外文文献Word文档格式.docx
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外文文献Word文档格式.docx
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Ottawa,Ontario,CanadaK1AEO9
E-mail:
kouba@geod.emr.ca;
popelar@geod.emr.ca
ABSTRACT
TheNAD83andWGS84referencecoordinateframeswereestablishedmorethanadecadeagotosatisfymostmapping,charting,positioningandnavigationapplications.Theyareconsistentatthe1-2metrelevelonacontinentalandglobalscalesrespectively,reflectingthelimitationsofavailabledataandtechniques.Withrapidimprovementsinpositioningaccuracy,mainlyduetoGPS,submetrenavigationhasbecomepracticalandreferenceframesatthecmtommlevelarerequiredbythemostdemandingusers.TheIERSTerrestrialReferenceFrame(ITRF)wasestablishedin1988bytheInternationalEarthRotationService(IERS)tofacilitateprecisemonitoringoftheEarthOrientationParameters(EOP)basedonstate-of-the-arttechniquessuchasVeryLongBaselineInterferometry(VLBI)andSatelliteLaserRanging(SLR).WiththeestablishmentoftheInternationalGPSServiceforGeodynamics(IGS)in1994,theITRFisdirectlyaccessibletousersworld-widebymeansofpreciseglobalGPSsatelliteorbit/clocksolutionsandalargenumberofIGSmonitoringstations.ThemostrecentITRFsolutions,designatedITRF92andITRF93,arebasedonspacegeodeticobservationsincludingGPSuptotheendof1993providingglobalconsistencyatthecmlevel.TheCanadianActiveControlSystem(CACS)facilitatesaccesstoITRFthroughactiveparticipationinIGSandVLBI.FiducialVLBIpointsincludedinNAD83provideadirectlinktoITRFandmakeitpossibletoupgradeNAD83coordinatesinordertosatisfypositioningandnavigationrequirementswithcmprecisioninthefuture.CACSfacilitatesthemostefficientconnectionstotheITRFandNAD83referenceframesforhighprecisionpositioningbyGPSaswellasforgeneralspatialreferencingneedsinCanada.
1.INTRODUCTION
Ingeodesyareferencecoordinateframeimpliesascale,orientationandcoordinateoriginaspartofareferencesystemwhichalsoincludesEarthplanetarymodelsandconstantsnecessaryforsatelliteorbitdetermination,ge-odynamicandgeophysicaldataanalysis.Satellitenavigationsystemsmadeitpossibletoestablishatrulyglobalgeocentricreferencesystemwhichwasquicklyadaptedforprecisegeodeticpositioning,especiallyoverlongdistances.Forthefirsttimeitwaspossibletodeterminedistortionsandmisorientationofclassicalgeodeticnetworksaroundtheworld.TheU.S.NavyNavigationSatelliteSystem(NNSS),alsocalledTransitorsimplyDoppler(KershnerandNewton,1962)becamethebasisfortheU.S.DepartmentofDefenseWorldGeodeticSystem1972(WGS72)andlaterWGS84whichdefineglobalgeocentricreferenceframesconsistentataboutthe1-2metrelevel.ToupgradeandcorrectdistortionsoftheclassicalNorthAmericanDatum1927(NAD27),areadjustmentofthegeodeticnetworksinCanada,USA,MexicoandGreenlandwasjointlyundertaken.Thisnewdatum,designatedNAD83,wasnominallymadecompatiblewithWGS84bybeinggeocentricandorientedaccordingtotransformedDopplerpositions,butinadditiontheNAD83adjustmentincludedVLBI(VeryLongBaselineInterferometry)baselines.Thusboth,WGS84andNAD83,areconsistentataboutonemetre,mainlyduetothelimitationsoftheDopplertechniques(Kouba,1993).GPSandotherspacebasedtechniquessuchasVLBIandSatelliteLaserRanging(SLR)providedatawithhigherprecisionstosupportstudiesofcrustaldynamicsandpolarmotionwhichrequireamoreaccurateglobalreferenceframe.TheIERSTerrestrialReferenceFrame(ITRF)wasestablishedin1988andisupdatedonanannualbasisbytheInternationalEarthRotationService(IERS)tokeepitcurrentandtoimproveknowledgeofstationvelocitieswhicharenecessaryformaintainingtheaccuracyofthisglobalreferenceframe.NAD83canberelatedtoITRFpreciselyforagivenepochbyatransformationbasedoncommonVLBIstations.TheCanadianActiveControlSystem(CACS)providesthemostefficientmethodtoupgradeNAD83coordinatesinCanadainordertomeetpositioningandnavigationrequirementswithcmprecisioninthefuture.
