Global Positioning System

Overview

GPSwasdevelopedbytheUnitedStatesinthe1970sandlasted20yearsandcost200Itisanewgenerationsatellitenavigationandpositioningsystemwithall-roundreal-timethree-dimensionalnavigationandpositioningfunctionsinsea,landandair.Theuseofsurveyingandmappingdepartmentsinmycountryinthepast10yearshasshownthatGPShaswonthetrustofsurveyingandmappingworkerswithitsall-weather,high-precision,automation,andhigh-efficiencyfeatures,andhasbeensuccessfullyappliedtogeodeticsurveying,engineeringsurveying,andaerialphotogrammetry.,Vehiclenavigationandcontrol,crustalmovementmonitoring,engineeringdeformationmonitoring,resourceprospecting,geodynamicsandotherdisciplines,thusbringingaprofoundtechnologicalrevolutiontothefieldofsurveyingandmapping.

GPSisthesecond-generationsatellitenavigationsystemintheUnitedStates.ItwasdevelopedonthebasisoftheMeridiansatellitenavigationsystem,anditadoptedthesuccessfulexperienceoftheMeridiansystem.Accordingtothecurrentplan,thespacepartofGPSuses24satelliteswithanaltitudeofapproximately20,200kilometerstoformasatelliteconstellation.The24satellitesareallinnearlycircularorbits,withanoperatingperiodofabout11hoursand58minutes,distributedon6orbitalplanes(4foreachorbitalplane),withanorbitalinclinationof55degrees.Thedistributionofsatellitesmakesitpossibletoobservemorethan4satellitesanywhereintheworldandatanytime,andtomaintainageometricfigurewithgoodpositioningaccuracy.Thisprovidescontinuousglobalnavigationcapabilitiesintime.

GPSismainlycomposedofthreemajorcomponents:thespacepart,thegroundmonitoringpartandtheuserequipmentpart.TheGPSsystemhasthecharacteristicsofhighprecision,all-weather,andextensiveuse.

TheinventionoftheGlobalPositioningSystem

Inthe1970s,theU.S.DepartmentofDefenseprovidedreal-time,all-weatherandglobalnavigationservicesforland,sea,andair.Somemilitarypurposes,suchasintelligencecollection,nuclearexplosionmonitoring,andemergencycommunications,havebeguntodevelopthe"NavigationSatelliteTimingandRangingGlobalPositioningSystem",referredtoastheGlobalPositioningSystem.In1973,theUSDepartmentofDefensebegantodesignandtest.OnFebruary4,1989,thefirstGPSsatellitewassuccessfullylaunched.Bytheendof1993,apracticalGPSnetwork,namelythe(21+3GPS)constellation,wasbuilt,andcommercialoperationsbegan.Aftermorethan20yearsofresearchandexperimentation,atacostof30billionU.S.dollars,byMarch1994,24GPSsatelliteconstellationswithaglobalcoveragerateof98%hadbeendeployed.

Developmenthistory

GPSreferstoaradionavigationpositioningsystemthatusesGPSsatellitestoprovideall-weather,real-time,three-dimensionalposition,three-dimensionalspeedandotherinformationtoallpartsoftheworld.ThepredecessorofGPSwasaTransitsatellitepositioningsystemdevelopedbytheU.S.militaryin1958.Itwasofficiallyputintousein1964.Thesystemusesastarnetworkcomposedof5-6satellitestoworkandbypasstheearthatmost13timesaday.Andtheheightinformationcannotbegiven,andthepositioningaccuracyisnotsatisfactory.However,theMeridiansatellitepositioningsystemenabledtheR&Ddepartmenttogainpreliminaryexperienceinsatellitepositioningandverifiedthefeasibilityofpositioningbythesatellitesystem,whichpavedthewayforthedevelopmentofGPS.AssatellitepositioninghasshowngreatadvantagesinnavigationandtheMeridiansystemhashugeshortcomingsinsubmarineandshipnavigation,theU.S.Army,Navy,andAirForceandciviliandepartmentsallfeelanurgentneedforanewsatellitenavigationsystem.

Inthe1970s,theland,navy,andairforcesjointlydevelopedanewgenerationofglobalpositioningsystem.ThemainpurposeofGPSistoprovidereal-time,all-weatherandglobalnavigationservicesfortheland,navy,andairforces.Formilitarypurposessuchasintelligencecollection,nuclearexplosionmonitoring,andemergencycommunications,aftermorethan20yearsofresearchandexperimentation,atacostof30billionU.S.dollars,by1994,24GPSsatelliteconstellationswithaglobalcoveragerateof98%hadbeendeployed.GPShasgonethroughthefirstandsecondgenerations,andhasnowbeenupgradedtothethirdgenerationtomaintainitsdominanceinnavigationandpositioningsystems.Fromthecurrentpointofview,GPSisthenavigationwiththehighestaccuracyandthewidestcoverageintheworld.GPS.

TheoriginalGPSplanwasdevelopedundertheleadershipoftheUnitedStatesJointPlanningBureau.Theplanplaced24satellitesinthreeorbitsthatare1200eachother.Thereare8satellitesineachorbit,and6-9satellitescanbeobservedatanypointontheearth.Inthisway,thecoarsecodeaccuracycanreach100m,andthefinecodeaccuracyis10m.Duetothecompressionofthebudget,theGPSplanhadtoreducethenumberofsatellitelaunches,andinsteaddistributed18satellitesonsixorbitsforming600eachother.However,thisschemecouldnotguaranteethereliabilityofthesatellites.Thelastmodificationwasmadein1988:21workingsatellitesand3sparesatelliteswereoperatingin6orbitsthatformed600eachother.ThisisalsotheworkingmethodcurrentlyusedbyGPSsatellites.

Workingprinciple

Positioningprinciple

GPSpositioningincludespseudo-rangesingle-pointpositioning,carrierphasepositioningandreal-timedifferentialpositioning.

1.Pseudo-rangemeasurementandpseudo-rangesingle-pointpositioning

Pseudo-rangemeasurementistodeterminethedistancefromthesatellitetothereceiver,thatis,thepropagationtimeoftherangingcodesignaltransmittedbythesatellitetotheGPSreceiverismultipliedbyThedistanceatthespeedoflight.Pseudo-rangesingle-pointpositioningistousetheGPSreceivertomeasurethepseudo-rangewithmorethan4GPSsatellitesatacertainmoment,andthesatelliteinstantaneouscoordinatesobtainedfromthesatellitenavigationmessage,andusethedistancerendezvousmethodtofindtheantennainWGS-84Three-dimensionalcoordinatesinthecoordinatesystem.

2.Carrierphasemeasurementandcarrierphasepositioning

CarrierphasemeasurementistomeasurethephasedelaybetweentheGPSsatellitecarriersignalandthereceiverantenna.TheGPSsatellitecarriermodulatestherangingcodeandnavigationmessage.Afterthereceiverreceivesthesatellitesignal,itfirstremovestherangingcodeandsatellitemessageonthecarriertoregainthecarrier,whichiscalledrebuildingthecarrier.TheGPSreceivercomparesthesatellitereconstructedcarrierwiththelocaloscillatorsignalgeneratedbytheoscillatorinthereceiverthroughaphasemetertoobtainthephasedifference.

