毕业外文资料原文与翻译测绘类Word格式文档下载.docx

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AutomaticPositionDetermination

1PositionDetermination

Thetraditionalmethodforthedirectdeterminationofpositionhasbeenbyastronomicalobservation.Therelativepositionsandmovementsofthestarsaswellcataloguedandsowithacombinationofaltitude,directionandtimeobservationstothestars,thepositionintermsoflatitudeandlongitudeofagroundstationcanbecalculated.Thereiflesscallfordirectpositiondeterminationnowadayssincemostcountriesarecoveredbyaprimaryhorizontalcontrolscheme,andtheabsolutepositionofanynewlocalsurveywordcanbeestablishedbyincludinganationalreferencepointinthelocalsurvey.

Inveryremoteareas,orwheretheterrainistotallyunsuitablefortheclassicsurveymethodsoftriangulationandtraverse,thereis,however,aneedfordirectpositiondetermination.Forthemajorityofthiswork,visualobservationtothestarshasbeenreplacedbyelectromagneticmeasurementstoorfromartificialearthsatellites.Inaddition,therelativepositionsofsurveystationscanbedetermineddirectlybyinertialtechniquesoriginallydevelopedforaircraftnavigation.

1.1InertialPositioning

ApairofgyrosareincorporatedinapositionfixingdeviceknownastheAuto-SurveyormanufacturedbyLittonSystems.Ithasoriginatedfromanaircraftnavigationsystem,whichwasdevelopedformilitaryapplicationsintoitspresentthree-dimensionalsurveysystem.‘Hardware’consistsof（i）theinertialmeasuringunit,（ii）acomputer,（iii）acassetterecorder,（iv）adisplayandcontrolunitand（v）thepowersupply.Theseitemscanbeplacedanywherewithinaroadvehicleorhelicopterprovidingthattheoperatoriscloseto（iv）.

Themeasuringunitcontainsagyro-stabilizedplatformwithtwoair-bearinggyroscopesinfourgimbalmountingswhichkeepitsthreeorthogonalaxisorientedinspaceinanorth-east=downwardsrelationship.Eachaxisalsohasasensor-torquertypeofacclerometerwhichdefinesaccelerationintheparticulardirectionviaaquantitizersystemfeedingthecomputer.Thisispre-programmedtocomputethesurveyasitiscarriedoutandtocontrolthesystem（seeFig.1）

Beforestaringthesurveyanalignmentdrillcarriedoutattheoriginstation.Thisissaidtohaveadurationofaboutonehour（thatforthesurveyisrestrictedtosomefourhoursorso）andtheplatformlevelsitselfwithrespecttothelocalverticalandalignsitsnothingaxiswiththelocalmeridiansothattheaccelerometersareinthedirectionsofnorth,eastanddown.SimultaneouslythecomputermonitorsthebiasesofthesystemandevaluatestheinitialconditionsforaKalmanfilterenablesanevaluationoftheperformanceofthesystembycomparisonoftheerrorsarisingduringthesurveytoaprioridatarelatingtothestatisticalnatureoferrorswhichnavigationinstrumentsarelikelytocontain.

Whenthealignmentiscompletetheknowncoordinatesandelevationofthestationarefedinbytheoperatorandthetraversecanbegin.Theorthogonalmovementsaresensedbytheaccelerometersand,atintervalsofsixteenmilliseconds,thesignalstothecomputeraredoublyintegratedintodistancesofdisplacementwithrespecttotheorigin.IntheAuto-Surveyor‘z-increments’accumulateaselevationdifferences,whilsttheothertwoincrementsareusedtocomputevaluesofgeodeticlatitudeandlongitudeonthepre-programmedreferencespheroid.Thecomputeralsodirectstheplatformgimbaltorquestoplacetheplatformtangentialtothespheroidwiththenorthaxiscorrectlyoriented.

Atapproximatelyfourminuteintervalsthevehicleishalteda‘zerovelocityupdating’iscarriedout.Thistakesaboutthirtysecondsandtheplatformisre-leveled,withrespecttolocalvertical,andeffectivelyresets’verticalzero‘.Asmentionedabove,previouscalculationsandplatformtorquingarewithrespecttothereferencespheroidsothattheamountoftorqueusedforthere-levelingisameasureofthedirectionalchangeofdeviationfromthevertical.Sincethevehicleisstationary,theaccelerometersshouldreadzeroasshouldthevelocitiesbymtegrationsothatwhentheKalmanfilterscrutinizesthesevaluesitcanestablishaccelerometererrorsduetotheirdrift,whichisnon-linear,anditusesthevelocityerrorstoestablishtherateofchangeofdrift;

preciseintegrationisnotpossibleafterfourminutesduetothatdrift.Platformre-levelingtakesplaceaftertheadjustment.

