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测绘外文文献
MODERNGEODETICREFERENCEFRAMESFORPRECISESATELLITEPOSITIONINGANDNAVIGATION
J.KoubaandJ.Popelar
GeodeticSurveyDivision,GeomaticsCanada,NaturalResourcesCanada(NRCan)615BoothStreet,
Ottawa,Ontario,CanadaK1AEO9
E-mail:
**************.ca;****************.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).Anewreferenceframewasrequiredtofacilitateuseofefficientandprecisesatellitegeodetictechniques