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1、英文文献TheGlobalPositioningSystemTheglobalPositioningSystem(GPS)isrevolutionizingsurveyingtechnology,Likeitspredecessor,theTEANSITDopplersystem,GPSshiftsthesceneofsurveyingoperationsfromground-to-groundmeasurementstoground-to-sky,withobviousimplications:int
2、ervisibilityofmarksisnolongeracriteionfortheirlocation;operationsarepossibleinnearlyallkindsofweatherandbeperformedduringdayornight;andtheskillsrequiredtoutilisethetechnologyaredifferentbothinfieldoperationsanddataprocessing.ButGPSisnotmerelyareplacement
3、forTRANSIT.Thesimultaneousvisibilityofmultiplesatellitesallowseffectivecancellationofthemajorsourcesoferrorinsatelliteobservations,withtheresultthatwithGPS,relativepositioningaccuraciesofonepartpermillion(ppm)orbetteroverdistancesfromonekilometertothousa
4、ndsofkilometersarepossible.ThismeansthatGPScancompetewithterrestrialtechniquesovershortdistances,andcanachievemoreaccurateresultsinlesstimethanTRANSITobservationsoverlongerdistances.GPSwasdesignedprimarilyasanavigationsystem,tosatisfybothmilitaryandcivil
5、ianneedsforreal-timepositioning.Thispositioningisaccomplishedthroughtheuseofcodedinformation,essentiallyclevertimingsignals,transmittedbythesatellites.EachGPSsatellitetransmitsauniquesignalontwoL-bandfrequencies:Aat1575.42MHzandBat1227.60MHz(equivalentto
6、wavelengthsofapproximately19and24cm,respectively).ThesatellitesignalsconsistoftheL-bandcarrierwavesmodulatedwitha"Standard"or Scode(formerlycalledtheC/Acode),a"Precise"orPcodeandaNavigationMessagecontaining,amongstotherthings,thecoordinatesofthesatellite
7、sasfunctionsoftime---the"BroadcastEphemerides".TheScodewhichisintendedmainlyforcivilianuse,yieldsarangemeasurementprecisionofabout10meter,ThenavigationserviceprovidedbythiscodeisreferredtoastheStandardPositioningService,Thepcodeisintendedformilitaryandse
8、lectedcivilianuseonlyandyieldsameasurementprecisionofabout1meter,ThenavigationserviceprovidedbythePcodeisthereforereferredtoasthePrecisePositioningService(PPS)Althoughbothcodescanbeusedforsurveying,amoreaccuratemethodistom
