Reference frames - floyd/courses/gg/201605_KIGAM/pdf/23-Ref_fra… · Reference frames M. A. Floyd...

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)

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).

2016/05/24 Referenceframes 2

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.


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


Examples•  ExpressingvelociEesin

ITRFisnotverymeaningfulorusefulwhenwewanttolookatthedeformaEonataplateboundary,e.g.theSanAndreasFaultsystem

•  BelertolookatvelociEeswithoneside“fixed”sowecanseewhattheothersideisdoingrelaEvetoit

PANA

PANA


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


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

αδ

i

o

o’

j

k

• Allreferencecoordinatesfreetoadjust(anomaliesmoreapparent);outliersareiteraEvelyremovedbyglorg• Networkcantranslateandrotatebutnotdistort• Worksbestwithstrongredundancy(numberand[ifrotaEon]geometryofcoordinatesexceedsnumberofparametersiloadingeffects


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.


StabilizaEonusingRegionalorLocalSites

•  IfyourareaofstudyhasarobustcGPSnetwork(10ormorewell-distributedsites)withaccurateapriorivelociEes,thenglorgstablizaEonisrobustandlillethoughtisinvolved(glorgwillautomaEcallydiscardtheoneortwositeswhichmaybeweakorinconsistent)

•  IfyourregionisshortoncGPSstaEonswithwell-knowncoordinates,youwillneedtothinkcarefullyaboutthechoiceofsitestoincludeinyoursoluEonandusetheiniEalstabilizaEon.AstabilizaEonsiteshouldhave–  highqualitydataoverthefullspanofyourstudy–  coordinateswell-knowninITRF2008–  Providesymmetriccoveragearoundyourstudyarea(exceptthatiftheregion

issmallenough,atranslaEon-onlystabilizaEonmaybepossibleanddistribuEonislessimportant)


IGS(IGb08)referenceframecorenetwork

hlp://igscb.jpl.nasa.gov/network/refframe_core.html


IGS(IGb08)referenceframenetwork

hlp://igscb.jpl.nasa.gov/network/refframe.html


FramerealizaEonsitesforPBO•  PBOusedanhierarchicallistbasedon500kmsitespacing

(sh_gen_statstabradopEon).(Bluedots)


160˚

170˚ 180˚

190˚ 200˚ 210˚ 220˚ 230˚ 240˚ 250˚ 260˚ 2

70˚ 280˚

290˚ 3

00˚

310˚

320˚

330˚

340˚

10˚ 20˚

30˚ 40˚

50˚ 60˚

70˚ 80˚

568 Ref, 1248 Non−Ref sites com18493.b.org

IssueswithEsEmaEngScale•  ManyGPSanalyses,automaEcallyincludescalewhen

aligningreferenceframes.SincetheEarthisalmostspherical,scalechangesaredirectlyproporEonaltotheaverageheightdifferencebetweenthereferencesitecoordinatesandtheirapriorivalues.

•  Whencomparingandanalyzingheightchanges,howscaleistreateddirectlyeffectstheresults.AspectsofthisissuearediscussedinawhitepaperthatshouldbeavailableshortlyfromtheGAGEanalysisdocumentaEonatUNAVCO.

•  ScaleesEmatesarerelatedthemeanheightdifferencesoverthereferencesites.ShouldthisberemovedornotisanopenquesEon.

•  ScaleesEmatesonnextslideshowmagnitudeofeffectforthePBOnetwork.


PBOnetworkscaleesEmates


2010 2010.2 2010.4 2010.6 2010.8 2011 2011.2 2011.4 2011.6 2011.8 2012−15

−10

−5

0

5

10

15

Year

Scal

e dH

(mm

)

Scale PBOMean dH PBOGlobal ReproNorthern

PBOnetworkesEmatescomparedtoglobalesEmatesconvertedtoaverageheightdifferences(6317kmEarth).Spikesinwinterareo]enduetoAlaskansites.

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


VelociEesandTimeSeries

•  ThecriteriaforstabilizaEonaredifferentforvelocitysoluEonsandEmeseries•  VelocitysoluEons:

–  Physicalreferenceisimportant–  NotsosensiEvetostaEondropout(soluEonholdstheframetogether)

•  Timeseries:–  Physicalreferenceisnotimportant–  SensiEvetostaEondropout–  BestrepresentaEonofthestaEsEcsofthevelocitysoluEonisstabilizaEon

usingALLthewell-determinedsitesfromthevelocitysoluEon,nowinacommonframe


Referencingtoahorizontalblock(‘plate’)

Appliedinglorg:firststabilizeintheusualwaywithrespecttoareferencesetofcoordinatesandvelociEes(e.g.ITRF-NNR),thendefineoneormore‘rigid’blocks

apr_fileitrf08.aprpos_orgxtranytranztranxrotyrotzrotstab_sitealgopie1nlibdraogoldsni1mkeachatcnd_hgtv10100.83.platenoamalgopie1nlibplatepcfcsni1mkeachatA]erstabilizaEon,glorgwillesEmatearotaEonvector(‘Eulerpole’)foreachplatewithrespecttotheframeofthefullstabilizaEonsetandprinttherelaEvepolesbetweeneachsetofplatesUsesh_org2veltoextractthevelociEesofallsiteswithrespecttoeachplate


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).


Summary

•  Atmosphericdelaytreatmentandissues–  VMF1approach(usedgriddedassimilaEonmeteorologicaldata)isprobablybestbutdoesrequiredlargeandregularlyupdated(ifnearreal-Eme)gridfiles

•  Loading–  NotstandardyetinIGSprocessing.Againneedslargegridfiles.ModelsusedinGAMITcanberemovedinGLOBK.

•  ReferenceframerealizaEon:VerEcaleffects–  TreatmentofscaleesEmatesandimpactonheightesEmatesshouldbecarefullyconsideredwhenheightsareoninterest.


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