reference frames - floyd/courses/gg/201605_kigam/pdf/23-ref_fra… · reference frames m. a. floyd...
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Referenceframes
M.A.FloydT.A.HerringMassachuse(sIns,tuteofTechnology
GAMIT/GLOBK/TRACKShortCourseforGPSDataAnalysisKoreaInsEtuteofGeoscienceandMineralResources(KIGAM)
Daejeon,RepublicofKorea23–27May2016
MaterialfromT.A.Herring,R.W.King,M.A.Floyd(MIT)andS.C.McClusky(nowANU)
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BasicissuesinreferenceframerealizaEon
• ConceptistoaligntheesEmatedsiteposiEonsandpossiblyvelocitytoasetofwelldefinedlocaEonsthathavephysicalsignificancefortheanalysisbeingperformed(e.g.,PBOwealigntoarealizaEonoftheNorthAmericaplatebasedonITRF2008.
• GLORGisthemodulewhichdoesthisandcomputesthecovariancematrixofthealignedsoluEoninthereferenceframechosen.
• TransformaEoniso]encalledanN-parameterHelmerttransformaEon:– N=3translaEononly(couldalsobejustrotaEon)– N=6translaEonandrotaEon– N=7translaEon,rotaEonandscale
• InGLOBKanalyses,youneedtodecide– Howmanyparameters(3/6/7)– Sitestousetodeterminetheparameters(sh_gen_stats)– ValuesoftheposiEons/velociEesofthereferenceframesites– WeighttobegiventoheightsincompuEngthetransformaEonparameters
(CND_HGTVcommand;firsttwoargumentsforposiEonandvelocity).
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Choicesofreferenceframe• ChooseyourreferenceframebasedonyourgeophysicalobjecEves
– VelociEesinITRFaredifficulttointerpretvisuallyfromageophysicalperspecEve• Localsurroundingsofavolcano• Onesideofafault• UpperplateofasubducEonzone
• Majorplatereferenceframe– Majorplatesareo]enchosentoconformwithconvenEonalperspecEvesof
velocitysoluEons– RelaEvetoEurasia,Nubia,NorthAmerica,SouthAmerica,etc.– Butdon’tfeelrestrictedbythis.SomeEmesyourgeophysicaldiscussionisbest
visualizedrelaEvetoanystableboundaryofadeformingregion• Regionalreferenceframe
– CentralValleyofCalifornia,non-deformingpartofAnatolia,smallercoherentregions,etc
• Localreferenceframe– Sitesnearbutoutsidetheinfluenceofavolcano,geothermalfield,etc.
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Waystodefineareferenceframe
• Createanapr-fileforusebyglorg1. ApplyknownrotaEonratetoapr-file(e.g.
itrf08_comb.apr→itrf08_comb_eura.apr)2. Zerovelocityapr-filerecords(anditerateusing
sh_exglktocreateupdatedapr-file)• Definesetofsites(mustbeincludedinGAMITprocessingorotherH-fileinputtoGLOBK)whichdefinestableregion3. plateinglobkcommandfile
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Examples• ExpressingvelociEesin
ITRFisnotverymeaningfulorusefulwhenwewanttolookatthedeformaEonataplateboundary,e.g.theSanAndreasFaultsystem
• BelertolookatvelociEeswithoneside“fixed”sowecanseewhattheothersideisdoingrelaEvetoit
PANA
PANA
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ReferenceframesinGeodeEcAnalyses
• OutputfromGAMIT– LooselyconstrainedsoluEons– RelaEveposiEonwelldetermined,“AbsoluteposiEon”weaklydefined– Needaproceduretoexpressedcoordinatesinawelldefined
referenceframe• Twoaspects
– TheoreEcal(e.g.,rigidblock,mantle-fixed,no-net-rotaEonofplates)– RealizaEonthroughasetofcoordinatesandvelociEes
• “finiteconstraints”:apriorisigmasonsitecoordinates• “generalizedconstraints”:minimizecoordinateresidualswhile
adjusEngtranslaEon,rotaEon,andscaleparameters• ThreeconsideraEonsindataprocessingandanalysis
– ConsistentwithGPSorbitsandEOP(NNR)• notanissueifnetworksmalloriforbitsandEOPesEmated
– PhysicallymeaningfulframeinwhichtovisualizesitemoEons– RobustrealizaEonforvelociEesand/orEmeseries
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FramedefiniEonwithgeneralizedconstraints
• Appliedinglorg:minimizeresidualsofreferencesiteswhileesEmaEngtranslaEon,rotaEon,and/orscale(3-7parameters)apr_fileitrf08.aprpos_orgxtranytranztranxrotyrotzrotstab_sitealgopie1drao…cnd_hgtv10100.83.stab_it40.52.5 j’
ij
k
k’θ
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• Allreferencecoordinatesfreetoadjust(anomaliesmoreapparent);outliersareiteraEvelyremovedbyglorg• Networkcantranslateandrotatebutnotdistort• Worksbestwithstrongredundancy(numberand[ifrotaEon]geometryofcoordinatesexceedsnumberofparametersiloadingeffects
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StabilizaEonusingaGlobalSetofSites
• Use40ormoresiteswithgoodvelociEesdeterminedintheITRF2008frame• Theitrf08_comb.aprfile,whenusedtogetherwithitrf08_comb.eqtoaccount
consistentlyforinstrumentalchangesoverEme,providesthewidestchoiceofsites,1992-2013.
