GAMIT/GLOBKforGNSST.A.HerringM.A.FloydR.W.KingMassachusetts InstituteofTechnology, Cambridge,MA,USA

GPSDataProcessing andAnalysiswithGAMIT/GLOBK/TRACKUNAVCOHeadquarters, Boulder, Colorado, USA

19–23June2017

http://web.mit.edu/mfloyd/www/courses/gg/201706_UNAVCO/MaterialfromR.W.King,T.A.Herring,M.A.Floyd(MIT)andS.C.McClusky (nowatANU)

Thepromise…

• WhenGPS,Glonass,Beidou,andGalileoaredeployedandmodernized,therewillbe>100satellitesand12distinct frequenciesavailablefortracking• Obviousadvantagesforkinematicpositioningandatmosphericstudies,butalsoameanstoseparateperiodicsignalsduetoaliasing intheGPSorbits• 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)2017/06/19 GAMIT/GLOBKforGNSS 16

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(2017-121 to2017-125)

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

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Summary

• GAMIT/GLOBKisnow(asof10.61)capableofprocessingalmostallGNSSdata,except• GLONASS,whichhasvariablefrequencies and,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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