RadarEquationforpointTarget雷達(dá)氣象學(xué)講義臺(tái)大周仲島教授課件

Chapter3:PrincipleofRadarObservation

雷達(dá)系統(tǒng)與觀測(cè)原理Radarsystem:Transmitter,Receiver,Antenna,andDisplayofdataScatteringtheory,MiescatteringChapter3:PrincipleofRadarOThestabilizedlocaloscillator(STALO)generatesacontinuouswave(cw)signalofnearlyperfectsinusoidform(coherentsignal)whichismodulated(pulseonandoff脈衝觸發(fā)器

)andamplifiedbyaklystrontoproduceintensemicrowavepower.Klystron(調(diào)速管)amplifierisphasecoherentandMagnetron(磁控管)amplifierisphaseincoherentoscillator.Radarpulsesarephasecoherentfrompulsetopulseifthephaseangleforeachpulseisfixed.MOPA:masteroscillatorandpoweramplifier:TransmitterThestabilizedlocaloscillatoBlockdiagramofacoherentpulseradarThechoiceofthetransmittedfrequency{f0

}resultsfromacompromise,takingintoaccounttheconditionsofpropagationintheatmosphereandthetechnologicalrequirementsinvolvedinimplementingtheequipment.Theemittedwave:S(t)=Atcosω0tThereceivedsignal:Pr(t)=acos(ω0t+φ[t]),φ[t]=2π(2r/λ)(PRF)BlockdiagramofacoherentpuTransmittedfrequencyW-band,90-100GHz,0.33-0.3cm,forexample:a94GHzcloudradar,3.2mmwavelength,byLhermitte(1987)K-band,18-26.5GHz,1.6-1.13cm(H2Oabsorptionband22.3GHZsitsinthecenter,thuswasabolished,thenuseaboveandunderKbands),Ka-band,26.5-40GHz,1.13-0.75cm(USairforceforcloudobservations),Ku-band,12.5-18GHz,2.4-1.6cmX-band,8-12.5GHz,3.7-2.4cm(installedonboardB-17bomber)afterWW2donatedtometeorologicalinstitutionsintheworld(wavelength3.2cm,APQ13andAPS15)C-band,4-8GHz,7.5-3.7cmS-band,2-4GHz,15-10cmL-band,1-2GHz,30-15cmUHF,0.3-1GHz,1to0.3m(clearairbackscatteringmotion)VHF,0.1-0.3GHz,3to1mTransmittedfrequencyW-band,9BlockdiagramofacoherentpulseradarI(t)=A0cos(ωdt)Q(t)=A0sin(ωdt)OrE(t)=A0exp[iωdt]Forthepulsen,En=A0exp[iψr,n]withψr=tan-1[Q/I]Fortwosamplesnandn-1correspondingtotwosuccessivepulses,wehaveΔψr/Δt={ψr,n-ψr,n-1}/PRT=ωd