3.Real-timedifferentialpositioning

TheprincipleofGPSreal-timedifferentialpositioningistoplaceaGPSreceiver(calledareferencestation)ontheexistingprecisegeocentriccoordinatepoint,andusetheknowngeocentriccoordinatesandTheephemeriscalculatesthecorrectionvalueoftheGPSobservationvalue,andsendsthecorrectionvaluetotheGPSreceiverinmotion(calledtherover)throughtheradiocommunicationdevice(calledthedatalink).TheroverusesthecorrectionvaluetocorrectitsownGPSobservationstoeliminatetheabove-mentionederrors,therebyimprovingtheaccuracyofreal-timepositioning.TherearemanyGPSdynamicdifferentialmethods,includingpositiondifferential,pseudo-rangedifferential(RTD),carrierphasereal-timedifferential(RTK),andwide-areadifferential.

PrinciplesofGPSsignalreceptionandmeasurement

Activerangingandpassiveranging

GPSisapassivesatellitenavigationsystem.Inthepassiverangingsystem,theuserTheantennaonlyneedstoreceivethenavigationandpositioningsignalsfromthesesatellites,sothatthedistanceordistancedifferencefromtheuser'santennatothesatellitecanbemeasured.Thisrangingmethodinwhichthesendingandreceivingrangingsignalsarelocatedintwodifferentplacesiscalledpassiveranging.Usingthemeasureddistancebetweenthestationandthesatelliteandtheknownpositionofthesatelliteinorbit,thethree-dimensionalpositionoftheuser'santennacanbecalculated.Thiskindofpositioningbasedontheprincipleofpassiverangingiscalledpassivepositioning.Iftherangingsignalemittedbythesendingdeviceisreflectedorforwardedbythereflector,itreturnstothesendingpointandisreceivedbythereceivingdevice,andthenthedistancetraveledbytherangingsignalismeasured.Thiskindofrangingprincipleinwhichthesendingandreceivingrangingsignalsarelocatedinthesameplaceiscalledactiveranging.Usingitsmeasuredstation-satellitedistanceandknownsatelliteorbitingposition,theuser'scurrentthree-dimensionalpositioncanalsobecalculated.Thistypeofpositioningbasedontheprincipleofactiverangingiscalledactivepositioning.

GPSpseudo-rangemeasurement

TheGPSglobalpositioningsystemadoptsamulti-satellitehigh-orbitrangingsystem,withdistanceasthebasicobservation,byperformingpseudo-rangemeasurementon4satellitesatthesametime,thatisThepositionofthereceivercanbecalculated.Sincerangingcanbecompletedinaveryshorttime,thatis,positioningiscompletedinaveryshorttime,itcanbeusedfordynamicusers.

Modernrangingisessentiallytheuseofradiosignalstomeasurethepropagationtimetocalculatedistance.Theround-trippropagationdelaycanbemeasured,andtheone-waypropagationdelaycanalsobemeasured.Theround-trippropagationrangingistheactiveranging,whichrequiresboththesatelliteandtheusertohavetheabilitytosendandreceive.Fortheuser,thisnotonlygreatlyincreasesthecomplexityoftheinstrument,butalsoisverydisadvantageousfromtheconcealmentpointofview,becausethetransmittedsignaliseasytocauseexposure.One-wayranging(iepassiveranging)avoidstheaboveshortcomingstoalargeextent.However,theone-wayrangingrequiresthatthesatellitebesynchronizedwiththeclockoftheuser'sreceiver.Ifthetwoclocksarenotsynchronized,themeasuredpropagationdelaytimeincludestheclockdifferencebetweenthetwoclocksinadditiontothepropagationdelaycausedbythedistancebetweenthesatelliteandtheuserreceiver.Itisdifficulttoachievesatelliteanduserclocksynchronizationinactualwork,butitcanbesolvedbyappropriatemethods.

Pseudo-randomcodeandpseudo-randomcoderanging

Inthecaseofnoiseinterference,comprehensiveconsiderationofrangingaccuracy,signalbandwidth,requiredpowerandidentificationofdifferentsatellites,etc.,Theglobalpositioningsystemusespseudo-randomcoderangingtechnology.Pseudo-randomcode,alsoknownaspseudo-noisecode,isabinarycodesequencethatcanbedeterminedinadvance,canberepeatedlygeneratedandcopied,andhasrandomstatisticalcharacteristics.Inthecaseofdeepspacecommunication,theuseofpseudo-randomcodedsignalscanachievelowsignal-to-noiseratioreception,whichgreatlyimprovesthereliabilityofcommunication,andcanrealizecodedivisionmultipleaccesscommunication.Inaddition,theuseofpseudo-randomcodedsignalscanachievehigh-performancesecurecommunication.ThesecharacteristicsareinlinewiththetechnicalrequirementsoftheGPSsystem.

Accordingtothegeneralresultsofsignaldetectiontheory,whenthenoiseiswhitenoisewithuniformpowerspectrum,thebestreceiverforrangingisacorrelationreceiver.Thiskindofreceivingmethodusesthecopysignalofthetransmittedsignal(calledthelocalsignal)andthesumofthereceivedsignalandnoiseforcorrelationcalculation,andthendeterminesthetargetdistancebymeasuringthepositionofthemaximumvalueofthecorrelationfunction.Fromtheperspectiveofcorrelationreception,therangingsignalisrequiredtohaveautocorrelationcharacteristicssimilartowhitenoise.Pseudo-randomcoderangingtechnologyistheembodimentofthisidea.

Usingpseudo-randomcodetomeasurethesignalpropagationdelay,itisnecessarytodetectthemaximumvalueoftherelevantoutput.Thiscanonlybedetectedbymovingthelocalcodebitbybit.Consideringthatthedetectionisperformedaftertheintegratorperformsintegration,theintegrationtimeshouldnotbetooshort,soittakesacertainamountoftimetodetectthemaximumcorrelationoutput,thatis,acertaincapturetimeisrequired.Inthecasethatthedistancetobemeasuredandthestationclockdifferencearenotknowninadvance,thelongerthecode,thelongertheacquisitiontimeisrequired.Inordertoshortentheacquisitiontime,GPSsatellitesalsobroadcastashortcode,namelyC/Acode,alsocalledrentalcode.SincetheC/AcodeisaGoldcodefamilycomposedoftwosequencesofthesamecodewithgoodcross-correlationcharacteristics,andissynchronizedwiththePcode,thePcodecanbeeasilycapturedaftertheC/Acodeiscaptured.

Components

Theglobalpositioningsystemconsistsofthefollowingthreeparts:thespacepart(GPSsatellites),thegroundmonitoringpartandtheuserpart.GPSsatellitescancontinuouslybroadcastrangingsignalsandnavigationmessagesfornavigationandpositioningtousers,andreceivevariousinformationandcommandsfromthegroundmonitoringsystemtomaintainthenormaloperationofthesystem.ThemainfunctionofthegroundmonitoringsystemistotrackGPSsatellites,measuretheirdistances,determinethesatellite'sorbitandsatelliteclockcorrections,makeaforecast,thencompileanavigationmessageaccordingtotheprescribedformat,andsendittothesatellitethroughtheinjectionstation.Thegroundmonitoringsystemcanalsoissuevariousinstructionstothesatellitethroughtheinjectionstation,adjustthesatellite'sorbitandclockreadings,repairfaultsoractivatespareparts,etc.TheuserusestheGPSreceivertomeasurethedistancefromthereceivertotheGPSsatellite,andcalculatesitsownthree-dimensionalposition,three-dimensionalmotionspeed,clockerrorandotherparametersbasedonthesatellite'spositioninspaceatthemomentofobservationgivenbythesatelliteephemeris..Atpresent,theUnitedStatesiscommittedtofurtherimprovingthefunctionsoftheentiresystem,suchasdeterminingsatelliteorbitsthroughmutualtrackingbetweensatellites,inordertoreducethedegreeofdependenceonthegroundmonitoringsystemandenhancetheautonomyofthesystem.