• CombiningyoursoluEonwiththeMITorSOPACglobalh-filesofferaccesstoover100siteswithouthavingtoincludetheminyourGAMITprocessing.– Youneedjust4-6commonsites,whichshouldbeofhighqualitybutneednot
bewellknowinITRF2008sincethese“Ee”sitesdonotneedtobeinyourframe-realizaEonlist.
• ForglobalITRFstabilizaEon,youcanusethehierarchicallistigb08_heirarchy.stab_siteingg/tables
• AlthoughaglobalframemaybeaconvenientwaytodothestabilizaEon,itisusuallynotnecessaryforregionalstudies.
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StabilizaEonusingRegionalorLocalSites
• IfyourareaofstudyhasarobustcGPSnetwork(10ormorewell-distributedsites)withaccurateapriorivelociEes,thenglorgstablizaEonisrobustandlillethoughtisinvolved(glorgwillautomaEcallydiscardtheoneortwositeswhichmaybeweakorinconsistent)
• IfyourregionisshortoncGPSstaEonswithwell-knowncoordinates,youwillneedtothinkcarefullyaboutthechoiceofsitestoincludeinyoursoluEonandusetheiniEalstabilizaEon.AstabilizaEonsiteshouldhave– highqualitydataoverthefullspanofyourstudy– coordinateswell-knowninITRF2008– Providesymmetriccoveragearoundyourstudyarea(exceptthatiftheregion
issmallenough,atranslaEon-onlystabilizaEonmaybepossibleanddistribuEonislessimportant)
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IGS(IGb08)referenceframecorenetwork
hlp://igscb.jpl.nasa.gov/network/refframe_core.html
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IGS(IGb08)referenceframenetwork
hlp://igscb.jpl.nasa.gov/network/refframe.html
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FramerealizaEonsitesforPBO• PBOusedanhierarchicallistbasedon500kmsitespacing
(sh_gen_statstabradopEon).(Bluedots)
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160˚
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568 Ref, 1248 Non−Ref sites com18493.b.org
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IssueswithEsEmaEngScale• ManyGPSanalyses,automaEcallyincludescalewhen
aligningreferenceframes.SincetheEarthisalmostspherical,scalechangesaredirectlyproporEonaltotheaverageheightdifferencebetweenthereferencesitecoordinatesandtheirapriorivalues.
• Whencomparingandanalyzingheightchanges,howscaleistreateddirectlyeffectstheresults.AspectsofthisissuearediscussedinawhitepaperthatshouldbeavailableshortlyfromtheGAGEanalysisdocumentaEonatUNAVCO.
• ScaleesEmatesarerelatedthemeanheightdifferencesoverthereferencesites.ShouldthisberemovedornotisanopenquesEon.
• ScaleesEmatesonnextslideshowmagnitudeofeffectforthePBOnetwork.
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PBOnetworkscaleesEmates
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2010 2010.2 2010.4 2010.6 2010.8 2011 2011.2 2011.4 2011.6 2011.8 2012−15
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Scal
e dH
(mm
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Scale PBOMean dH PBOGlobal ReproNorthern
PBOnetworkesEmatescomparedtoglobalesEmatesconvertedtoaverageheightdifferences(6317kmEarth).Spikesinwinterareo]enduetoAlaskansites.