(PRF)BlockdiagramofacoherentpuRadarEquationforpointTarget雷達(dá)氣象學(xué)講義臺(tái)大周仲島教授課件Theelectrongunproducesaflowofelectrons.Thebunchingcavitiesregulatethespeedoftheelectronssothattheyarriveinbunchesattheoutputcavity.Thebunchesofelectronsexcitemicrowavesintheoutputcavityoftheklystron.Themicrowavesflowintothewaveguide,whichtransportsthemtotheaccelerator.Theelectronsareabsorbedinthebeamstop.InaklystronTheelectrongunproducesaflKlystrongalleryinStanfordU.KlystrongalleryinStanfordUStanfordUniversity-KlystrongalleryStanfordUniversity-KlystrongInamagnetronInamagnetronThenucleusofthehigh-voltagesystemisthemagnetrontube.Themagnetronisadiode-typeelectrontubewhichisusedtoproducetherequired2450MHzofmicrowaveenergy.Itisclassedasadiodebecauseithasnogridasdoesanordinaryelectrontube.Amagneticfieldimposedonthespacebetweentheanode(plate)andthecathodeservesasthegrid.Whiletheexternalconfigurationsofdifferentmagnetronswillvary,thebasicinternalstructuresarethesame.Theseincludetheanode,thefilament/cathode,theantenna,andthemagnetsThenucleusofthehigh-voltagRadarEquationforpointTarget雷達(dá)氣象學(xué)講義臺(tái)大周仲島教授課件SolidstatetransmitterSolidStateFMExiter/Amplifier/Transimittersfrom500wto20kwTube(ZeroBiastriode)type,longlifeeconomicalFMTransmitterfrom1kwto32.5kwSTLsystems(CompositeStudioTransmitterLinks)SolidStateAMTransimitterfrom250wattsto100kwSolidstatetransmitterSolidSSpectralpurity:theabsenceofpoweratfrequenciesotherthantheintendedones.發(fā)射訊號(hào)頻率純度對(duì)地面雜波去除以及大範(fàn)圍強(qiáng)回波區(qū)內(nèi)偵測(cè)弱回波訊號(hào)（龍捲）都非常重要。Groundcluttercancellation:suppressingechoesfromstationaryobjectsontheground.MicrowavepulsesPulserepetitiontime(frequency):PRT(PRF)Ts(fs)Pulseduration(pulsewidth)~1μs(definedasthetimebetweeninstanceswhenthepowerisone-quarterofthepeak).Listeningperiod:Ts-,receivingsignal,PowerdensityofanidealizedpulseS(r,,)U(t-r/c)，whereU(t-r/c)=1,r/ct(r/c+)andU(t-r/c)=0,otherwise.Spectralpurity:theabsenceoAntenna天線天線由輻射器(radiator)與反射器(reflector)組合而成輻射器是由波導(dǎo)管(wave-guide)以及波導(dǎo)管擴(kuò)展而成之喇叭口(hornfeed)組合而成經(jīng)放大之高能微波脈衝經(jīng)由波導(dǎo)管引至喇叭口放射至反射器上反射器為一拋物面之圓形天線,喇叭口位於此反射器之焦點(diǎn)上,產(chǎn)生窄波束之波鋒Antenna天線天線由輻射器(radiator)與反AntennaThereareseveralcriticalparametersthataffectanantenna'sperformanceandcanbeadjustedduringthedesignprocess.Theseareresonantfrequency,impedance,gain,apertureorradiationpattern,polarization,efficiencyandbandwidth.Transmitantennasmayalsohaveamaximumpowerrating,andreceiveantennasdifferintheirnoiserejectionproperties.AntennaThereareseveralcritResonantfrequencyTheresonantfrequencyisrelatedtotheelectricallengthoftheantenna.Thisisusuallythephysicallengthofthewiremultipliedbytheratioofthespeedofwavepropagationinthewire.Typicallyanantennaistunedforaspecificfrequency,andiseffectiveforarangeoffrequenciesusuallycenteredonthatresonantfrequency.ResonantfrequencyTheresonantImpedanceandSWRImpedanceissimilartorefractiveindexinoptics.Astheelectricwavetravelsthroughthedifferentpartsoftheantennasystem(radio,feedline,antenna,freespace)itmayencounterdifferencesinimpedance.Ateachinterface,somefractionofthewave'senergywillreflectbacktothesource,formingastandingwaveinthefeedline.Theratioofmaximumpowertominimumpowerinthewavecanbemeasuredandiscalledthestandingwaveratio(SWR).ASWRof1:1isideal.ASWRof1.5:1isconsideredtobemarginallyacceptableinlowpowerapplicationswherepowerlossismorecritical,althoughanSWRashighas6:1maystillbeusablewiththerightequipment.Minimizingimpedancedifferencesateachinterface(impedancematching)willreduceSWRandmaximizepowertransferthrougheachpartoftheantennasystem.ImpedanceandSWRImpedanceisAntennagain:dBiordBd.Itisimportanttonotethatantennagainisdifferentthanamplifiergain.Antennasdonothaveapowersourcethatallowstheantennatocreateadditionalenergytoboostthesignal.Anantennaissimilartoareflectivelensinprinciple-ittakestheenergyavailablefromthesourceandfocusesitoverawiderornarrowerarea.