Spacepart

GPSsatellite

ThemainbodyoftheGPSsatelliteiscylindrical,withsolarpanelsonbothsides,whichcanautomaticallyorienttothesun.Solarcellsprovidepowerforthesatellitestowork.Eachsatelliteisequippedwithmultipleatomicclocks,whichcanprovidesatelliteswithhigh-precisiontimestandards.Thesatelliteisequippedwithfuelandnozzles,whichcanadjustitsorbitunderthecontrolofthegroundcontrolsystem.ThebasicfunctionsofGPSsatellitesare:receiveandstorenavigationmessagesfromthegroundcontrolsystem;automaticallygeneraterangingcodesandcarrierwavesunderthecontroloftheatomicclock;modulatetherangingcodesandnavigationmessagesonthecarrierandbroadcastthemtousers;followgroundcontrolThesystemcommandstoadjusttheorbit,adjustthesatelliteclock,repairfaultsoractivatesparepartstomaintainthenormaloperationoftheentiresystem.Theshapesofdifferenttypesofsatellitesarealsodifferent.

GPSsatellitescanbedividedintotestsatellitesandworkingsatellites.Thebasiccharacteristicsofeachtypeareasfollows:

(1)Testsatellite

Testsatellitesarealsocalledprototypesatellites.Thesatelliteweighs774kg(including310kgoffuel)andhasadesignlifeof5years.Inordertomeettheneedsofprogramdemonstrationandtheentiresystemtestandimprovement,theUnitedStatessuccessivelylaunched11testsatelliteswithAtlasrocketsfromVandenbergAirForceBaseinCaliforniafrom1978to1985.Theseventhsatellitefailedtolaunchanddidnotentertheintendedorbit.Attheendof1995,thelasttestsatellitestoppedworking.

(2)Workingsatellite

TheBlockIIsatelliteweighsabout1.5tandhasadesignlifeof7.5years.Eachsatellitecosts48millionUSdollars.BetweenFebruary1989andOctober1990,9BlockIIsatelliteswerelaunchedwithDeltaIIrocketsfromtheKenaveralSpaceBaseinFlorida.Comparedwiththetestsatellite,theBlockIIsatellitehasmademanyimprovements.Thesatellitecanstorenavigationmessagesfor14daysandhastheabilitytoimplementSAandAS.BlockⅡAsatellite(A:Advanced)weighsabout17t,thesatellitedesignlifeis7.5years,andthesatelliteshavetheabilitytocommunicatewitheachother.Thesatellite’sabilitytostorenavigationmessageshasbeenincreasedto180days.SVN35andSVN36satellitesareequippedwithlaserreflectionprisms,whichcananalyzetheerrorsofsatelliteclockandsatelliteephemeristhroughlaserranging,andtesttheaccuracyofGPSranging.Thesizeofthereflectingprismis24cm×20cm.

Controlpart

Thegroundmonitoringpartiscomposedoffivegroundstationsdistributedallovertheworld.Accordingtotheirfunctions,theycanbedividedintothreetypes:monitoringstation,maincontrolstationandinjectionstation.Themonitoringstationisequippedwithdual-frequencyGPSreceivers,high-precisionatomicclocks,meteorologicalparametertesters,computersandotherequipment.ThemaintaskistocompletecontinuousobservationofGPSsatellitesignals,andtransmitthecollecteddataandlocalmeteorologicalobservationdatatoMastercontrolstation.Inadditiontocoordinatingandmanagingthegroundmonitoringsystem,themaincontrolstationisalsoresponsibleforthejointprocessingoftheobservationdataofthemonitoringstation,calculatingthesatelliteephemeris,satelliteclockoffsetandatmosphericcorrectionparameters,andcompilingthesedataintonavigationmessagestotheinjectionstation;,Itcanalsoadjustoff-orbitsatellitestomovethemalongapredeterminedorbit,anddispatchbackupsatellitestoreplacefailedsatellitestocarryoutwork.Themaintaskoftheinjectionstationistoinjectthenavigationmessagescompiledbythemaincontrolstation,thecalculatedsatelliteephemerisandsatelliteclockerrorcorrections,etc.,intothecorrespondingsatellitesthroughanantennawithadiameterof3.6m.

Userpart

UserequipmentismainlycomposedofGPSreceiver,hardwareanddataprocessingsoftware,microprocessorandterminalequipment;GPSreceiveriscomposedofhost,antennaandpowersupply.Itsmaintaskistocapture,trackandlocksatellitesignals;toprocessthereceivedsatellitesignalstomeasurethepropagationtimeofGPSsignalsfromthesatellitetothereceiverantenna;todecodethenavigationmessagestransmittedbytheGPSsatellitesandtocalculatethereceiverantennainrealtimeThree-dimensionalposition,speedandtime.

GPSFeatures

GPSiscurrentlythemostsuccessfulsatellitepositioningsystem,andisknownasamilestoneinhumanpositioningtechnology.Insummary,thesystemhasthefollowingcharacteristics:

(1)Global,all-weathercontinuousnavigationandpositioningcapabilities.GPScanprovidecontinuous,all-weathernavigationandpositioningcapabilitiesforallkindsofusersinanylocationornear-Earthspaceintheworld.Usersdonotneedtotransmitsignals,soitcanmeettheneedsofmultipleusers.

(2)Real-timenavigation,highpositioningaccuracyandshortobservationtime.WhenGPSpositioningisused,positiondatacanbeobtainedseveraltimeswithin1s.Thisnear-real-timenavigationcapabilityisofgreatsignificanceforhighlydynamicusers,anditcanprovideuserswithcontinuousthree-dimensionalposition,three-dimensionalspeed,andaccuratetimeinformation.Atpresent,thereal-timepositioningaccuracyusingC/Acodecanreach20-50m,thespeedaccuracyis0.1m/s,thespecialprocessingcanreach0.005m/s,andtherelativepositioningaccuracycanreachmillimeterlevel.

WiththecontinuousimprovementoftheGPSsystemandthecontinuousupdateofthesoftware,thecurrentrelativestaticpositioningwithin20kmonlytakes15-20min.Duringrapidstaticrelativepositioningmeasurement,wheneachroveris15kmawayfromthereferencestationWhenitiswithin,theobservationtimeoftheroveronlyneeds1-2minutes,andthenitcanbelocatedatanytime,andtheobservationofeachstationonlytakesafewseconds.

(3)Thestationdoesnotneedtobevisible:GPSmeasurementonlyrequiresthattheskyabovethestationisopen,anddoesnotrequiremutualvisibilitybetweenstations,soitcansavealotofbiddingcosts(generally,thecostofbiddingaccountsforthetotal30%,50%ofthefunding).Sincethereisnoneedtoseethroughthepoints,thepositionofthepointscanbesparseordenseaccordingtotheneeds,whichmakestheselectionofpointsveryflexible,andcanalsosavethemeasurementworkoftransferpointsandtransitionpointsintheclassicmeasurement,

(4)Globallyunifiedthree-dimensionalgeocentriccoordinatescanbeprovided:GPSmeasurementcanaccuratelydeterminetheplanepositionofthestationandtheelevationoftheearthatthesametime.ThecurrentGPSlevelcanmeettheaccuracyofthefourth-levellevelingmeasurement.Inaddition,theGPSpositioningiscalculatedinthegloballyunifiedWGS-84coordinatesystem,sothemeasurementresultsofdifferentlocationsaroundtheworldareinterrelated.

