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1、英文文献TheGlobalPositioningSystemTheglobalPositioningSystem(GPS)isrevolutionizingsurveyingtechnology,Likeitspredecessor,theTEANSITDopplersystem,GPSshiftsthesceneofsurveyingoperationsfromground-to-groundmeasurementstoground-to-sky,withobviousimplications:intervisibilityofm
2、arksisnolongeracriteionfortheirlocation;operationsarepossibleinnearlyallkindsofweatherandbeperformedduringdayornight;andtheskillsrequiredtoutilisethetechnologyaredifferentbothinfieldoperationsanddataprocessing.ButGPSisnotmerelyareplacementforTRANSIT.Thesimultaneousvisi
3、bilityofmultiplesatellitesallowseffectivecancellationofthemajorsourcesoferrorinsatelliteobservations,withtheresultthatwithGPS,relativepositioningaccuraciesofonepartpermillion(ppm)orbetteroverdistancesfromonekilometertothousandsofkilometersarepossible.ThismeansthatGPSca
4、ncompetewithterrestrialtechniquesovershortdistances,andcanachievemoreaccurateresultsinlesstimethanTRANSITobservationsoverlongerdistances.GPSwasdesignedprimarilyasanavigationsystem,tosatisfybothmilitaryandcivilianneedsforreal-timepositioning.Thispositioningisaccomplishe
5、dthroughtheuseofcodedinformation,essentiallyclevertimingsignals,transmittedbythesatellites.EachGPSsatellitetransmitsauniquesignalontwoL-bandfrequencies:Aat1575.42MHzandBat1227.60MHz(equivalenttowavelengthsofapproximately19and24cm,respectively).Thesatellitesignalsconsis
6、toftheL-bandcarrierwavesmodulatedwitha"Standard"or Scode(formerlycalledtheC/Acode),a"Precise"orPcodeandaNavigationMessagecontaining,amongstotherthings,thecoordinatesofthesatellitesasfunctionsoftime---the"BroadcastEphemerides".TheScodewhichisintendedmainlyforcivilianuse
7、,yieldsarangemeasurementprecisionofabout10meter,ThenavigationserviceprovidedbythiscodeisreferredtoastheStandardPositioningService,Thepcodeisintendedformilitaryandselectedcivilianuseonlyandyieldsameasurementprecisionofabout1meter,ThenavigationserviceprovidedbythePcodeis
8、thereforereferredtoasthePrecisePositioningService(PPS)Althoughbothcodescanbeusedforsurveying,amoreaccuratemethodistom