Antennagainisthenameasureoftheamountoffocusthatanantennacanapplytotheincomingsignalrelativetooneoftworeferencedispersionpatterns.MaxStreamspecifiesallantennagainsindBi.

dBiistheamountoffocusappliedbyanantennawithrespecttoan"IsotropicRadiator"(adispersionpatternthatradiatestheenergyequallyinalldirectionsontoanimaginaryspheresurroundingapointsource).Thusanantennawith2.1dBiofgainfocusestheenergysothatsomeareasonanimaginaryspheresurroundingtheantennawillhave2.1dBmoresignalstrengththanthestrengthofthestrongestspotonthespherearoundanIsotropicRadiator.

dBdreferstotheantennagainwithrespecttoareferencedipoleantenna.Areferencedipoleantennaisdefinedtohave2.15dBiofgain.SoconvertingbetweendBianddBdisassimpleasaddingorsubtracting2.15accordingtotheseformulas:dBi=dBd+2.15dBd=dBi-2.15SpecifyingantennagainindBdmeansthattheantennainquestionhastheabilitytofocustheenergyxdBmorethanadipole.

Antennagain:dBiordBd.ItiIsotropicanddipoleantennaTheisotropicradiatorisapurelytheoreticalantennathatradiatesequallyinalldirections.Itisconsideredtobeapointinspacewithnodimensionsandnomass.Thisantennacannotphysicallyexist,butisusefulasatheoreticalmodelforcomparisonwithallotherantennas.Mostantennas'gainsaremeasuredwithreferencetoanisotropicradiator,andareratedindBi(decibelswithrespecttoanisotropicradiator).Thedipoleantennaissimplytwowirespointedinoppositedirectionsarrangedeitherhorizontallyorvertically,withoneendofeachwireconnectedtotheradioandtheotherendhangingfreeinspace.Sincethisisthesimplestpracticalantenna,itisalsousedasreferencemodelforotherantennas;gainwithrespecttoadipoleislabeledasdBd.Generally,thedipoleisconsideredtobeomnidirectionalintheplaneperpendiculartotheaxisoftheantenna,butithasdeepnullsinthedirectionsoftheaxis.Variationsofthedipoleincludethefoldeddipole,thehalfwaveantenna,thegroundplaneantenna,thewhip,andtheJ-pole.IsotropicanddipoleantennaThBeamWidthBecausehighergainantennasachievetheextrapowerbyfocusinginonasmallerareaitisimportanttorememberthatthegreaterthegain,thesmallertheareacoveredasmeasuredindegreesofbeamwidth(thinkofanadjustablebeamflashlight).Inmanycasesahighgainantennaisadetrimenttothesystemperformancebecausethesystemneedstohavereceptionoveralargearea.BeamWidthBecausehighergainDipoleantennaradiationpatternDipoleantennaradiationpatteIsotropicantenna(dBi),half-wavelengthdipoleantenna(actuallynogainantenna,dBd),omnivsdirectionalantenna,parabolicantennaThevastmajorityofantennasaresimpleverticalrodsaquarterofawavelengthlong.Suchantennasaresimpleinconstruction,usuallyinexpensive,andbothradiateinandreceivefromallhorizontaldirections(omnidirectional).Onelimitationofthisantennaisthatitdoesnotradiateorreceiveinthedirectioninwhichtherodpoints.Thisregioniscalledtheantennablindconeornull.Isotropicantenna(dBi),halfApertureistheshapeofthe"beam"crosssectioninthedirectionofhighestgain,andistwo-dimensional.(Sometimesapertureisexpressedastheradiusofthecirclethatapproximatesthiscrosssectionortheangleofthecone.)andradiationpatternApertureistheshapeofthe"BandwidthandpolarizationThebandwidthofanantennaistherangeoffrequenciesoverwhichitiseffective,usuallycenteredaroundtheresonantfrequency.Thebandwidthofanantennamaybeincreasedbyseveraltechniques,includingusingthickerwires,replacingwireswithcagestosimulateathickerwire,taperingantennacomponents(likeinafeedhorn),andcombiningmultipleantennasintoasingleassemblyandallowingthenaturalimpedancetoselectthecorrectantenna.Smallantennasareusuallypreferredforconvenience,butthereisafundamentallimitrelatingbandwidth,sizeandefficiency.ThepolarizationofanantennaororientationoftheradiowaveisdeterminedbytheelectricfieldorE-plane.Theionospherechangesthepolarizationofsignalsunpredictably,soforsignalswhichwillbereflectedbytheionosphere,polarizationisnotcrucial.However,forline-of-sightcommunications,itcanmakeatremendousdifferenceinsignalqualitytohavethetransmitterandreceiverusingthesamepolarization.Polarizationscommonlyconsideredarelinear,suchasverticalandhorizontal,andcircular,whichisdividedintoright-handandleft-handcircular.BandwidthandpolarizationTheAntennaefficiencyEfficiencyEfficiencyistheratioofpoweractuallyradiatedtothepowerputintotheantennaterminals.AdummyloadmayhaveaSWRof1:1butanefficiencyof0,asitabsorbsallpowerandradiatesheatbutnotRFenergy,showingthatSWRaloneisnotaneffectivemeasureofanantenna'sefficiency.Radiationinanantennaiscausedbyradiationresistancewhichcanonlybemeasuredaspartoftotalresistanceincludinglossresistance.Lossresistanceusuallyresultsinheatgenerationratherthanradiation,andtherefore,reducesefficency.OverviewofantennaparametersOftheparametersabove,SWRismosteasilymeasured.Impedancecanbemeasuredwithspecializedequipment,asitrelatestothecomplexSWR.Measuringradiationpatternrequiresasophisticatedsetupincludingsignificantclearspace(enoughtogetintotheantenna'sfarfield)orananechoicchamberdesignedforantennameasurements,carefulstudyofexperimentgeometry,andspecialisedmeasurementequipmentthatrotatestheantennaduringthemeasurements.Bandwidthdependsontheoveralleffectivenessoftheantenna,soalloftheseparametersmustbeunderstoodtounderstandbandwidth.However,typicallybandwidthismeasuredbyonlylookingatSWR,i.e.,byfindingthefrequencyrangeoverwhichtheSWRislessthanagivenvalue.Bandwidthoverwhichanantennaexhibitsaparticularradiationpatternmightalsobeconsidered.AntennaefficiencyEfficiencyTransmissionandreceptionAlloftheseparametersareexpressedintermsofatransmissionantenna,butareidenticallyapplicabletoareceivingantenna,duetoreciprocity.Impedance,however,isnotappliedinanobviousway;forimpedance,theimpedanceattheload(wherethepowerisconsumed)ismostcritical.Foratransmittingantenna,thisistheantennaitself.Forareceivingantenna,thisisatthe(radio)receiverratherthanattheantenna.Antennasusedfortransmissionhaveamaximumpowerrating,beyondwhichheating,arcingorsparkingmayoccurinthecomponents,whichmaycausethemtobedamagedordestroyed.Raisingthismaximumpowerratingusuallyrequireslargerandheaviercomponents,whichmayrequirelargerandheaviersupportingstructures.Ofcourse,thisisonlyaconcernfortransmittingantennas;thepowerreceivedbyanantennararelyexceedsthemicrowattrange.Antennasdesignedspecificiallyforreceptionmightbeoptimizedfornoiserejectioncapabilities.Thiscanbedonebyselectinganarrowbandwidthsothatnoisefromotherfrequenciesisrejected,orselectingaspecificradiationpatterntorejectnoisefromaspecificdirection,orbyselectingapolarizationdifferentfromthenoisepolarization,orbyselectinganantennathatfavorseithertheelectricormagneticfield.Forinstance,anantennatobeusedforreceptionoflowfrequencies(belowabouttenmegahertz)willbesubjecttobothmanmadenoisefrommotorsandothermachinery,andfromnaturalsourcessuchaslightning.Successfullyrejectingtheseformsofnoiseisanimportantantennafeature.Asmallcoilofwirewithmanyturnsismoreabletorejectsuchnoisethanaverticalantenna.However,theverticalwillradiatemuchmoreeffectivelyontransmit,whereextraneoussignalsarenotaconcern.TransmissionandreceptionAllParabolicantennaParabolicantennaRadarscanmodeElevationangleθeAzimuthangleφRangerHorizontaldistancedHeighthZenithzRadarscanmodeElevationangleVarioustypicalantennareflectorsandassociatedbeamsMostweatherradarsusesymmetricalbeamswithwidthsof1to3degrees.Narrowverticalandwiderhorizontalbeamsisusedtomeasureheightsandverticaldimensionsofsmalltargets.Searchradarfordetectingaircraftgenerallyusewideverticalbeams.Theprecisionofthemeasurementsofazimuthalandelevationanglestoatargetislargelyafunctionoftheshapeofthebeamproducedbytheantenna.Theshapeofthebeamisgovernedbytheshapeandsizeoftheantennareflectorandthewavelengthoftheradarset.VarioustypicalantennareflecReflectorandhornfeedReflectorandhornfeed3-dBbeamwidth當(dāng)脈衝離開(kāi)天線以天線直徑大小平行電磁波束行進(jìn)。由於繞射緣故，此一平行電磁波束在到達(dá)第一個(gè)繞射零點(diǎn)(距離為r=Da2/)後開(kāi)始發(fā)散成圓錐狀。雷達(dá)波束的束寬(beamwidth)定義為能流密度不小於最大值的一半之波束寬，或稱之為3-dB束寬。由於雷達(dá)天線發(fā)射出之雷達(dá)束的能流密度可以S(r,,)表示。天線無(wú)法將能量完全集中在一個(gè)窄束中，因此脈衝能流密度分佈呈現(xiàn)主瓣(mainlobe)和幾個(gè)副瓣(sidelobe)現(xiàn)象?，F(xiàn)有一般雷達(dá)設(shè)計(jì)，副瓣的能流密度沒(méi)有大於主瓣百分之一的。事實(shí)上，所有副瓣能流密度的總和僅佔(zhàn)主瓣的百分之幾而已。3-dBbeamwidth當(dāng)脈衝離開(kāi)天線以天線直徑大小平