(5)Theinstrumentiseasytooperate:WiththecontinuousimprovementofGPSreceivers,thedegreeofautomationofGPSmeasurementisgettinghigherandhigher.Intheobservationtowel,thesurveyoronlyneedstoinstalltheinstrument,connectthecable,measuretheheightoftheantenna,andmonitortheworkingstatusoftheinstrument.Otherobservationtasks,suchassatellitecapture,tracking,observationandrecording,areautomaticallycompletedbytheinstrument.Whenthemeasurementiscompleted,Youonlyneedtoturnoffthepowerandputthereceiverintocompletethefielddatacollectiontask.

Ifyouneedtodoalong-termcontinuousobservationonameasuringstation,youcanalsousethedatacommunicationmethodtocollectthecollecteddata.Thedataistransmittedtothedataprocessingcentertorealizefullyautomateddatacollectionandprocessing.Inaddition,thevolumeofthereceiverisgettingsmallerandsmaller,andthecorrespondingweightisgettinglighterandlighter,whichgreatlyreducesthelaborintensityofthesurveyauthorandmakesfieldworkeasier.

(6)Stronganti-interferenceabilityandgoodconfidentiality:GPSadoptsspreadspectrumtechnologyandpseudocodetechnology,usersonlyneedtoreceiveGPSsignals,theywillnottransmitsignalsthemselves,andwillnotreceiveothersignalsfromtheoutsideworld.Sourceofinterference.

(7)Multiplefunctionsandwideapplications:GPSisadual-usesystemformilitaryandcivilianuse,anditsapplicationrangeisverywide.Specificapplicationexamplesinclude:carnavigationandtrafficmanagement,linepatrolvehiclemanagement,roadengineering,personalpositioning,andnavigator,etc.

GPSreceiver

ThemainfunctionoftheGPSreceiveristoreceiveGPSsatellitesignalsandundergosignalamplification,frequencyconversion,andphaselockprocessingtodeterminetheGPSsignalfromthesatellitetothereceiverantenna.Propagationtime,interpretationofnavigationmessages,real-timecalculationofGPSantennalocation(three-dimensionalcoordinates)andoperatingspeed,etc.GPSreceiverisakindofpassiveradiopositioningequipment.Accordingtodifferentpurposes,itisdividedintonavigationreceiver,geodeticreceiver,timingreceiverandattitudemeasurementreceiver;accordingtothenumberofreceiverchannels,itcanbedividedintomulti-channelreceiver,Sequentialchannelreceiverandmultiplexedchannelreceiver.

TheGPSreceiverismainlycomposedofGPSreceivingantenna,GPSreceiverhostandpowersupply.

1.GPSreceiverantenna

TheGPSreceiverantennaconsistsofanantennaunitandapreamplifier.ThefunctionoftheantennaistoconverttheweakelectromagneticwaveenergyoftheGPSsatellitesignalintocorrespondingcurrent,andamplifythereceivedGPSsignalthroughthepre-amplifier.

2.GPSreceiverhost

Thereceiverhostiscomposedoffrequencyconverter,signalchannel,microprocessor,memoryanddisplay.ThemaintaskofthefrequencyconverteristomakethereceivedL-bandradiofrequencysignalintoalow-frequencysignal.Thesignalchannelisacircuitthatcombinessoftwareandhardware.Itisthecorepartofthereceiver.Itsfunctionistosearchfor,pullandtracksatellites,despreadanddemodulatethebroadcastmessagesignalintoabroadcastmessage,performpseudorangemeasurement,carrierphasemeasurementandmultiplePullerfrequencyshiftmeasurement.Thememoryisusedtostorethesatelliteephemeris,satellitealmanac,codephasepseudorangeobservations,carrierphaseobservationsandDopplerfrequencyshiftscollectedbythereceiveronceanhour.ThemicroprocessoristhecoreoftheGPSreceiver'swork,andtheGPSreceiver'sworkiscarriedoutundertheunifiedcoordinationoftheinstructionsofthemicrocomputer.GPSreceivershaveLCDscreenstoprovideworkinginformationoftheGPSreceivers,andareequippedwithacontrolkeyboardsothatuserscancontroltheworkofthereceiver.

3.GPSreceiverPowersupply

TherearetwotypesofpowersuppliesforGPSreceivers.RAMmemoryispowered;theotherisanexternalpowersupply,commonlyusedrechargeable12VDCnickel-cadmiumbatterypack.

ClassificationofGPSreceivers

GPSreceiverscanbedividedintodifferenttypesaccordingtodifferentclassificationstandards.

(1)Accordingtotheworkingprinciple,itcanbedividedintocodecorrelationtypereceiver,squaretypereceiverandhybridtypereceiver.Thecode-relatedreceivercangenerateaduplicatecodethatisexactlythesameasthemeasuredsatelliterangingcodestructure,usingC/AcodeorPcode,providedthattherangingcodestructureismastered.Thesquaringreceiverusesthesquaringtechniqueofthecarriersignaltoremovethemodulationcodetoobtainthecarriersignalnecessaryforcarrierphasemeasurement.Themachineonlyusessatellitesignalswithoutdecodingormasteringtherangingcodestructure.Itisalsocalledacodelessreceiver.Hybridreceiverscomprehensivelyutilizetheadvantagesofcodecorrelationtechnologyandsquaringtechnology,andobtaincodephaseandprecisecarrierphaseobservationsatthesametime,andarecurrentlywidelyused.

(2)Accordingtothetypeofreceiversignalchannel,itcanbedividedintomulti-channelreceiver,sequentialchannelreceiverandmultiplexchannelreceiver.Themulti-channelreceiverhasmultiplesatellitesignalchannels,andeachchannelonlycontinuouslytracksonesatellitesignal,whichisalsocalledacontinuoustrackingreceiver.Sequentialchannelreceivershaveonly1-2signalchannels.Inordertotrackmultiplesatellites,underthecontrolofthecorrespondingsoftware,thesatellitesignalsaretrackedandmeasuredinsequenceaccordingtothetimesequence.Ittakesalongtimetomeasureonecycleinturn(morethan20m)./s),thetrackingofsatellitesignalsisdiscontinuous.Themultiplexchannelreceiverissimilartothesequentialchannelreceiver,andonlyhas1-2signalchannels.Underthecontrolofthecorrespondingsoftware,thesatellitesignalsaretrackedandmeasuredinsequenceaccordingtothesequence.Short(lessthan20m/s),whichcankeepcontinuoustrackingofsatellitesignals.

(3)Accordingtothefrequencydivisionofthereceivedsatellitesignal,itcanbedividedintosingle-frequencyreceiveranddual-frequencyreceiver.Thesingle-frequencyreceiver(L1)onlyreceivesthemodulatedL1signal.Althoughtheparametersprovidedbythenavigationmessagecanbeusedtocorrecttheionosphericinfluenceontheobservations,thecorrectionmodelisnotyetcompleteandtheaccuracyispoor.Itismainlyusedforlessthan20km.Precisepositioningoftheshortbaseline.Thedual-frequencyreceiver(L1+L2)acceptsbothL1andL2signalsatthesametime.Usingdual-frequencytechnology,itcaneliminateorweakentheinfluenceofionosphericrefractionontheobservationandthepositioningaccuracyishigh.Theclassificationofsatellitesignalsbysignalfrequencyisthemostcommonlyusedtype.

(4)Accordingtothepurposeofthereceiver,itcanbedividedintonavigationtype,measurementtypeandtimingtype.Thenavigationtypeismainlyusedtodeterminethereal-timepositionandspeedofships,vehicles,airplanes,etc.,toensurenavigationaccordingtoapredeterminedrouteortoselectthebestroute.Itusessingle-pointreal-timepositioningordifferentialGPSpositioningwithcodedpseudo-rangesasobservations.Theprecisionislow,thestructureissimple,thepriceischeap,andtheapplicationiswide.Themeasurementreceiverusescarrierphaseobservationforrelativepositioning,withhighaccuracy.Theobservationdatacanbeprocessedaftermeasurementorreal-timeprocessing(RTK).Itneedstobeequippedwithcompletedataprocessingsoftware.Comparedwiththenavigationtype,thestructureiscomplicatedandthepriceisexpensive.Timingreceiversaremainlyusedinobservatoriesorgroundmonitoringstationsfortime-frequencysynchronizationmeasurements.

