GAMIT/GLOBKforGNSSM.A.Floyd

MassachusettsInstituteofTechnology,Cambridge,MA,USA

GPSDataProcessingandAnalysiswithGAMIT/GLOBKEarthObservatoryofSingapore

17–21July2017

http://geoweb.mit.edu/~floyd/courses/gg/201707_EOS/MaterialfromR.W.King,T.A.Herring,M.A.Floyd(MIT)andS.C.McClusky (nowatANU)

Thepromise…

• WhenGPS,Glonass,Beidou,andGalileoaredeployedandmodernized,therewillbe>100satellitesand12distinctfrequenciesavailablefortracking• Obviousadvantagesforkinematicpositioningandatmosphericstudies,butalsoameanstoseparateperiodicsignalsduetoaliasingintheGPSorbits• Fulldeploymentexpectedby2020,butconstellations>20SVsbymid-2018shouldprovidehighlyusefulresults

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GNSSavailableinGAMIT/GLOBK

• AsofGAMIT/GLOBKrelease10.61(2017-04-01),thefollowingGlobalNavigationSatelliteSystems(GNSSs)maybeprocessed✔ GPS(USA),BeiDou (China),Galileo(Europe),IRNSS(India)✘ GLONASS(Russia),QZSS(Japan)

• GNSScodeisintrialandweneedtesters• Bug-fixedandalpha-testedcodeisintheupdates/sourcedirectoryasgnss_170616.tar.gz,tobeuntarredatthegg levelafter thecurrentincremental_updates.

GNSSdatamustbeprocessedseparately foreachsysteminGAMIT,i.e.onecannotprocessGPSdataandGalileodatasimultaneously

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Whyprocessseparately?

• Dual-frequencyobservationsarefundamentalforGNSStoremovetheionosphereandareeasilyimplementedunderthecurrentstructureofGAMIT,butprocessingdifferentsystemsacrossmorethantwofrequenciessimultaneouslyrequiresadifferentalgorithmicapproachandwilltakesometimetoimplement

• Solution(h-)filesfrommultiplesystems(aswithmultiplesub-nets)canberigorouslycombinedinGLOBKtoestimatesitecoordinatesandvelocitiesforstaticobservations

• Basedonresearchthusfar,itisnotclearthatjointprocessingwillImproveresultsforthelong-sessionsusedformm-levelmeasurements,thoughthatmaychangeasthesystemsmature(improvedorbitsandknowledgeofinter-systemsignalbiases)

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ProcessingGNSSinGAMIT/GLOBK

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Versionawarenessandwarnings

• AmajorchangebetweenGAMIT/GLOBK10.5and10.6wastheformatofmanytables(e.g.dcb.dat,svnav.dat,etc.)toaccommodatecodechangesforGNSS• GAMIT/GLOBK10.61nowbuildsuponthenewfilestructurestodeliverthedataprocessingcapability• Giventhesemajorchanges,manytablesusedinGAMIT/GLOBK10.6and10.61arenot backwardscompatiblewithGAMIT/GLOBK10.5andpriorreleases• Youcannot usemanytablesthatcamewithGAMIT/GLOBK10.5andpriortoprocess(GPS-onlyorGNSS)datausingGAMIT/GLOBK10.6andlater

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Suggestionsforprocessingstrategies

• IfyouwishtocombinedatafromdifferentGNSS,processeachsysteminaseparateexperimentdirectory,e.g./2017gand/2017eforGPSandGalileo• DownloadtheRINEXfilesinadvancetocheckforavailabilityofGNSSsignals• Forsh_gamit usethe“-gnss”optiontospecifytheGNSS;andCOM1 for“-orbit”(IGSF okforGPS)• Checktheorbit-fitrms filesinthe/igs directorytoassesstheorbitquality• Combinetheresultingh-filesinGLOBKtoproduceasingleresult(timeseriesorvelocities)

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RINEXfiles• RINEX2,whichisstillbyfarthemostcommonformatofRINEXfile,wasdesignedinanerawhenonlyGPSwasviableforobservation

• SincetheredesignofGPStobroadcastasecondcodeonL2(“L2C”)specificallyforcivilianuse,therestorationofGLONASSandtheintroductionofothernavigationsatellitesystems,RINEX2nolongersufficestotrackallavailableobservations

• Beverycarefulwithhowyoutranslateanduseotherpeople’sRINEX2fileswithL2C(seeBerglundetal.,2010;Blumeetal.,2012;andhttp://kb.unavco.org/kb/article/the-effects-of-l2c-signal-tracking-on-high-precision-carrier-phase-gps-postioning-689.html)

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GAMIT

• Severalscriptsnowhaveanadditionaloption(“-gnss”)thatsetsthetypeofGNSS• Mostlikelytousedirectly:sh_gamit,sh_get_orbits,sh_sp3fit

• Lesslikelytousedirectly: sh_preproc,sh_bcfit,sh_rxscan,sh_get_times,sh_makexp

• Validargumentsare(onlyoneof)• G (GPS)• R (GLONASS;notyetcodedoravailabletouse)• C (BeiDou-2/COMPASS)• E (Galileo)• J (QZSS;notyetcodedoravailabletouse)• I (IRNSS)

