Legacy GPS signalsWord下载.docx

Legacy GPS signalsWord下载.docx

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LegacyGPSsignalsLegacyGPSsignalsTheoriginalGPSdesigncontainstworangingcodes:

@#@theCoarse/Acquisition（C/A）code,whichisfreelyavailabletothepublic,andtherestrictedPrecision（P）code,usuallyreservedformilitaryapplications.Coarse/AcquisitioncodeTheC/Acodeisa1,023bitdeterministicsequencecalledpseudorandomnoise（alsopseudorandombinarysequence）（PNorPRNcode）which,whentransmittedat1.023megabitspersecond（Mbit/s）,repeatseverymillisecond.Thesesequencesonlymatchup,orstronglycorrelate,whentheyareexactlyaligned.EachsatellitetransmitsauniquePRNcode,whichdoesnotcorrelatewellwithanyothersatellitesPRNcode.Inotherwords,thePRNcodesarehighlyorthogonaltooneanother.Thisisaformofcodedivisionmultipleaccess（CDMA）,whichallowsthereceivertorecognizemultiplesatellitesonthesamefrequency.PrecisioncodeTheP-codeisalsoaPRN;@#@however,eachsatellitesP-codePRNcodeis6.18711012bitslong（6,187,100,000,000bits,720.213gigabytes）andonlyrepeatsonceaweek（itistransmittedat10.23Mbit/s）.TheextremelengthoftheP-codeincreasesitscorrelationgainandeliminatesanyrangeambiguitywithintheSolarSystem.However,thecodeissolongandcomplexitwasbelievedthatareceivercouldnotdirectlyacquireandsynchronizewiththissignalalone.ItwasexpectedthatthereceiverwouldfirstlockontotherelativelysimpleC/Acodeandthen,afterobtainingthecurrenttimeandapproximateposition,synchronizewiththeP-code.WhereastheC/APRNsareuniqueforeachsatellite,theP-codePRNisactuallyasmallsegmentofamasterP-codeapproximately2.351014bitsinlength（235,000,000,000,000bits,26.716terabytes）andeachsatelliterepeatedlytransmitsitsassignedsegmentofthemastercode.Topreventunauthorizedusersfromusingorpotentiallyinterferingwiththemilitarysignalthroughaprocesscalledspoofing,itwasdecidedtoencrypttheP-code.TothatendtheP-codewasmodulatedwiththeW-code,aspecialencryptionsequence,togeneratetheY-code.TheY-codeiswhatthesatelliteshavebeentransmittingsincetheanti-spoofingmodulewassettotheonstate.TheencryptedsignalisreferredtoastheP（Y）-code.ThedetailsoftheW-codearekeptsecret,butitisknownthatitisappliedtotheP-codeatapproximately500kHz,1whichisaslowerratethanthatoftheP-codeitselfbyafactorofapproximately20.Thishasallowedcompaniestodevelopsemi-codelessapproachesfortrackingtheP（Y）signal,withoutknowledgeoftheW-codeitself.NavigationmessageGPSmessageformatSubframesWordsDescription112Telemetryandhandoverwords（TLMandHOW）310Satelliteclock,GPStimerelationship2/312Telemetryandhandoverwords（TLMandHOW）310Ephemeris（precisesatelliteorbit）4/512Telemetryandhandoverwords（TLMandHOW）310Almanaccomponent（satellitenetworksynopsis,errorcorrection）InadditiontothePRNrangingcodes,areceiverneedstoknowdetailedinformationabouteachsatellitespositionandthenetwork.TheGPSdesignhasthisinformationmodulatedontopofboththeC/AandP（Y）rangingcodesat50bit/sandcallsittheNavigationMessage.Thenavigationmessageismadeupofthreemajorcomponents.ThefirstpartcontainstheGPSdateandtime,plusthesatellitesstatusandanindicationofitshealth.Thesecondpartcontainsorbitalinformationcalledephemerisdataandallowsthereceivertocalculatethepositionofthesatellite.Thethirdpart,calledthealmanac,containsinformationandstatusconcerningallthesatellites;@#@theirlocationsandPRNnumbers.Whereasephemerisinformationishighlydetailedandconsideredvalidfornomorethanfourhours,almanacinformationismoregeneralandisconsideredvalidforupto180days.Thealmanacassiststhereceiverindeterminingwhichsatellitestosearchfor,andoncethereceiverpicksupeachsatellitessignalinturn,itthendownloadstheephemerisdatadirectlyfromthatsatellite.Apositionfixusinganysatellitecannotbecalculateduntilthereceiverhasanaccurateandcompletecopyofthatsatellitesephemerisdata.Ifthesignalfromasatelliteislostwhileitsephemerisdataisbeingacquired,thereceivermustdiscardthatdataandstartagain.Thenavigationmessageitselfisconstructedfroma1,500bitframe,whichisdividedintofivesubframesof300bitseachandtransmittedat50bit/s.Eachsubframe,therefore,requires6secondstotransmit.EachsubframehastheGPStime.Subframe1containstheGPSdate（weeknumber）andinformationtocorrectthesatellitestimetoGPStime,plussatellitestatusandhealth.Subframes2and3togethercontainthetransmittingsatellitesephemerisdata.Subframes4and5containcomponentsofthealmanac.Eachframecontainsonly1/25thofthetotalalmanac;@#@areceivermustprocess25wholeframesworthofdatatoretrievetheentire15