Theworkingprincipleofthereceiver

Thebasicfunctionoftheantennaistoconverttheenergyfromthesatellitesignalintothecorrespondingcurrent,andperformthefrequencythroughthepreamplifier.Transformationinordertotrack,processandmeasurethesignal.

WhenGPSsatellitesriseintheuser’shorizon,thereceivercancapturethesatellitestobemeasuredselectedaccordingtoacertainsatellitealtitudecut-offangle,andcantracktheoperationofthesesatellites;,Withthefunctionsoftransformation,amplificationandprocessing,inordertomeasurethepropagationtimeoftheGPSsignalfromthesatellitetothereceivingantenna,interpretthenavigationmessagesentbytheGPSsatellite,andcalculatethei-dimensionalpositionofthestationinrealtime,andeventhethree-dimensionalvelocityandtime.GPSsignalreceiversnotonlyneedpowerfulin-machinesoftware,butalsoamulti-functionalGPSdatapost-processingsoftwarepackage.ThereceiverplustheprocessingsoftwarepackageisthecompleteGPSsignaluserequipment.

Theerrorofthereceivingdevice

Theerrorofthereceivingdevicemainlyincludestheobservationerror,thereceiverclockerror,theinfluenceoftheuncertaintyofthewholecycleofthecarrierphaseobservationandtheantennaPhasecentererror.

(1)Observationerror.Inadditiontotheresolutionerror,italsoincludestheplacementerrorofthereceivingantennarelativetothestation.Theresolutionerrorisgenerallyconsideredtobeabout1%ofthesignalwavelength.Theplacementerrormainlyincludesthelevelingandcenteringerroroftheantennaandtheerrorofmeasuringtheheightoftheantennaphasecenter(antennaheight).Forexample,whentheantennais1.6mhighandthelevelingerroris0.10,thecenteringerroris2.10mm.

(2)Receiverclockerror.GPSreceiversaregenerallyequippedwithhigh-precisionquartzclockswithadailyfrequencystabilityof10-11.Ifthesynchronizationdifferencebetweenthereceiverclockandthesatelliteclockis1s,theequivalentdistanceerrorcausedis300m.Themethodofprocessingthereceiverclockerrorisasanunknownnumber,whichissolvedindataprocessing;themethodofusingtheobservationvaluetoreducetheinfluenceofthereceiverclockerror;whenthepositioningaccuracyishigher,anexternalfrequencystandard,suchasrubidiumandcesiumatomicclock,canbeused.Improvetheaccuracyofreceivertimestandards.

(3)Thewholeweekunknownofthecarrierphaseobservation.Itisnotpossibletodirectlydeterminethenumberofcompletecyclesinthepropagationpathofthecarrierphasecorrespondingtothestartingepoch.Atthesametime,thesatellitesignalisblockedandinterfered,resultinginsignaltrackinginterruptionandfullcyclechange.

(4)Antennaphasecenterpositiondeviation.InGPSpositioning,theobservationvalueisbasedonthephasecenterpositionofthereceiverantenna.Intheory,thephasecenteroftheantennashouldbeconsistentwiththegeometriccenteroftheinstrument.Inpractice,itvarieswiththeintensityanddirectionofthesignalinput.Thechangeisalsorelatedtothequalityoftheantenna,whichcanreachseveralmillimeterstoseveralcentimeters.Howtoreducetheoffsetofthephasecenterisanurgentprobleminantennadesign.

Fourmajornavigation

U.S.GlobalPositioningSystem

TheU.S.GlobalPositioningSystem(GPS)wasajointNewspacesatellitenavigationandpositioningsystemdeveloped.Itsmainpurposeistoprovidereal-time,all-weatherandglobalnavigationservicesforthethreemajorareasofland,seaandair,andformilitarypurposessuchasintelligencecollection,nuclearexplosionmonitoring,andemergencycommunications.ItisanimportantpartoftheUnitedStates'globaldominancestrategy.Aftermorethan20yearsofresearchandexperimentation,atacostof30billionU.S.dollars,byMarch1994,all24GPSsatelliteswithaglobalcoveragerateof98%werecompleted.

GPSisaglobal,all-weather,all-weather,high-precisionnavigationpositioningandtimetransfersystem.Asamilitary-civiliandual-usesystem,itprovidestwolevelsofservice.Inrecentyears,inordertostrengthenitscompetitivenessintheglobalnavigationmarket,theU.S.governmenthascancelledtheSAinterferencetechnologyagainstGPS.Thepositioningaccuracyofthestandardpositioningservicecanactuallybeincreasedto20mandthetimingaccuracyto40nsduringdual-frequencyoperation,therebyrestrainingothercountries.EstablishaparallelsystemandadvocatetheuseofGPSandtheU.S.government’senhancedsystemasinternationalstandards.

TheglobalpositioningsystemoftheUnitedStatesincludes27satellites(24orbits,3standby)orbitingtheearth,whichareevenlydistributedin6orbits.Eachsatelliteisabout17,000kilometersawayfromthegroundandcancontinuouslytransmitradiosignalsofacertainfrequency.Aslongasyouhaveaportablesignalreceiver,youcanreceivespecificsignalsfromsatellitesnomatteryouareonland,atseaorintheair.Thecomputerinthereceiveronlyneedstoanalyzethesignalsfrom4ormoresatellitestodeterminethelocationofthereceiver.Inadditiontonavigation,GPSalsohasmanyotheruses.Forexample,scientistscanuseittomonitorthetinymovementsoftheearth'scrusttohelppredictearthquakes;surveyingandmappingpersonneluseittodeterminegroundboundaries;cardriverscanuseittofinddirectionswhentheyarelost;themilitaryRelyonittoensuretherightdirection.

TheEuropeanUnion’s“Galileo”system

TheEuropeanUnion’sCalileosystemistheworld’sfirstcivilian-basedglobalsatellitenavigationandpositioningsystem.TheglobalnavigationsatellitesystemdevelopedbythemonopolyinthefieldiscalledtheEuropeanversionof"GPS".

OnJanuary7,2010,theEuropeanCommissionstatedthattheGalileosystemwillbeputintooperationin2014atacostof3billioneuros.SouthKorea,China,Japan,Argentina,Australia,Russiaandothercountrieshaveparticipatedintheplan,Theoriginaltargetcompletiontimewas2008,butduetovariousreasonssuchastechnology,progresswasveryslow,andtheoriginalkeyplantimenodewasdelayedagainandagain,andthelatestnewswasextendedto2014.ThegoaloftheGalileoplanistobuildanindependent,globalcivilnavigationandpositioningsystem.Chinahasalsoinvested2.96millionUSdollarsintheGalileoplan.TheGalileosystemwillprovideim-accuracypositioningandnavigationservicesforroad,rail,airandoceantransportationandevenhikersinEUmemberstatesandChina,whichwillalsobreaktheUSdominanceofglobalsatellitenavigationsystems.

TheGalileosystemadoptsanewtechnologywithextremelyadvancedperformance,maintainstheindependenceofthesystem,andconsidersthecompatibilityandinteroperabilitywithothersatellitenavigationsystems(thefocusisonGPScompatibilityandinteroperability.

