RCS meausre.docx
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RCS meausre.docx
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RCSmeausre
August2005
RCSMeasurementsinaCompactRange
byJeffFordham,VicePresident,andMarionBaggett,SoftwareEngineeringManager,
MITechnologies
Introduction
TheRadarCrossSection(RCS)ofanobjectisdefinedas,"theareainterceptingthatamountofpowerwhich,whenscatteredisotropically,producesareturnattheradarequaltothatfromthetarget."Insimplerterms,RCSistheprojectedareaofaspherethathasthesameradarreturnasthetarget.Theunitofmeasureforanobject'sRCSis"decibelspersquaremeter,"ordBsm.Thepowerreceivedbyaradarforatargetindicateshowwelltheradarcandetectortrackthattarget.Forthisreason,muchresearchandefforthasbeenputintoreducingthe"signature"ofvariousaircraft,shipsandotherobjects.SeeFigure1foraschematicanddefinitionofRCS.
Figure1:
TheDefinitionofRCS.
Thetaskofcreatingdesignsbasedupontheoreticalmodelingandsimulationmustbeprovenintheend.WithRCSdesigns,thisinvolvestakingtheobjecttoanRCSmeasurementfacilityandmeasuringtheradarreturnoftheobject.TherearemanytypesofRCSmeasurementtechniquesandRCSmeasurementrangepossibilities.CompactrangesareoneofthemostpopularmethodsformeasuringtheRCSofvariousobjects.CompactRanges(CR)havetheadvantagesofindoortestinginacontrolledenvironment,directwhole-bodymeasurementsofthetargetandRFsignalsecurityifthecompactrangeisinashieldedenclosure.SeeFigure2forapictureofaCRusedforRCSmeasurements.
Figure2:
CompactRangeReflectorandFeedlocatedattheInstitutoNacionaldeTecnicaAeroespacial(INTA)inSpain.ThisCRiscapableoftestingtheRCSofobjectsupto5.5meterswideby5meterstall.
Inshort,aCompactRangeconsistsofalargeoffsetfedreflectorandafeedtoilluminatethereflector.Onceproperlyaligned,aCRwillproduceaplane-wavezonewithminimalphasetaper,minimalamplituderippleandapproximately0.5to1.0dBamplitudetaperoverthedesignedplane-wavezone.Formoreinformationonthedesignandcapabilitiesofcompactranges,thereadershouldconsulttheliteraturebaseregardingcompactranges.ThisarticlewillfocusinsteadonconsiderationsinvolvedformakinggoodRCSmeasurementsinaCRRCSmeasurementfacility.
RFMeasurementConsiderations
TherearemanydesignchallengesinvolvedinRCSrangesthataffectsystemperformance.
Criticaldesignfactorsinclude:
•ClutterRejectionorReduction-Targetresponseandreturnsfromtherangemustbeseparated.Thesereturnscaninclude:
objectsintherange,suchaswallsandtargetmount;interactionofthetargetwiththeseobjectsandanyunwantedsignalsintherangeitself.SeeFigure3foranillustrationofclutterinameasurement.
Figure3:
IllustrationofClutterandLeakagesourcesinaCRvs.time.
•DynamicRange-Areceiverwithhighdynamicrange(ontheorderof85to90db)isrequiredtoisolatelargetargetresponsesfromclosesmallerones.Thisrequirementisparticularlytrueforwholebodyevaluations.SomecomplextargetscanexhibitchangesinRCSonthisorderoverafewhundredthsofadegree.SeeFigure4foratypicaldynamicrangemeasurement.
Figure4:
DynamicRangeTestdatacollectedontheMI-1797receiverat10GHz:
Inputsignalsetatthe.1dBcompressionpointfortheMI-1797(toptrace)
TheotherfourtracesarewiththeTransmitandReceivecables
terminatedinaloadandaveragingsetat1,8,32and1024samples
•FrequencyCoverageandSwitchingSpeed-AnRCSinstrumentationsystemusuallyrequiresawideoperatingbandwidth.Manytestfrequenciesmayberequired.Afastswitchingspeedwillgreatlyincreaserangethroughputbyallowingmanyfrequenciestobecollectedinasingletargetrotation.
Figure5:
RCSvs.Angle
•MeasurementCapabilities-Atarget'sRCSisafunctionoftargetposition,frequency,polarizationandotherfactors.Someofthemorecommonmeasurementsare:
RCSvs.Angle(SeeFigure5)
›RCSvs.CrossRange(SeeFigure6)
›RCSvs.anglevs.frequency(SeeFigure7)
›RCSvs.RangeAxisvs.Angle(SeeFigure8)
›RCSimaging(SeeFigure9)
›RCSPowerSpectrum(Dopplermeasurements)
›Polarimetricmeasurements
›RCSvs.Bi-staticangle
•Ringdown-CharacterizedbyRFenergystoragewithinthesystem,ringdownhassignificantimpactuponoverallmeasurementperformance.ThetwomostsignificantcontributorstoringdownarerelatedtotheimpulseresponseoftheCRfeedsandenergyreflectionsbetweencomponentconnectionsduetoimpedancemismatch.Whenanenergypulseistransmittedtoanantenna,theVSWRatthefeedreflectssomeenergybacktothetransmittedoutputoftheradar.TheVSWRatthatpointwillreflectsomeoftheenergybacktothefeed.Thecyclecontinuesuntilalltheenergyisradiatedorabsorbedbythecable.SeeFigure10showingmeasuredreceivedpowervs.timeforanexampleoffeedringdowntime.Inthefigure,notethatbetweenthe30nsecand180nsecpoints,thereflectionoftheCRrisesabovetheringdownandVSWR.
