通信传播技术设计(1)

RadioScience,Volume,Number,Pages1??,SimulationstudiesofGPSradiooccultationmeasurementsG.Beyerle1,M.E.Gorbunov2,C.O.Ao3G.Beyerle,GeoForschungsZentrumPotsdam(GFZ),Division1,KinematicsandDy-namicsoftheEarth,Telegrafenberg,D-14473Potsdam,Germany(e-mail:[e-mail])M.E.Gorbunov,InstituteforAtmosphericPhysics,Moscow,Russia(e-mail:[e-mail])C.O.Ao,JetPropulsionLaboratory,CaliforniaInstituteofTechnology,Pasadena,USA1GeoForschungsZentrumPotsdam(GFZ),Potsdam,Germany2InstituteforAtmosphericPhysics,Moscow,Russia3JetPropulsionLaboratory,CaliforniaInstituteofTechnology,Pasadena,USADRAFTOctober18,2002,1:36pmDRAFT2BEYERLEETAL.:RADIOOCCULTATIONSIMULATIONSAbstract.TheatmosphericpropagationofGPSsignalsundermultipathconditionsandtheirdetectionaresimulated.UsingthemultiplephasescreenmethodC/A-codemodulatedL1signalsarepropagatedthroughaspheri-callysymmetricrefractivityfieldderivedfromahigh-resolutionradiosondeobservation.ThepropagatedsignalsaretrackedbyGPSreceiversimplementedinsoftwareandconvertedtorefractivityprofilesbythecanonicaltransformtechniqueandtheAbelinversion.Ignoringnoiseandassuminganidealre-ceivertrackingbehaviourthetruerefractivityprofileisreproducedtobet-terthan0.1%ataltitudeabove2km.Thenon-idealcaseissimulatedbyaddingbetween14and24dBofGaussianwhitenoisetothesignalandtrackingthesignalwithreceiversoperatingat50and200Hzsamplingfrequencyusingtwodifferentcarrierphasedetectors.Intheuppertroposphereandstrato-spherethereceivermodelsusedinthisstudyyieldcomparablerefractivityprofiles.However,inthemid-tropospheredowntoaltitudesofabout2kmaCostas-typephase-lockedlooptrackinginducesnegativerefractivitybiasesontheorderof¡1to¡2%at50Hzsamplingfrequency.Modificationstothereceivertrackingalgorithmsignificantlyimprovestheretrievalresults.Inparticular,replacingtheCostas-loop’stwo-quadrantphaseextractorwithafour-quadrantdiscriminatorreducestherefractivitybiasesbyafactorof5;increasingthesamplingfrequencyfrom50to200Hzgainsanotherfactorof2.DRAFTOctober18,2002,1:36pmDRAFTBEYERLEETAL.:RADIOOCCULTATIONSIMULATIONS31.IntroductionAtmosphericsoundingsoftemperatureandwatervaporbyGlobalPositioningSys-tem(GPS)radiooccultation(RO)measurementsareincreasinglybeingconsideredfornumericalweatherpredictionandclimatechangestudies.To-dateadatasetofmorethan100,000temperatureprofileshasbeencollectedbytheproof-of-conceptGPS/Meteorology(GPS/MET)mission[Wareetal.,1996]aswellasthecurrentCHAMP[Reigberetal.,2000,2002]andSAC-Cmissions.FirstvalidationstudiesbasedonCHAMPobservationsindicatethattheobservedtemperaturebiaswithrespecttoEuropeanCentreforMedium-RangeWeatherForecasts(ECMWF)globalanalysesislessthan1Kabovethetropopauseandlessthan0:5Kbetween12to20kmatmidandhighlatitudes[Wickertetal.,2001].ThesevaluesareconsistentwithvalidationresultsofGPS/METdata[Rockenetal.,1997].However,inthelowertroposphereatmidandlowlatitudesanegativerefractiv-itybiasofmorethan1%isobservedinpastandcurrentROdata[seee.g.,Rockenetal.,1997;Marquardtetal.,2001].Thenegativebiasiscommonlyattributedtothereceivers’inadequatesignaltrackingbehaviorwithinzonesofmultipathsignalpropa-gation[seee.g.,Gorbunov,2002b].Thesemultipathzonesarecausedbycomplicatedstructuresinthetroposphericrefractivityfieldgeneratedbyspatialvariationsofthewatervapordistribution.Theanalysisofradiooccultationdataaffectedbymul-tipathpropagationisextensivelydiscussedintheliterature[Gorbunovetal.,1996;KarayelandHinson,1997;Hockeetal.,1999].Recently,thecanonicaltransform(CT)methodwasdevisedtosolvetheproblemofcalculatingbendinganglepro-DRAFTOctober18,2002,1:36pmDRAFT4BEYERLEETAL.:RADIOOCCULTATIONSIMULATIONSfilesfromphaseandamplitudedataobservedwithinmultipathregions[Gorbunovetal.,2000;Gorbunov,2002a,b].CTprocessingofalargenumberofGPS/METandCHAMPobservations,however,didnotsucceedinremovingtherefractivitybiassug-gestingthatthequalityofthelowertroposphericdataisdegraded.Thisconclusioniscorroboratedbyaslidingspectralanalysis[Gorbunov,2002b].Thepresentstudyinvestigatestheobservedrefractivitybiasusingend-to-endsim-ulations.TheatmosphericpropagationofaC/A-codemodulatedGPSsignalismodelled.Forsimplicityonlytheneutralatmosphereisconsideredinthefollowing,dispersionduetoionosphericpropagationisnottakenintoaccount.GPSsignalde-modulationisperformedbymultiplicationwithaC/A-codereplica.Inmultipathzones,however,thesignalcannotbedemodulatedunambiguouslysincemultiplein-terferingraysmayhavedifferentphasedelaysandDopplershifts.Theobjectiveofthepresentstudyistoinvestigatetheinfluenceofthereceivertrackingprocessonretrievedamplitudeandphasedataandexploretheirrelationtothenegativerefractivitybias.Thepaperisorganizedintotwoparts:first,themethodsemployedinthesimulationchainareintroducedanddescribed.BrieflyreviewedarethemultiplephasescreenmethodwhichisusedtosimulateGPSsignalpropagationthroughtheatmosphereandtheCTmethodwhichtransformstheamplitudeandphasedataintobendingangleprofiles;thereceivertrackingmoduleisdiscussedinmoredetail.Second,thesimulationstudiesaredescribedandtheresultsarediscussed.DRAFTOctober18,2002,1:36pmDRAFTBEYERLEETAL.:RADIOOCCULTATIONSIMULATI