TheGalileosystemismainlycomposedofthreeparts:thespaceconstellationpart,thegroundmonitoringandservicefacilitypart,andtheuserequipmentpart.

Thespaceconstellationpartiscomposedof30satellitesdistributedinthreeorbits.Itiscomposedofcircularorbitsatellites.Thesatellitesaredistributedinthreeorbitswithanaltitudeof23,616kmandaninclinationangleof56degrees.Itenablesanyonetoaccuratelylocateatanytimeandatanyplacewithanerrorofnomorethan3m.TheGalileoconstellationhasbetterDOPvaluedistributioncharacteristics,andthepositioningaccuracyisbetterthanGPSpositioningaccuracy.ThegroundmonitoringandservicefacilitiesincludetwoGalileolocatedintheEuropeanUnion.ControlCenter(GalileoControlCenter)and20GalileoSensorStationsdistributedaroundtheworld.Inaddition,thereare5S-banduplinkstationsand10C-bandsfordataexchangebetweensatellitesandcontrolcenters.Downlinkstation,theGalileocontrolcentermainlycontrolstheoperationofsatellitesandthemanagementofnavigationtasks.20sensorstationstransmitdatatothecontrolcenterthroughredundantcommunicationnetworks.Theuserequipmentpartismainlycomposedofnavigationandpositioningmodulesandcommunicationmodules.

Galileosystemcansendreal-timehigh-precisionpositioninginformation,whichisnotavailableinexistingsatellitenavigationsystems.Atthesametime,Galileosystemcanguaranteetoprovideservicesundermanyspecialcircumstances,andcannotifycustomerswithinafewsecondsifitfails.ComparedwiththeUSGPS,theGalileosystemismoreadvancedandmorereliable.ThesatellitesignalprovidedbytheUSGPScanonlydetectobjectsabout10mlongontheground,whilethesatellitesoftheGalileosystemcandetecttargets1mlong.AmilitaryexpertAvividanalogyisthatGPScanonlyfindstreets,whileGalileosystemcanfindhomes.

Russian"GLONASS"system

RussianGLONASSwasfirstdevelopedintheSovietUnion.1993.Russiabegantobuilditsownglobalsatellitenavigationsystemin1988.Theoriginalplanwastostartoperationsbeforetheendof2007andexpandtheservicetotheworldbeforetheendof2009.However,duetovariousreasonssuchasfunding,thesystemisstillintheongoingstage.GLONASSatleastIttakes18satellitestoprovidepositioningandnavigationservicesfortheentireterritoryofRussia.Ifyouwanttoprovideglobalservices,youneed24satellitestobeinorbitandanother6satellitestobeinorbitforbackup.AccordingtoofficialRussianreports,afterthesystemisfullycompleted,Itspositioningandnavigationerrorrangeisonly23m,whichwillbetheworld'sleadinglevelintermsofaccuracy.

GLONASSissimilartoGPS.Italsoconsistsofaspaceconstellationpart,agroundmonitoringpartandauserequipmentpart.Thespaceconstellationpartismainlycomposedof2-4satellites,evenlydistributedonthreenearlycircularorbitalplanes.Eachorbitalplanehas8satellites.Theorbitalheightis19100km,theoperationperiodis11h15min,andtheorbitalinclinationisslightlylargerthanthatofGPS.Itis64.8degrees.ThegroundmonitoringpartandtheuserequipmentpartaresimilartoGPS.

Atpresent,themaindifferencebetweenGLONASSandGPSisthesignalstructure.GLONASSusesFrequencyDivisionMultipleAccess(FDMA)technology,whichusesthesamecodetobroadcastnavigationsignalsondifferentcarrierfrequencies.GLONASSisdistinguishedbythefrequencyoftheirrespectiveorbitalsignals,with24intervals,named1-24.GPSusescodedivisionmultipleaccess(CDMA)technology.ThecarrierfrequencyofallGPSsatellitesisthesame,andtheyaredistinguishedbytheirrespectivepseudo-randomcodes(PRN).Itspseudo-randomcodes1-32areused.24.Recently,RussiaofficiallyannouncedthatitwilladdCDMAsignals(1575.42MHzand1176.45MHz,respectivelycorrespondingtoGPSL1andL5carriersignals)inthenewgenerationofGLONASS-Msatellites,increasethenumberofsatellites,expandthegroundenhancementsystem,andupgradeGroundcontrolandintegritymonitoringtoexpandthemarkettomeettheneedsofmoreusers.

ChinaBeiDouSatelliteNavigationSystem

China'sBeiDousatellitenavigationsystemBeiDou(COMPASS)navigationsatellitesystem(CNSS).China'sself-developedandindependentlyoperatedglobalsatellitenavigationandcommunicationsystem.TogetherwiththeUSGPS,Russia'sGLONASS,andtheEU'sGalileosystem,theyarecalledtheworld'sfourlargestsatellitenavigationsystems.

TheBeidousatellitenavigationsystemiscomposedofthreeparts:thespaceterminal,thegroundterminalandtheuserterminal.Thespaceterminalincludes3geostationaryorbitsatellitesand30non-geostationaryorbitsatellites.The30non-geostationaryorbitsatellitesaresubdividedinto27mediumorbit(MEO)(including3backupsatellites)satellitesand3tiltedgeosynchronousorbit(IGSO)satellites.)Satellitecomposition,27MEOsatellitesareevenlydistributedonthreeplaneswithaninclinationangleof55degrees,withanorbitalheightof21500km.Thegroundterminalincludesseveralgroundstationssuchasthemaincontrolstation,injectionstationandmonitoringstation.UserterminalsincludeBeidouuserterminalsandterminalscompatiblewithothersatellitenavigationsystems.TheBeidousatellitenavigationsystemcanprovideallkindsofuserswithhigh-precisionandhigh-reliabilitypositioning,navigation,andtimingservicesaroundtheworld,allweatherandallday,anditalsohasshortmessagecommunicationcapabilities.ThegoaloftheBeidousatellitenavigationsystemistobuildanindependent,openandcompatible,technologicallyadvanced,stableandreliablesatellitenavigationsystemthatcoverstheworld.TheBeidousatellitenavigationsystemprovidestwoservices:openserviceandauthorizationservice.Theopenserviceistoprovidefreepositioning,speedmeasurementandtimingservicestousersaroundtheworld.Thepositioningaccuracyis10m,andthespeedmeasurementaccuracyis0.2m/s.Thetimingaccuracyis50ns;theauthorizationserviceistoprovidepositioning,speedmeasurement,timingandcommunicationservicesandsystemintegrityinformationforuserswithhigh-precisionandhigh-reliabilitysatellitenavigationrequirements.BeidounavigationterminalandGPS,GAL.ComparedwithGLONASS,ILEO'sadvantageliesinthecombinationofshortmessageserviceandnavigation,whichaddscommunicationfunctions;fastpositioningaroundtheclock,fewcommunicationblindspots,andaccuracyequivalenttoGPS,whileintheenhancedregion,theAsia-Pacificregion,theaccuracyevenexceedsGPS;XiangQuanTheservicesprovidedbytheworldarefree.Whenprovidingpassivenavigationandpositioningandtimingservices,thereisnolimitonthenumberofusers,anditiscompatiblewithGPS;autonomoussystem,high-strengthencryptiondesign,safe,reliable,andstable,suitableforkeysectorapplications.

Practicalapplication

GlobalpositioningsystemsgenerallymainlyrefertoGPS.AlthoughRussia,EuropeandChinahavealsoestablishedtheirownglobalpositioningsystems,intermsoftechnology,theGPSestablishedbytheUnitedStatesTechnologyisinanadvancedposition.TheapplicationofthepositioningsystemreferredtointhissectionismainlybasedontheapplicationofGPS.Webelievethatinthenearfuture,otherpositioningsystemswillalsoplaytheirownrolesandcanalsoachieveapplicationsinthecorrespondingfields.