天線-反射器經(jīng)常成拋物圓形,能流密度(輻射強(qiáng)度)與離束軸(beamaxis)距離()成[1-4(/Da)2]2的關(guān)係。在此情形下,常態(tài)化能流密度形狀(或稱為輻射形狀函數(shù)radiationpattern)與偏角的關(guān)係可寫(xiě)成：

?2()=S()/S(0)={8J2[(Dasin)/]/[(Dasin)/]2}2

其中為離束軸之偏角大小,為雷達(dá)發(fā)射電磁波波長(zhǎng),Da為天線直徑,J2是Besselfunctionofsecondorder。雷達(dá)發(fā)射之微波脈衝可視為厚度為c(脈衝寬)的球形殼以光速往外傳播並擴(kuò)張。因此,在一封閉球體內(nèi)所發(fā)射之脈衝能量（Pt’)為常數(shù),但是散射物所接收到的能流密度大小與距離平方成反比。此外由於天線本身,傳輸線,天線罩等造成的電磁波能量減損,進(jìn)入天線之脈衝能量Pt要大於實(shí)際發(fā)射出的Pt’。天線-反射器經(jīng)常成拋物圓形,能流密度(輻射強(qiáng)度)與AntennabeampatternAntennabeampatternBeampattern束形天線(雷達(dá)發(fā)射之微波脈衝)束形,脈衝之主瓣以及多個(gè)副瓣清楚可見(jiàn)。Beampattern束形天線(雷達(dá)發(fā)射之微波脈衝)束形BeampatternWithaparabolicreflectorandGaussianillumination(氣象常用)Widthofthebeamat3dB,1/2=70/DaWidthofthemainlobe(betweennulls)null=170/DaLeveloffirstsidelobe:-23dBbelowthevalueontheaxisBeampatternWithaparabolicr天線增益AntennaGainThemaximumdirectionalgainoftheantenna

