsituationsinvolvingGNSSsignalproblemsthatarenotconnectedwiththelocalenvironmentlikeseveresignaldegradationduetoionosphericpropagationorproblemsatsystemlevel.CurrentlysystemsotherthanGPSarenotmonitoredbyEGNOS,buttheycanbeeasilyincludedinthealgorithmtoincreaseitsreliabilityonceEGNOSwillincludeotherGNSS.
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