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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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
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