gt’Sp/[Pt’/4r2]Pt’isdifficulttomeasure,sousuallyPt(thepowerdeliveredtotheantenna’sinputport)ismeasured.Spisthepeakpowerandmeasuredatdistancefar(r>2Da2/)fromtheantenna.Thus,thecomputedgaingtaccountsforlossesofenergyassociatedwiththeantennasystem(radomeandwaveguide).Theincidentradiationpowerdensityatrangerwithoutconsideringattenuationisgivenby

Si(,)=Ptgt?2(,)/4r2?2(,)isthenormalized[i.e.,?(,)=1at0,0]powergainpatternandgtistheantennapowergainalongthebeamaxis.天線增益AntennaGainThemaximumdSignalprocessor(receiver)Signalprocessor(receiver)Separatingcluttersignalandweathersignal:anexampleSeparatingcluttersignalandRadarEquationforpointTarget雷達(dá)氣象學(xué)講義臺(tái)大周仲島教授課件RadarEquationforpointTarget雷達(dá)氣象學(xué)講義臺(tái)大周仲島教授課件The(back)-scatteringcrosssection(反)散射截面Thecrosssectionofascatterer(hydrometeor)isanapparentareathatinterceptsapowerSiwhich,ifradiatedisotropically,producesatthereceiverapowerdensitySr=Si/4r2equaltothatscatteredbytheactualhydrometeor.注意:目標(biāo)物的散射截面並不等於目標(biāo)物的幾何截面ThebackscatteringcrosssectionbofasphericalwaterdropofdiameterDsmallcomparedto(I.e.,Rayleighscatteringorsmalldropscatteringapproximation,D/16)iswellapproximatedby

b(5/4)Km2D6，WhereKm=(m2-1)/(m2+2)andm=n-jnisthecomplexrefractiveindexofwater,nisrefractiveindex,istheattenuationindex.Someauthorsdefinetheabsorptioncoefficientk=n.The(back)-scatteringcrosssThescatteringphasefunction

P(θ)=(2/82)(D/)6Km2(1+cos2θ)Foθistheanglebetweentheincidentbeamandthescatteredbeam.ThescatteringphasefunctionMieandRayleighScattering米散射和雷利散射MieandRayleighScattering米散Complexrefractiveindex:iceandwater=10cmandlonger,Forwater,n=9(independentofT),k=0.63~1.47for20~0℃Forice,n=1.78(alsoindependentofT),k=2.4x10-3~5.5x10-4for0~-20℃Forwaterdrops,Km2