• Thedefaultisstill“G”(GPS)

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GLOBK

• GLOBKshouldworkinthenormalmannerexceptthatyoucannotincludeorbitsintheh-file(useBASELINE inGAMITsestbl.)• SeeGLOBKlectures,includingthoseoncreatingtimeseriesusingglred andcombinationorvelocitysolutionsusingglobk

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Resultsofinitialtests

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Systemsprocessed:GPS(L1andL2)Galileo(E1andE5)BeiDou (C2andC7)

5 daysprocessed(2017-121to2017-125)

SkytracksDay121atSpanishsiteVILLEachcirclecoversa4-hrwindow

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GPS

Galileo

Beidou

PhaseRMS(for2017-121)

Fromsh_gamit_121[gec].summary(mm):

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Site GPS Galileo BeiDou

ASCG 7.8 9.1 4.8

BOR1 5.6 5.9 6.7

BRST 8.5 9.7 7.0

BRUX 4.4 5.0 4.7

DJIG 5.9 8.0 17.1

GANP 5.9 6.6 8.4

KIRU 6.8 8.0 10.7

KOUR 10.6 12.6 5.5

Site GPS Galileo BeiDou

LAMP 4.5 6.2 6.2

MAL2 7.7 10.0 12.7

MAS1 7.1 8.1 3.1

NICO 6.1 9.1 22.1

REYK 8.4 11.1 8.3

STJ3 5.3 6.0 2.3

VILL 8.1 9.6 5.1

ALL 7.1 8.6 11.3

Ambiguityresolution

Bestfromsh_gamit_[gec].summary:

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System Widelane(WL) Narrow lane(NL)

GPS 98% 91%

Galileo 99% 78%

BeiDou 80% 50%

Timeseriesstabilization

From“POSSTAT”linesin.org-file(s)(mm:

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System Sites E N U

GPS 10 1-2 1–2 6–10

Galileo 9-10 2-3 2–4 11-16

BeiDou 6–9 1–7 3–9 6–18

Exampletimeseries

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GPS Galileo BeiDou

Positiondifferences(GPSversusGalileo)

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Initialimpressions

• Galileohasrobustphasetrackingandcould,forsmallregionalnetworkswithgoodsatellitecoverage,nowproducenearlyGPS-qualityresults• TheBeidou constelletion willneedtobefilledoutsignificantly,expectedbymid-2018• BeiDou alsoappearstobepronetosinglecycleslips,resultinginpoordetectionandcleaningoftrackedphase• Thismaybeimprovedbytuningautcln.cmd

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Summary• GAMIT/GLOBKisnow(asof10.61)capableofprocessingalmostallGNSSdata,except• GLONASS,whichhasvariablefrequenciesand,assuch,requiresaredesignof

thesoftware’sstructureandflow• QZSS,whichisasmall,regional,high-altitudesystemoflittleusebeyondthe

narrowdesignregionoronaglobalscale(sameistrueofIRNSS,althoughthisiscoded)

• GNSSdataareavailablebutfewusersareactuallycollectingorprocessingsuchdata

• Asaresult,globalorbitsarepoorlyconstrainedbygroundstationswithaccuratecoordinatesintheterrestrialreferenceframe• SatelliteorbitalmodelsandantennadesignsarelesswellknownthanGPS• ManyGNSSsotherthanGPSareinasimilar“weak”statetowhereGPSwasin

theearly1990sbeforetheadventoftheIGS• ItisdifficulttopredictatwhattimetheothersystemswillenhanceratherthandegradeGPSresultsbutweshouldseerapidimprovementwiththelaunchofmoresatellitesandtheexpansionofthetrackingnetworkinthenext18months

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ReferencesBerglund,H.,F.Blume,L.H.Estey,andA.A.Borsa (2010),The

EffectsofL2CSignalTrackingonHigh-PrecisionCarrierPhaseGPSPositioning,AbstractG11B-0640 presentedat2010FallMeeting,AGU,SanFrancisco,Calif.,13-17Dec.

Blume,F.,H.Berglund,andL.Estey (2012),TheEffectsofL2CSignalTrackingonHigh-PrecisionCarrierPhaseGPSPositioning:ImplicationsfortheNextGenerationofGNSSSystems,AbstractG52B-07presentedat2012FallMeeting,AGU,SanFrancisco,Calif.,3-7Dec.[http://acc.igs.org/trf/agu12_blume_l2c.pdf]

http://kb.unavco.org/kb/article/the-effects-of-l2c-signal-tracking-on-high-precision-carrier-phase-gps-postioning-689.html

Montenbruck,O.,R.Schmid,F.Mercier,P.Steigenberger,C.Noll,R.Fatkulin,S.Kogure,andA.S.Ganeshan (2015),Adv.SpaceRes.,56,1015–1029,doi:10.1016/j.asr.2015.06.019.

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