,000bitalmanacmessage.Atthisrate,12.5minutesarerequiredtoreceivetheentirealmanacfromasinglesatellite.Theorbitalpositiondata,orephemeris,fromthenavigationmessageisusedtocalculatepreciselywherethesatellitewasatthestartofthemessage.Amoresensitivereceiverwillpotentiallyacquiretheephemerisdatamorequicklythanalesssensitivereceiver,especiallyinanoisyenvironment.2Eachsubframeisdividedinto10words.ItbeginswithaTelemetryWord（TLM）,whichenablesthereceivertodetectthebeginningofasubframeanddeterminethereceiverclocktimeatwhichthenavigationsubframebegins.Thenextwordisthehandoverword（HOW）,whichgivestheGPStime（actuallythetimewhenthefirstbitofthenextsubframewillbetransmitted）andidentifiesthespecificsubframewithinacompleteframe.34Theremainingeightwordsofthesubframecontaintheactualdataspecifictothatsubframe.Afterasubframehasbeenreadandinterpreted,thetimethenextsubframewassentcanbecalculatedthroughtheuseoftheclockcorrectiondataandtheHOW.ThereceiverknowsthereceiverclocktimeofwhenthebeginningofthenextsubframewasreceivedfromdetectionoftheTelemetryWordtherebyenablingcomputationofthetransittimeandthusthepseudorange.Thereceiverispotentiallycapableofgettinganewpseudorangemeasurementatthebeginningofeachsubframeorevery6seconds.AlmanacThealmanac,providedinsubframes4and5oftheframes,consistsofcoarseorbitandstatusinformationforeachsatelliteintheconstellation,anionosphericmodel,andinformationtorelateGPSderivedtimetoCoordinatedUniversalTime（UTC）.Eachframecontainsapartofthealmanac（insubframes4and5）andthecompletealmanacistransmittedbyeachsatellitein25framestotal（requiring12.5minutes）.5Thealmanacservesseveralpurposes.Thefirstistoassistintheacquisitionofsatellitesatpower-upbyallowingthereceivertogeneratealistofvisiblesatellitesbasedonstoredpositionandtime,whileanephemerisfromeachsatelliteisneededtocomputepositionfixesusingthatsatellite.Inolderhardware,lackofanalmanacinanewreceiverwouldcauselongdelaysbeforeprovidingavalidposition,becausethesearchforeachsatellitewasaslowprocess.Advancesinhardwarehavemadetheacquisitionprocessmuchfaster,sonothavinganalmanacisnolongeranissue.ThesecondpurposeisforrelatingtimederivedfromtheGPS（calledGPStime）totheinternationaltimestandardofUTC.Finally,thealmanacallowsasingle-frequencyreceivertocorrectforionosphericerrorbyusingaglobalionosphericmodel.ThecorrectionsarenotasaccurateasaugmentationsystemslikeWAASordual-frequencyreceivers.However,itisoftenbetterthannocorrection,sinceionosphericerroristhelargesterrorsourceforasingle-frequencyGPSreceiver.DataupdatesSatellitedataisupdatedtypicallyevery24hours,withupto60daysdataloadedincasethereisadisruptionintheabilitytomakeupdatesregularly.Typicallytheupdatescontainnewephemerides,withnewalmanacsuploadedlessfrequently.TheControlSegmentguaranteesthatduringnormaloperationsanewalmanacwillbeuploadedatleastevery6days.Satellitesbroadcastanewephemeriseverytwohours.Theephemerisisgenerallyvalidfor4hours,withprovisionsforupdatesevery4hoursorlongerinnon-nominalconditions.Thetimeneededtoacquiretheephemerisisbecomingasignificantelementofthedelaytofirstpositionfix,becauseasthehardwarebecomesmorecapable,thetimetolockontothesatellitesignalsshrinks;@#@however,theephemerisdatarequires18to36secondsbeforeitisreceived,duetothelowdatatransmissionrate.FrequencyinformationGPSbroadcastsignalFortherangingcodesandnavigationmessagetotravelfromthesatellitetothereceiver,theymustbemodulatedontoacarrierfrequency.InthecaseoftheoriginalGPSdesign,twofrequenciesareutilized;@#@oneat1575.42MHz（10.23MHz154）calledL1;@#@andasecondat1227.60MHz（10.23MHz120）,calledL2.TheC/AcodeistransmittedontheL1frequencyasa1.023MHzsignalusingabi-phaseshiftkeying（BPSK）modulationtechnique.TheP（Y）-codeistransmittedonboththeL1andL2frequenciesasa10.23MHzsignalusingthesameBPSKmodulation,howevertheP（Y）-codecarrierisinquadraturewiththeC/Acarrier（meaningitis90outofphase）.Besidesredundancyandincreasedresistancetojamming,acriticalbenefitofhavingtwofrequenciestransmittedfromonesatelliteistheabilitytomeasuredirectly,andthereforeremove,theionosphericdelayerrorforthatsatellite.Withoutsuchameasurement,aGPSreceivermustuseagenericmodelorreceiveionosphericcorrectionsfromanothersource（suchastheWideAreaAugmentationSystemorEGNOS）.AdvancesinthetechnologyusedonboththeGPSsatellitesandtheGPSreceivershasmadeionosphericdelaythelargestremainingsourceoferrorinthesignal.Areceivercapableofperformingthismeasurementcanbesig