=0.91~0.93,

=1-10cm,indep.ofTForicespheres,Km2

=018,=0.917gcm-3,indep.ofTandNote,whenD~,b/[D2/4]=(m-1)2/(m+1)2

IfD<<b(ice)=0.20b(water)(-6.989dB)IfD~b(ice)=0.12b(water)(-9.2dB)Complexrefractiveindex:ice米散射Miescattering米散射Miescattering懸浮粒子散射的相函數(shù)型態(tài),硫酸氨粒子,相對(duì)溼度80%,入射電磁波(由圖左方)波長(zhǎng)為550nm.懸浮粒子散射的相函數(shù)型態(tài),硫酸氨粒子,相對(duì)溼度80%,UniversityofNorthDakotaradarTheradarisamodifiedEnterpriseElectronicsCorporationC-band(5cmwavelength)Dopplerradar.InJune1989,anewSigmetModelPPP-02pulse-pairsignalprocessorwasaddedtothesystem,replacingtheolderDopplersignalprocessor.Thepulse-pairprocessorprovideshighqualityreflectivity,Dopplerradialvelocity,andvelocityspectrum-widthmeasurementsatasmanyas1024rangegates.Thesystemiscapableofrecordingthesedatadigitallyaswellasdisplayingtheminreal-timeonanumberofcolordisplays.Theradarelectronicsandcomputersupportequipmentarehousedina48fttransportvan.Theradarelectronicsandcomputertrailer(REACT)isaself-containedresearchlaboratorywithenvironmentalcontrolsformaintainingtemperature,humidity,anddustlevels,providingacomfortableworkenvironmentforuptofourscientistsorengineersatthefieldprojectsite.REACTprovidesacomputerroomenvironmentforthesystem'sConcurrent3280XPcomputer,Ramtekdisplayprocessor,ChormaticsdisplaysystemsandtheRadarSystem's(RDS)HP100/A900computerequipment.Allelectronicsarepoweredfromacomputer-gradeisolatedandregulatedpowersystem.Thetotalsystemelectricalloadof75-kVAmaybesplitforconnectiontotwo50-kVAexternaldieselgeneratorsorcanbeoperatedusingcommercialpowersources.TheinteriorofREACTisdividedintofourcompartments:ananalysisarea,acomputerarea,aradaroperationsarea,andatransmitter/powerdistributionandregulationarea.Allcompartmentsareinterconnectedwithahigh-qualitycommunicationssystem,allowingtelephone,intercom,andVHFradiocontrol.Avoicerecordercanbeactivatedfromtheuser'sworkstations.Inadditiontothetrailerhousingtheradaranditsdisplayandprocessingunits,theUniversityalsohasastoragetrailerusedtotransportandstoresparepartsandotheritemsusedonfieldprojects.Theantennafortheradaroperatesinafiberglassradomeandismountedona50fttower.Dependinguponthesiterequirements,theradarhasbeenmountedaslowas20ftorashighas60ft.Technicalspecificationsaregivenonthetablesbelow:Antenna,Diameter3.66m12ftBeamwidth.99degreesGain43.75dBPolarization=Horizontal,Rotationrateupto24degrees/secTransmitter,BandCFrequency5.55GHzWavelength5.4cmPeakpower250KWPulselength.6&2PRF250-1200HzReceiver,Bandwidth.75MHz(SP)NoisePower-106.5dBmNoisefigure4dBA/Dconverter10bitCluttersuprression30,40dBNumberofgates1024GateSpacing25,50,75..MRadarConstant64.5dBZ(min)at50km-8.0dBZusinglongpulseUniversityofNorthDakotaradChapter3:PrincipleofRadarObservation

雷達(dá)系統(tǒng)與觀測(cè)原理Radarsystem:Transmitter,Receiver,Antenna,andDisplayofdataScatteringtheory,MiescatteringChapter3:PrincipleofRadarOThestabilizedlocaloscillator(STALO)generatesacontinuouswave(cw)signalofnearlyperfectsinusoidform(coherentsignal)whichismodulated(pulseonandoff脈衝觸發(fā)器

)andamplifiedbyaklystrontoproduceintensemicrowavepower.Klystron(調(diào)速管)amplifierisphasecoherentandMagnetron(磁控管)amplifierisphaseincoherentoscillator.Radarpulsesarephasecoherentfrompulsetopulseifthephaseangleforeachpulseisfixed.MOPA:masteroscillatorandpoweramplifier:TransmitterThestabilizedlocaloscillatoBlockdiagramofacoherentpulseradarThechoiceofthetransmittedfrequency{f0