Thesurveynowcontinueswithsuccessiveupdatingstopsandalsowithhaltsatthesurveystationswherethesameprocessoccurs.Attheterminalstationknowncoordinatesandelevationsareenteredasupdatinginformationandazerovelocityupdateiscarriedout;

theKalmanfiltercarriesoutasmoothingadjustmentdataandresultsofthesmoothingareregisteredontherecorder.

Traverselinesarenormallymeasuredintwodirections,occupyingthesamestations,discrepanciesthenindicatingeffectsofgyrodriftonplatformalignment.Standarderrorsof±

0.20minpositionofpointsat10kmseparationareachievablewithasimilarvalueforheightingwhenaroadvehicleisused.

FerrantimanufactureacorrespondingsystemintheUK,andreferencecanbemadetoareportintheNewCivilEngineer,17January1980,onitsvaluetothehighwayengineer.ThisreportrefersspecificallytotheirInertialRoadSurveyoralthoughitindicatesthatsystemsforlandsurveys,boreholesurveysandunderwatersurveysareavailabletoo.

2GPSINNAVIGATIONALAPPLICATIONS

2.1INTRODUCTION

GlobalPositioningSystem（GPS）hasforseveralyearsbeenabuzzwordforprofessionalsinmanyfieldsincludingsurveying,geodesy,GIS,meteorology,andgeodynamics.ThereasonforthisGPSwonderperhapsliesinthesuperiorcapabilityofGPS:

itofferssolutionstomanyproblemsthatwecouldnotorfeltdifficulttosolve,andalsoenablesustodomanythingsbetterthanbefore.Navigationisoneofthesethings,whichhasbeengreatlychangedfromthedevelopmentofGPS.

ThispaperwillprovideanoverviewofGPSasappliedtonavigation.ItwillfirstdescribebrieflytheprinciplesofGPS.ThedifferentGPSbasedpositioningmethodsinnavigationwillthenbediscussed,followedbyanreviewofGPSbasedsystemsforair,landandmarinenavigation.

2.2PRINCIPLESOFGPSPOSITIONING

GPSisasatellitebasedpassivepositioningsystemthatwasinitiallydesignedprimarilyformilitaryuse.ItwasdevelopedandhasbeenmaintainedbytheUnitedStatesDepartmentofDefense（USDoD）.Thesystemisnowusedbyboththemilitaryandcivilianuserstoobtainhighaccuracyposition,velocityandtimeinformation,24hoursaday,underallweatherconditions,andanywhereintheworld.Thesystemwas1993andfulloperationalcapability（FOC）inApril1995.

2.2.1TheComponentsofGPS

OnecommonwaytolookatGPSistoresolveitintothreesegments:

ThespacesegmentreferstoGPSsatellitesthatareorbitingatanaltitudeofabout20,200kmabovetheearthsurface.ThefulloperationalcapacityofGPSisachievedwith24activesatellites.Therearecurrently27operationalsatellites,threeofthataretheactivesparesthatcanbeusedasreplacementswhentheactivesatellitesareoutofservices.Thekeycomponentsinsatellitearetheantennassendingandreceivingsignals,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）andcanbiaccessedonlybyauthorizeduserssuchastheUSmilitaryanditsallies.Theserviceprovidesapositionalaccuracyofabout15mhorizontallyand25mverticallyatthe95%probabilitylevel.

TheGPSNavigationMessagecontainssuchinformationastheorbitalelementsofthesatellites,clockbehavior,andanalmanacthatgivestheapproximatedataforeachactivesatellite.TwocarrierfrequenciesonL-band,L1andL2areusedtocarrythesignalsdescribedabove.L1hasawave-lengthofabout19cm（1575.42MHz）andL2awave-lengthofabout24cm（1227.60MHz）.BothL1andL2aremicrowavefrequenciesandcanpenetratetheatmosphere.L1carriesboththeC/AandthepcodesandL2thepcodeonly.TheNavigationMessageiscarriedonbothofthetwofrequencies.

Tothemoresophisticateduserssuchasthesurveyors,positioningusingthecodeinformationcannotfulfilltheiraccuracyrequirements,sayatthecentimeterormillimeterlevel.Inthiscase,theL1orL2,orbothL1andL2carrierphasesarealsoobservedandusedforpositioning.

2.2.2TheWorkingPrinciplesofGPS

GPSmeasurespositionsbymeasuringdistances.GPSsatelliteshaveknownorbits