Section 7 Automatic Position DeterminationWord文档格式.docx
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Section 7 Automatic Position DeterminationWord文档格式.docx
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InertialPositioning
ApairofgyrosareincorporatedinapositionfixingdeviceknownastheAuto-SurveyormanufacturedbyLittonSystems.Ithasoriginatedfromanaircraftnavigationsystem,whichwasdevelopedformilitaryapplications,intoitspresentthree-dimensionalsurveysystem.'
Hardware'
consistsof(i)theinertialmeasuringunit,(ii)acomputer,(iii)acassetterecorder,(iv)adisplayandcontrolunitand(v)thepowersupply.Theseitemscanbeplacedanywherewithinaroadvehicleorhelicopterprovidingthattheoperatoriscloseto(iv).
Themeasuringunitcontainsagyro-stabilizedplatformwithtwoair-bearinggyroscopesinfourgimbalmountingswhichkeepitsthreeorthogonalaxesorientedinspaceinanorth-east-downwardsrelationship.Eachaxisalsohasasensor-torquertypeofaccelerometerwhichdefinesaccelerationintheparticulardirectionviaaquantitizersystemfeedingthecomputer.Thisispre-programmedtocomputethesurveyasitiscarriedoutandtocontrolthesystem(seeFig.1).
Beforestartingthesurveyanalignmentdrilliscarriedoutattheoriginstation.Thisissaidtohaveadurationofaboutonehour(thatforthesurveyisrestrictedtosomefourhoursorso)andtheplatformlevelsitselfwithrespecttothelocalverticalandalignsitsnorthingaxiswiththelocalmeridiansothattheaccelerometersareinthedirectionsofnorth,eastanddown.SimultaneouslythecomputermonitorsthebiasesofthesystemandevaluatestheinitialconditionsforaKalmanfilterprogramwhichisincludedinthe'
software'
forthepurposeofadjustmentofoutputs.TheKalmanfilterenablesanevaluationoftheperformanceofthesystembycomparisonoftheerrorsarisingduringthesurveytoaprioridatarelatingtothestatisticalnatureoferrorswhichnavigationinstrumentsarelikelytocontain.
Whenthealignmentiscompletetheknowncoordinatesandelevationofthestationarefedinbytheoperatorandthetraversecanbegin.Theorthogonalmovementsaresensedbytheaccelerometersand,atintervalsofsixteenmilliseconds,thesignalstothecomputeraredoublyintegratedintodistancesofdisplacementwithrespecttotheorigin.IntheAuto-Surveyor'
z-increments'
accumulateaselevationdifferences,whilsttheothertwoincrementsareusedtocomputevaluesofgeodeticlatitudeandlongitudeonthepre-programmedreferencespheroid.Thecomputeralsodirectstheplatformgimbaltorquestoplacetheplatformtangentialtothespheroidwiththenorthaxiscorrectlyoriented.
Atapproximatelyfourminuteintervalsthevehicleishaltedanda'
zerovelocityupdating'
iscarriedout.Thistakesaboutthirtysecondsandtheplatformisre-leveled,withrespecttolocalvertical,andeffectivelyresets'
verticalzero'
.Asmentionedabove,previouscalculationsandplatformtorquingarewithrespecttothereferencespheroidsothattheamountoftorqueusedforthere-levelingisameasureofthedirectionalchangeofdeviationfromthevertical.Sincethevehicleisstationary,theaccelerometersshouldreadzeroasshouldthevelocitiesbyintegrationsothatwhentheKalmanfilterscrutinizesthesevaluesitcanestablishaccelerometererrorsduetotheirdrift,whichisnon-linear,anditusesthevelocityerrorstoestablishtherateofchangeofdrift:
preciseintegrationisnotpossibleafterfourminutesduetothatdrift.Platformre-levelingtakesplaceaftertheadjustment.
Thesurveynowcontinueswithsuccessiveupdatingstopsandalsowithhaltsatthesurveystationswherethesameprocessoccurs.Attheterminalstationknowncoordinatesandelevationsareenteredasupdatinginformationandazerovelocityupdateiscarriedout;
theKalmanfiltercarriesoutasmoothingadjustmentsoastodistributeanymisclosuresbackthroughthemeasuredpoints.Themeasurementdataandresultsofthesmoothingareregisteredontherecorder.
Traverselinesarenormallymeasuredintwodirections,occupyingthesamestations,discrepanciesthenindicatingeffectsofgyrodriftonplatformalignment.Standarderrorsof±
0.20minpositionofpointsat10kmseparationareachievablewithasimilarvalueforheightingwhenaroadvehicleisused.