}resultsfromacompromise,takingintoaccounttheconditionsofpropagationintheatmosphereandthetechnologicalrequirementsinvolvedinimplementingtheequipment.Theemittedwave:S(t)=Atcosω0tThereceivedsignal:Pr(t)=acos(ω0t+φ[t]),φ[t]=2π(2r/λ)(PRF)BlockdiagramofacoherentpuTransmittedfrequencyW-band,90-100GHz,0.33-0.3cm,forexample:a94GHzcloudradar,3.2mmwavelength,byLhermitte(1987)K-band,18-26.5GHz,1.6-1.13cm(H2Oabsorptionband22.3GHZsitsinthecenter,thuswasabolished,thenuseaboveandunderKbands),Ka-band,26.5-40GHz,1.13-0.75cm(USairforceforcloudobservations),Ku-band,12.5-18GHz,2.4-1.6cmX-band,8-12.5GHz,3.7-2.4cm(installedonboardB-17bomber)afterWW2donatedtometeorologicalinstitutionsintheworld(wavelength3.2cm,APQ13andAPS15)C-band,4-8GHz,7.5-3.7cmS-band,2-4GHz,15-10cmL-band,1-2GHz,30-15cmUHF,0.3-1GHz,1to0.3m(clearairbackscatteringmotion)VHF,0.1-0.3GHz,3to1mTransmittedfrequencyW-band,9BlockdiagramofacoherentpulseradarI(t)=A0cos(ωdt)Q(t)=A0sin(ωdt)OrE(t)=A0exp[iωdt]Forthepulsen,En=A0exp[iψr,n]withψr=tan-1[Q/I]Fortwosamplesnandn-1correspondingtotwosuccessivepulses,wehaveΔψr/Δt={ψr,n-ψr,n-1}/PRT=ωd

(PRF)BlockdiagramofacoherentpuRadarEquationforpointTarget雷達(dá)氣象學(xué)講義臺(tái)大周仲島教授課件Theelectrongunproducesaflowofelectrons.Thebunchingcavitiesregulatethespeedoftheelectronssothattheyarriveinbunchesattheoutputcavity.Thebunchesofelectronsexcitemicrowavesintheoutputcavityoftheklystron.Themicrowavesflowintothewaveguide,whichtransportsthemtotheaccelerator.Theelectronsareabsorbedinthebeamstop.InaklystronTheelectrongunproducesaflKlystrongalleryinStanfordU.KlystrongalleryinStanfordUStanfordUniversity-KlystrongalleryStanfordUniversity-KlystrongInamagnetronInamagnetronThenucleusofthehigh-voltagesystemisthemagnetrontube.Themagnetronisadiode-typeelectrontubewhichisusedtoproducetherequired2450MHzofmicrowaveenergy.Itisclassedasadiodebecauseithasnogridasdoesanordinaryelectrontube.Amagneticfieldimposedonthespacebetweentheanode(plate)andthecathodeservesasthegrid.Whiletheexternalconfigurationsofdifferentmagnetronswillvary,thebasicinternalstructuresarethesame.Theseincludetheanode,thefilament/cathode,theantenna,andthemagnetsThenucleusofthehigh-voltagRadarEquationforpointTarget雷達(dá)氣象學(xué)講義臺(tái)大周仲島教授課件SolidstatetransmitterSolidStateFMExiter/Amplifier/Transimittersfrom500wto20kwTube(ZeroBiastriode)type,longlifeeconomicalFMTransmitterfrom1kwto32.5kwSTLsystems(CompositeStudioTransmitterLinks)SolidStateAMTransimitterfrom250wattsto100kwSolidstatetransmitterSolidSSpectralpurity:theabsenceofpoweratfrequenciesotherthantheintendedones.發(fā)射訊號(hào)頻率純度對(duì)地面雜波去除以及大範(fàn)圍強(qiáng)回波區(qū)內(nèi)偵測(cè)弱回波訊號(hào)（龍捲）都非常重要。Groundcluttercancellation:suppressingechoesfromstationaryobjectsontheground.MicrowavepulsesPulserepetitiontime(frequency):PRT(PRF)Ts(fs)Pulseduration(pulsewidth)~1μs(definedasthetimebetweeninstanceswhenthepowerisone-quarterofthepeak).Listeningperiod:Ts-,receivingsignal,PowerdensityofanidealizedpulseS(r,,)U(t-r/c)，whereU(t-r/c)=1,r/ct(r/c+)andU(t-r/c)=0,otherwise.Spectralpurity:theabsenceoAntenna天線天線由輻射器(radiator)與反射器(reflector)組合而成輻射器是由波導(dǎo)管(wave-guide)以及波導(dǎo)管