(1)Applicationinprecisionengineering,surveyinganddeformationmonitoring

ThecontrolnetworkestablishedbytheapplicationofGPSsatellitepositioningtechnologyiscalledGPSnetwork.GPSnetworkisdividedintotwocategories,oneOnecategoryisaglobalornationalhigh-precisionGPSnetwork;theothercategoryisaregionalGPSnetwork.Thescientificresearchtaskofgeodeticsurveyistostudytheshapeoftheearthanditschangesovertime,andestablishahigh-precisiondynamiccoordinateframebyusingtheglobalcoverageofthehigh-precisionGPSnetwork.TheregionalGPSnetworkreferstothenationalC,D,E-classGPSnetworkortheengineeringGPSnetworkspeciallydesignedforengineeringprojects.

(2)Applicationintransportationsystem

Thepositioningofthecurrentpositionandthepositioningofthetargetaretwokeytechnologiesofthegroundvehiclenavigationsystem.TheformerrequiresGPStoobtainthepointlocationbasis,whilethelatterfocusesondeterminingthepointlocationbasedonadigitalmap.Thisisactuallyaproblemofmap-relatedanalysis.

Withtheexpansionofthescaleofurbanconstructioninourcountry,theincreasingnumberofvehicles,themanagementandrationaldispatchoftransportation,thecommandandsafetymanagementofpolicevehicleshavebecomeanimportantissueinthepublicsecurityandtransportationsystems.TheemergenceofGPSnavigationandpositioningtechnologyprovidesspecificreal-timepositioningcapabilitiesforthenavigationandpositioningofvehicles,shipsandothervehicles.Mainlyusedinpublicsecurityandtransportationsystemsare:acommandandmanagementsystemthatcombinesvehicleGPSpositioningandradiocommunicationsystems;andacommandandmanagementsystemthatusesGPSdifferentialtechnology.GPSvehiclenavigationhasawiderangeofapplications,suchastransportationroutenavigation,emergencyvehiclenavigation,vehicledispatch,etc.Generally,intelligentvehiclesandhighwaysystemscanbedividedintofourtypesaccordingtotheirfunctions:automaticsystems,fleetmanagementsystems,Consultativenavigationsystemandcensussystem.Thefleetmanagementsystemcanoperateonmultiplevehiclesandhasadegreecenter;theconsultingnavigationsystemintegratestheautomaticsystemandthefleetmanagementsystem;thecensus-typesystemusuallyconsistsofanautomaticvehicleloadingcameraoradigitalcamera,whichcanbeusedforObtainroadinformationwithtimeandpointcharacteristics,

(3)ApplicationinGeodynamics

UseGPStomonitorglobalandregionalplatemovement,monitorregionalcrustalmovement,Researchonthecauseofformationandpowermachine

system.Studytheactivitypatternsofundergroundfaults,changesinstressfields,andestimateandpredictearthquakeriskvalues.Inordertomonitormycountry’scrustaldeformation,the“NinthFive-Year”majorscientificprojectundertakenbytheEarthquakeAdministration,theGeneralStaffSurveyingandMappingBureau,theNationalBureauofSurveyingandMapping,andtheChineseAcademyofSciences,the“ZhongtongCrustalMovementMonitoringNetworkProject”,wascompletedin2000.WuhanTechnicalUniversityofSurveyingandMappingusedthetwophasesofGPSmonitoringdatainwesternYunnanandYunnantoinvertthelowerfaultactivitymodeloftheHonghefaultzone,andmadearelativelyaccuratemid-termpredictionofthe1996LijiangearthquakeinYunnan.Thepositionerroris27kmandthefocaldeptherroris0.-6km.Themagnitudeiscompletelyaccurate,revealingthepossibilityofusingGPSmonitoringdataformid-termearthquakeprediction.

(4)ApplicationinMeteorology

UsingGPStheoryandtechnologytoremotelymeasuretheearth’satmosphereandconductmeteorologicaltheoryandmethodresearchiscalledGPSMeteorology(GPSIMET)iGPSThestudyofmeteorologybeganinthelate1910s,firststartedintheUnitedStates,afterobtainingidealtestresultsintheUnitedStates,theapplicationofGPSinmeteorologyhasgraduallybeguninothercountriessuchasJapan.GPS/METdetectiondatahasthecharacteristicsofwidecoverage(global),highverticalresolution,highprecisionandhighlong-termstability.

Theearth’satmosphereisequivalenttoalens.Thetransmissionpathofelectromagneticwavesignals(suchascarrierwaves)intheatmospherewillbetortuousduetoatmosphericrefraction.Thegreaterthegasdensity,thestrongerthesignalrefractionandtheslowerthewavetransmission.Thelow-orbitGPSreceiverreceivestherefractedsignalandcancalculatetheintegratedsignalrefractiveindex.Afterproperconversion,theverticalstructureoftherefractionpathofelectromagneticwavesineachlayeroftheatmospherecanbereconstructed.RadiousesmicrowaveconverterstotransmitsignalsfromGPSsatellitestoreceivers,whichneedtopassthroughacertainmediumduringpropagation.Researchonthenatureofsignaltransmissioncanrevealthenatureofthemedium.TherearethreesourcesofGPSsignalrefractionintheatmosphere,namelydrymatter,moistureandionosphericmatter.

(5)Applicationsinthemilitary

Militarycanbeusedforcoordinatedoperations,missileguidance,searchandrescuepersonnelfieldpositioning.Intermsofcoordinatedoperations,GPScanprovidecommandsystemsatalllevelswiththetimeandlocationofvarioustargetsandevents.Intermsofmissileguidance,about70%oftheUS-IranwarusedGPS-assistedguidance,enablingtheTomahawkcruisemissiletoaccuratelystrikeatargetinasmallhousefromadistanceof600km.Intermsoffieldpositioningforsearchandrescuepersonnel,therearenosignsinthevastdesert.Theymainlyrelyonnavigationsatellitesforpositioningtoknowwheretheyare.

(6)Applicationinagriculture

TheGPSsystemisextremelyimportantintheimplementationofprecisionagriculture:itcanaccuratelylocatevariousinformationonthefarmland,includingthenavigationofagriculturalmachineryandvehicles,flatland,Preciseseeding,spraying,spreadingfertilizer,datamanagement,andcropvigordetectionandvariablecontrol.Themorematureandeffectiveapplicationsinprecisionagricultureinclude:autonomousdriving,fertilization,sprayingandseeding,etc.GPSismuchhigherinefficiencyandaccuracyforsoilnutrientdistributioninvestigation,cropyielddetectionandfarmlandmanagementthanhumanmanagement;monitorsandGPSreceiversareconfiguredonthecombineharvestertoformacropyieldmonitoringsystem;throughandsoilnutrientcontentdistributionThroughcomprehensiveanalysis,itispossibletofindouttherelevantfactorsthataffectcropyields,soastocarryoutspecificfieldfertilizationandothermanagementtasks.Usingthedifferenceinthewavelengthoflightreflectedfrombrownsoilandgreencropleaves

todistinguishsoil,cropsandweeds.Usethedifferenceinreflectedlightwavestoidentifycropleavesthatlacknutrientsorareinfectedwithpestsanddiseases.Therearetwowaystoapplyherbicides:①Useweeddetectionsensorstocollectfieldweedinformation,andcontroltheamountofherbicidesprayedthroughthecontrolsystemofvariablesprayequipment;②UseweedsensorstomapfieldweedpatchesinadvanceDistributionmap,outputprescriptionfromelectronicmap,implementedbyvariablesprayingmachinery.