2.NORTHAMERICANGEODETICDATUM:
NAD83
TheNorthAmericanDatum1927(NAD27)wasestablishedatthebeginningofthiscenturyusingcontinentaltriangulationwithacentrallylocateddatumpointatMeadesRanchinKansas,USA(Ross,1936).Satellitegeodesyinthe60'
sand70'
sdetectedtheapproximately100moffsetoftheNAD27originwithrespecttothegeocenteraswellasdistortionsexceedingtensofmetersinsomepartsofthegeodeticcontrolnetwork(Mueller,1974).Anewreferenceframewasrequiredtofacilitateuseofefficientandprecisesatellitegeodetictechniquesinsurveyingandnavigation.SatelliteDopplerpositionsandseveralVLBIbaselineswhichhadbeenestablishedbeforetheendof1986,wereusedtoprovideaframeworkandtodefinethegeodeticdatuminanewway.TheNorthAmericanDatum1983(NAD83)wasbasedonDopplerstationcoordinatestransformedtoconformwiththeinternationalconventionforgeocentricorigin,scaleandorientationofthereferenceellipsoid(NOAA,1989).Classicalgeodeticobservationsformorethan260,000controlpointshavebeenreadjustedandintegratedwithintheframeworktoprovidetheNAD83coordinatesofthehorizontalcontrolnetworkmonumentsforpracticaluse.Thus,NAD83initsoriginalversionprovidesareferenceframeforhorizontalpositioningwithaccuraciesattheonemeterlevelcorrespondingtosatelliteDopplerprecisionsomewhatdilutedbyerrorsintheclassicaltriangulationarcsincludedintheNAD83networkadjustment.AtthislevelofprecisiontherewasnoneedtointroducestationvelocitiesandNAD83isconsideredtobeattachedtotheNorthAmericantectonicplate.TheNAD83referenceframesatisfiesmostpracticalneedsformapping,charting,navigationandspatialreferencinginNorthAmericawheresub-meteraccuracyisnotrequired.However,todaytheincreasedprecisionofgeodeticGPSmeasurementsrequiresareferenceframeconsistencyatacmlevelwhichwouldfacilitatestudiesofcrustaldynamicsrelatedtoplatetectonicsandnaturalhazardsassociatedwithseismicorvolcanicactivities,etc.TheaccuracyoftheVLBIbaselineswhichcontributedtothedefinitionofNAD83notonlyprovidesaneffectivewaytorelateNAD83tomoreaccuratereferenceframesata2cmlevel(Soleretal.,1992)butalsofacilitatesprecisionupgradesusingaccurategeodeticspacetechniques.SuchanapproachwillassurecontinuousimprovementsofpositioningaccuracyaswellastraceabilitytoNAD83whichisofgreatpracticalimportance.
3.WORLDGEODETICSYSTEM:
WGS84
WGS84isaglobalgeodeticreferencesystemwhichhasbeenestablishedandmaintainedbytheU.S.DepartmentofDefensetofacilitatepositioningandnavigationworldwide(DMA,1991).TheterrestrialcoordinatereferenceframecorrespondingtoWGS84hasbeenupdatedtokeeppacewithincreasingprecisionofGPSpositioningandnavigationtechnologyingeneraluse.
3.1ORIGINALWGS84TERRESTRIALEFERENCEFRAME
WGS84worldwideterrestrialreferenceframewasinitiallybasedonlyonsatelliteDopplercoordinatestransformedinthesamewayasforNAD83.However,adifferentsetofDopplerstationswasusedandnoVLBIbaselinemeasurementswereincludedinthenetworkadjustment.Thisapproachproducedagloballyhomogeneousgeodeticreferenceframewithanaccuracyof1-2mreflectingthelimitationsoftheDopplertechnique.Stationvelocitieswereignoredastheywereoflittleimportance.AlthoughtheDopplerWGS84referenceframeiscomparablewiththatofNAD83inNorthAmerica,thelackofpreciseVLBIframeworkmakesitimpossibletorelateWGS84tocurrent,moreaccuratereferenceframeswithaprecisionbetterthan1m.SignificantimprovementcanbeachievediftheWGS84frameworkadoptedforGPSoperationsisconsidered.ThisWGS84(GPS)terrestrialreferenceframeisbasedonWGS84coordinatesof10GPStrackingstationsusedbytheU.S.DoDforgenerationofoperational(broadcast)satelliteorbitsandclockparameters.
3.2REVISEDWGS84(G730)TERRESTRIALREFERENCEFRAME
TheWGS84(GPS)coordinatesofthe10GPStrackingstationshavebeenrevisedusingseveralweeksofGPSobservationsfromaglobalnetworkof32stations(10DoD+22IGS)inasimultaneousadjustmentofsatelliteorbitsandstationcoordinates;
thecoordinatesof8IGSstationswereconstrainedtothevaluesadoptedbytheInternationalEarthRotationService(IERS)andtheIERSvalueofthegeocentricconstantofgravitationwasused.ThisimprovedreferenceframeforGPS,designatedWGS84(G730)torefertoGPSweek730,showsglobalconsistencyataboutthe10cmlevelandusesNUVEL-1platemotionmodelforstationvelocities(Swift,1994;
DeMetsatal.,1990).Sincethebeginningof1994,DMAhasusedWGS84(G730)inpost-processinganditisexpectedtobeadoptedforthecomputationofoperational(broadcast)GPSsatelliteorbitsinthenearfuture(MalysandSlater,1994).
4.IERSTERRESTRIALREFERENCEFRAME:
ITRF
InordertofacilitatepreciseEarthrotationandpolarmotionmonitoringbymodernspacegeodetictechniquestheBureauInternationaldel'
Heure(BIH)establishedin1984theBIHTerrestrialSystem(BTS84)basedmainlyonVLBI,SLRandsatelliteDopplerobservations.In1988whenBIHwassupersededbyIERStheIERSTerrestrialReferenceFrame(ITRF88)wascreatedtomeetthefollowingrequirements(Boucher,1990):
Figure1.ResidualdifferencesbetweenNAD83andITRF92(1994.0)fortheCACS
monitoringstations.
(a)itisgeocentricwiththeoriginatthecenterofmassofthewholeEarthincludingtheoceansandt
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