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UseofGlobalbinaryH-files
• Includeglobalh-files…ornot?Forpost-2000datanotneededfororbits• Advantages– AccesstoalargenumberofsitesforframedefiniEon– Can(should)allowadjustmenttoorbitsandEOP– EasescomputaEonalburden
• Disadvantages– Mustuse(mostly)thesamemodelsastheglobalprocessing– OrbitsimpliedbytheglobaldataworsethanIGSF.Once-per-revoluEonradiaEonmodelparameters(looseinglobalh-files)shouldbetreatedcarefully.
– Somebaddatamaybeincludedinglobalh-files(canremove)– Greaterdatastorageburden
• MIThfilesavailableat]p://everest.mit.edu/pub/MIT_GLL/HYYWhenusingMITfiles,addapr_svantallFFFtoglobkcommandfiletofixthesatelliteantennaoffsets
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VelociEesandTimeSeries
• ThecriteriaforstabilizaEonaredifferentforvelocitysoluEonsandEmeseries• VelocitysoluEons:
– Physicalreferenceisimportant– NotsosensiEvetostaEondropout(soluEonholdstheframetogether)
• Timeseries:– Physicalreferenceisnotimportant– SensiEvetostaEondropout– BestrepresentaEonofthestaEsEcsofthevelocitysoluEonisstabilizaEon
usingALLthewell-determinedsitesfromthevelocitysoluEon,nowinacommonframe
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Referencingtoahorizontalblock(‘plate’)
Appliedinglorg:firststabilizeintheusualwaywithrespecttoareferencesetofcoordinatesandvelociEes(e.g.ITRF-NNR),thendefineoneormore‘rigid’blocks
apr_fileitrf08.aprpos_orgxtranytranztranxrotyrotzrotstab_sitealgopie1nlibdraogoldsni1mkeachatcnd_hgtv10100.83.platenoamalgopie1nlibplatepcfcsni1mkeachatA]erstabilizaEon,glorgwillesEmatearotaEonvector(‘Eulerpole’)foreachplatewithrespecttotheframeofthefullstabilizaEonsetandprinttherelaEvepolesbetweeneachsetofplatesUsesh_org2veltoextractthevelociEesofallsiteswithrespecttoeachplate
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RulesforStabilizaEonofTimeSeries• Small-extentnetwork:translaEon-onlyinglorg,mustconstrainEOPin
globk• Large-extentnetwork:translaEon+rotaEon,mustkeepEOPloosein
globk;• ifscaleesEmatedinglorg,itmustesEmatescaleinglobk• 1stpassforediEng:
– “Adequate”stab_sitelistofstaEonswithaccurateaprioricoordinatesandvelociEesandavailablemostdays
– Keepinminddeficienciesinthelist• FinalpassforpresentaEon/assessment/staEsEcs
– Robuststab_sitelistofall/moststaEonsinnetwork,withcoordinatesandvelociEesdeterminedfromthefinalvelocitysoluEon
• Systemiso]eniterated(velocityfieldsoluEon,generateEmeseries,ediEngandstaEsEcsofEmeseries;re-generatevelocityfield).
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GLOBKReporEngofmodels• Whenbinaryh-filesfromGAMITh-filesareused(asopposedtoSINEXfiles),GLOBKandGLISTcanreportmodelsused.CanbecriEcalwhencombiningwithMITh-files.
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GPS System Information : Time GPST Frame J2000 Precession IAU76 Radiation model BERNE Nutation IAU00 Gravity EGR08 EarthRad NCLE1 AntThrust ANTBK
MODELS Used in Analysis: SD-WOB | SD-UT1 | IERS10 | E-Tide | K1-Tide | PoleTide| OC-Load | MPT2010 | OceanPT | LOAD Models Used : NCEP CM USED |
ATM Delay Models Used : UFL | GP25 | VMF1 | VMF1 | 2nd Order Ion | GMAP | IGRF11 | Theselinesarewrapped.
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Summary
• Atmosphericdelaytreatmentandissues– VMF1approach(usedgriddedassimilaEonmeteorologicaldata)isprobablybestbutdoesrequiredlargeandregularlyupdated(ifnearreal-Eme)gridfiles
• Loading– NotstandardyetinIGSprocessing.Againneedslargegridfiles.ModelsusedinGAMITcanberemovedinGLOBK.
• ReferenceframerealizaEon:VerEcaleffects– TreatmentofscaleesEmatesandimpactonheightesEmatesshouldbecarefullyconsideredwhenheightsareoninterest.
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