FerrantimanufactureacorrespondingsystemintheUK,andreferencecanbemadetoareportintheNewCivilEngineer,17January1980,onitsvaluetothehighwayengineer.ThisreportrefersspecificallytotheirInertialRoadSurveyoralthoughitindicatesthatsystemsforlandsurveys,boreholesurveysandunderwatersurveysareavailabletoo.
GPSINNAVIGATIONALAPPLICATIONS
INTRODUCTION
GlobalPositioningSystem(GPS)hasforseveralyearsbeenabuzzwordforprofessionalsinmanyfieldsincludingsurveying,geodesy,GIS,meteorology,andgeodynamics.ThereasonforthisGPSwonderperhapsliesinthesuperiorcapabilityofGPS:
itofferssolutionstomanyproblemsthatwecouldnotorfeltdifficulttosolve,andalsoenablesustodomanythingsbetterthanbefore.Navigationisoneofthesethings,whichhasbeengreatlychangedfromthedevelopmentofGPS.
ThispaperwillprovideanoverviewofGPSasappliedtonavigation.ItwillfirstdescribebrieflytheprinciplesofGPS.ThedifferentGPSbasedpositioningmethodsinnavigationwillthenbediscussed,followedbyanreviewofGPSbasedsystemsforair,landandmarinenavigation.
PRINCIPLESOFGPSPOSITIONING
GPSisasatellitebasedpassivepositioningsystemthatwasinitiallydesignedprimarilyformilitaryuse.ItwasdevelopedandhasbeenmaintainedbytheUnitedStatesDepartmentofDefense(USDoD).Thesystemisnowusedbyboththemilitaryandcivilianuserstoobtainhighaccuracyposition,velocityandtimeinformation,24hoursaday,underallweatherconditions,andanywhereintheworld.Thesystemwasdeclaredtoreachtherequirementofaninitialoperationalcapability(IOC)inDecember1993andfulloperationalcapability(FOC)inApril1995.
TheComponentsofGPS
OnecommonwaytolookatGPSistoresolveitintothreesegments:
ThespacesegmentreferstoGPSsatellitesthatareorbitingatanaltitudeofabout20,200kmabovetheearthsurface.ThefulloperationalcapacityofGPSisachievedwith24activesatellites.Therearecurrently27operationalsatellites,threeofthataretheactivesparesthatcanbeusedasreplacementswhentheactivesatellitesareoutofservices.ThekeycomponentsinaGPSsatellitearetheantennassendingandreceivingsignals,twolargewingscoveredwithsolarcellstogeneratepowerforthesatellitetoconsume,andatomicclocksthatareaccuratetoabout1secondin3,000,000years.
Thecontrolsegmentconsistsof5monitorstations,3groundantennas,and1mastercontrolstation.Themonitorstationspassivelytrackallsatellitesinview,accumulatingrangingdata.Thetrackeddataareprocessedatthemastercontrolstationtodeterminesatelliteorbitsandtoupdateeachsatellite'
sNavigationMessage.Theupdatedinformationistransmittedtoeachsatelliteviathegroundantennas.
TheusersegmentisanybodywhohasaGPSreceiver.Thesurveyors,thenavigatorsandtheGISdatacollectorsareexamplesoftheusers.
ThesignalsthatGPSsatellitessendoutconsistoftwocodes,thecoarseacquisition(C/A)codeandtheprecise(P)code,andaNavigationMessage.TheGPScodesarejustlikeaseriesof1'
sand0'
sthatarearrangedintocertainsequences,Figure1.TheC/AcodeisusedfortheStandardPositioningService(SPS)availabletoallusers.Theserviceoffersapositionalaccuracyofabout100mhorizontallyand156mverticallyatthe95%probabilitylevel.ThePcodeisusedforthePrecisePositioningService(PPS)andcanbeaccessedonlybyauthorizeduserssuchastheUSmilitaryanditsallies.Theserviceprovidesapositionalaccuracyofabout15mhorizontallyand25mverticallyatthe95%probabilitylevel.
TheGPSNavigationMessagecontainssuchinformationastheorbitalelementsofthesatellites,clockbehavior,andanalmanacthatgivestheapproximatedataforeachactivesatellite.TwocarrierfrequenciesonL-band,L1andL2areusedtocarrythesignalsdescribedabove.L1hasawave-lengthofabout19cm(1575.42MHz)andL2awave-lengthofabout24cm(1227.60MHz).BothL1andL2aremicrowavefrequenciesandcanpenetratetheatmosphere.L1carriesboththeC/AandthePcodesandL2thePcodeonly.TheNavigationMessageiscarriedonbothofthetwofrequencies.
Tothemoresophisticateduserssuchasthesurveyors,positioningusingthecodeinformationcannotfulfilltheiraccuracyrequirements,sayatt
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