Figure6:
RCSvs.CrossRange
•Singlevs.DualFeeds-ThepreferredtechnicalapproachimplementsadualfeedsysteminordertominimizeRCSperformancerisk.Singlefeedsystemshaveasomewhatlowercomplexity,buthavenotabletechnicaldrawbacks.Withsinglefeedsystems,therearealimitedamountofpracticalmodificationstobemadeinordertoreduceringing.Primaryreductionstypicallyinvolvecharacterizationoffeedringingperformance,potentiallyleadingtoexpensiveandtimeconsumingfeeddesignchanges.Also,feedringingcharacterizationinitself,isaninvolvedandhighlytechnicalactivitywhichmustbefactoredintoprogramrisk.Dualfeedsystems,offerasignificantincreaseintransmittoreceiveisolationattheexpenseofincreasedsystemcomplexityincontrollingthereceivegatingpathway.
Figure7:
RCSvs.FrequencyandAngle
•Sensitivity-TypicalRangespecificationsof-80dBsmprovideachallengingrequirementandrequiresignificantattentiontodetail.AdetailedRCSlinkbudgetforacompactrangesystemutilizingtheradarrangeequationisrequired.SubtledesignimplementationssuchasminimizationofRFcablelengths,selectionofappropriateRFcabletype,utilizationofreceiverintegrationgainandprudentmaximizingofdutycycletominimizeassociatedlosshavetobeconsidered.Individuallyeachofthesedesignelementsmaynotseemcritical,butwhenviewedinthecompositetheyformasignificantcontributiontooverallsystemsensitivity.
•IsolationAndMeasurementRepeatability-Tomeeta-80dBsmrequirementwillrequirecareinselectingRFcomponents.Selectingadualfeedconfigurationwithitsbetterisolationcombinedwithhighcomponentswitchisolationprovidesformorerobustsystemperformance.MITechnologies'provengatingsystemcascadesmultipleelementstoachievesignificantimprovementinisolation,upto170dB.
Attentiontodetailinthermalpackagingandproximityplacementofcomponentsisrequired.Backgroundsubtractionofartifactsandnoiseisonlypossibleifconsistentandrepeatablemeasurementscanbeachieved.FocuseddesignattentiontothermalheatsinksandforcedaircoolingofactiveRFcomponentsaswellasselectionofultraphasestablecablesaresignificantkeystoimprovingbackgroundsubtraction.
Figure8:
RCSvs.RangeAxisvs.Angle.(a)AngularScanAxisshownasX-Axis.
(b)AngularScanAxisshownalongPolarAxis
•RangeThroughput&HumanFactors-Considerationforthroughputandhumanfactorsarerequiredtosupportbothproductionandengineeringdevelopmentprojects.AdequateconsiderationmustbegiventooptimizingthroughputofproductionantennasandeaseofconversionbetweentheAntenna-RCSconfigurations.Hightechnicalperformancewithoutconsiderationforthepracticalday-to-dayuseoftherangeisnotacceptable.Severalkeyaspectsofthesystemarchitecturemustbeconsideredtoensureefficientconfigurationchanges.Aheavy-dutyquick-changearbormountdesignandmanliftcanbeutilizedtoimprovehumanfactors.
Figure9:
RCSofthreesphereswithanRCSof-60dBsmontopofafoamcolumn.
(a)RCSofSpheresandColumn.(b)RCSoftheSpheresafterusingBackground
Subtractiontoremoveinfluenceofthefoamcolumn.
ApplicableRCSLinkBudgetEquation
Inordertomakehighqualitymeasurementswithrepeatableresults,acarefulexaminationoftheexpectationonreceivedpowermustbemade.ThiscarefulexaminationismadebyreviewingtheexpectedlinkbudgetoftheRCSmeasurementrange.
ThelinkbudgetsarederivedfromtheRCSrangeequation,whichisshownbelow.
Where:
Pr=Receivepower
Pt=Transmitpower
G=Antennagain
s=RCS
l=Wavelength
N=IntegrationGain
ktB=Boltzman'sConstanttimesReceiverBandwidth
Lt=LossesbetweenTransmitterandFeed
Lr=LossesbetweenFeedandLNA
Lg=GatingLoss
NF=LNANoiseFigure
R=RangeLength(EqualtoCRFocalLength)
Theradarrangeequationassumesthatthepotentialerrorsourcesassociatedwiththefeed'sisolation,ringdownandVSWRareinconsequential.Therangeequationalsoassumesthattherearenoresidualreflectionsfromotheritemsinthechamberpresentwithinameasuredpulsewindow.InaphysicallyrealizableRCSrange,therearepracticallimitationstospecificdevicesthatcancauseperformancetobelessthanindicatedbytheRCSrangeequation.
Figure10:
PlotofReceivedPowervs.Timeshowingsystemringdown.TheX-axisisrangedelayinnsecandtheY-axisisrelativereceivepower.Thisplotwascreatedbyshiftingthereceivegateintimeandplottingthereceivedpower.
TypicalEquipmentandtheMISolution
MITechnologies'approachforth
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