(7)ApplicationinWildlifeConservation

TheUnitedStatesBureauofLandManagementdevelopedaGPS-basedGeographicInformationSystem(GIS)inthemiddleofthe1910s,theUnitedStatesTheBureauofLandManagementcollectsrelevantinformationonwildliferesourcesonaquarterlybasis,suchasecologicalconditions,thedistributionofwildlifehabitats,andthenumberofwildlife.TensofthousandsofwildhorsesandbisonliveinthewesternUnitedStates.Themainpurposeofestablishingawildlifemanagementsystemistousevoiceinputtoreplacetheoldmanualinputmethod;itcandisplaycurrentandhistoricalflightroutesandaccuratelylocatethedistributionofwildlife.Inactualwork,thesystemisinstalledonairplanesorvehiclestosearchforwildanimalsintheair,andrelevantinformationisrecordedbyvoicerecording,andGPSpointdataisautomaticallystoredinthedatabase.Whennecessary,acomparativeanalysisofdifferentperiodscanbecarriedoutaccordingtohistoricalroutes,andcurrentandhistoricaldatacanbecombinedtocarryoutdynamicmonitoringofwildanimalactivityareas.

(8)Applicationinemergencies

Thelengthofresponsetimetoemergenciesaffectsthelossoftheeventtosomeextent.GPSandGISplayadecisiveroleintherapidresponsetoemergenciessuchasmedicalcare,firealarms,andtrafficaccidents.Generally,theestablishmentofanemergencyrescuesystemcanroughlyinclude:①Thevehicle-mountedpart.,CarryingGPSequipment,throughappropriatesoftwareprocessing,toobtaingeographiccoordinates;②Communicationsystem.Usingdual-channelcellularcommunicationnetwork,afteranincident,thedriveractivatesthehelp-seekingsystem,requestingthetypeofhelp(alarm,medical,etc.)andthedegreeofinjury;③Theprocessingcenter.Itcanreceiveusersignals,andwiththeassistanceofGIS,itcanquicklyinquireandlocatethelocationoftheincident,anddeterminethecorrespondingfirstaidmeasuresaccordingtothetypeofhelpinformationprovidedbytheuser.GISincludesdigitalmapsandattributeinformation,whichcanqueryandlocatethelocationofemergenciesondigitalmapsandtheselectionoftheshortestroutebydispatchers.GPScanfeedbackinformationsuchasthelocationofemergencyvehiclestothemaincontrolintime.Centerforscheduling.

(9)Applicationintourism

Withtheimprovementofpeople’squalityoflife,GPSservicesaregraduallyappliedForfieldsurveysinprimitiveforests,snow-cappedmountainsandvalleysorlargedesertareas,theGPSreceiverinstalledinthecarwillgivefullplaytoitsglobalpositioningfunctionandbecomethedriver'smostloyalguide.Onthewaytoexplorethescenerybycar,theridercanknowthelocationofthevehicleandthespeedanddirectionatanytime,soastoavoidlosingtheway.Evenifthereistroubleontheway,theGPSmonitoringservicecenterwillprovidetimelyinstructions,connecttothenearestrescueagency,andtakeactiveactions.ThroughthefriendlyremoteserviceprovidedbytheGPSmonitoringcenter,evenifthepassengerstravelthousandsofmiles,theystillfeelathome.Providingtravelrouteplanningisanimportantauxiliaryfunctionofcarnavigationsystems,includingautomaticrouteplanningandroutedesign.Automaticrouteplanningisautomaticallydesignedbycomputersoftwareaccordingtorequirements.Thebestdrivingrouteincludesthecalculationofthefastestroute,thesimplestroute,andtheroutewiththeleastnumberofpassesthroughthehighwaysection.Manualroutedesignisdesignedbythedrivertodesignthestartingpoint,endingpointandpassingpointsaccordingtohisdestination,andautomaticallyestablisharoutelibrary.

Theapplicationofglobalpositioningsystemalsoincludesaerialphotogrammetry,routesurveyandtunnelpenetrationsurvey,terrain,cadastralandrealestatesurvey,oceansurveyingandmapping,engineeringconstructionsurvey,bridgeconstructioncontrolnetworkestablishment,offshoreexplorationplatformsettlementmonitoring,Bridgedynamicreal-timedeformationmonitoring,real-timedeformationmonitoringofhigh-risebuildings.GPS'smoreefficientserviceswillbeappliedinvariousfieldsofproductionandlife.

TheapplicationofGPSinthelogisticsfieldismainlyreflectedinthefollowingaspects:

1.TheGPScomputerinformationmanagementsystemforcargotrackingcancollectreal-timedynamicinformationoftrains,locomotives,vehicles,containers,andtransportedgoodsthroughGPSandcomputernetworks,soastorealizethetrackingmanagementoflandandwatertransportgoods.Aslongasyouknowthetruckmodel,vehiclenumber,orshipnumber,youcanimmediatelyfindthetruckorshipfromtherailwaynetworkorwaterwaynetwork,knowwheretheyare,thedistancetothedestinationoftransportation,andtheinformationofallshipments.Theuseofthistechnologycangreatlyimprovetheaccuracyandtransparencyofoperationsandprovidehigh-qualityservicestoshippers.

2.Combinewithgeographicinformationsystem(GIS)tosolvelogisticsdistribution.Logisticsincludesordermanagement,transportation,warehousing,loadingandunloading,delivery,customsdeclaration,returnprocessing,informationservicesandvalue-addedservices.Thewholeprocesscontrolisthecoreissueoflogisticsmanagement.Suppliersmustcomprehensively,accuratelyanddynamicallygrasptheflowofproductsscatteredinvarioustransitwarehouses,distributors,retail,automobiles,trains,airplanes,shipsandothertransportationlinksacrossthecountry,andformulateproductionandsalesplansaccordingly.Adjustmarketstrategyintime.Therefore,forlarge-scalesuppliers,itisimpossibletoestablishaneffectivedistributionnetworkwithouttheentireprocessoflogisticsmanagement;forlarge-scalechainretailers,itisimpossibletoestablishasupplyanddistributionsystemwithouttheentireprocessoflogisticsmanagement;Three-partylogisticsserviceprovidersandwarehousinglogisticscenterscannotobtaincustomers’logisticsbusinesswithoutfull-processlogisticsmanagementservices;forordinaryusers,thereisnofast,accurate,safe,andreliablelogisticsdeliveryservice.Onlineprocurementisalmostimpossible.unimaginable.Theprocessoflogisticsdistributionismainlytheprocessoftransferringthespatiallocationofgoods.Intheprocessoflogisticsdistribution,itinvolvesbusinesslinkssuchastransportation,warehousing,loadingandunloading,anddeliveryofgoods.Theissuesinvolvedineachlinkincludethechoiceoftransportationroutesandthelocationofwarehouses.Effectivemanagementanddecision-makinganalysissuchastheselectionofwarehousecapacity,reasonableloadingandunloadingstrategies,transportationvehicleschedulinganddeliveryrouteselectionwillhelplogisticsdistributioncompaniestoeffectivelyuseexistingresources,reduceconsumption,andimproveefficiency.Infact,acarefulanalysisoftheproblemsineachoftheabovelinkscanrevealthattheaboveproblemsallinvolvegeographicelementsandgeographicdistribution.GIStechnologycanbeappliedtoallareasinvolvinggeographicdistribution,andGPS/GIStechnologyisanindispensablepartofthewholeprocessoflogisticsmanagement.