RCS-计算平板或圆球的手算公式

0.093m1m1m44in0.093mSmallFlatplateRCS=1m2at10GHzor0.01m2at1GHzFlatPlateRCS=14,000m2at10GHzor140m2at1GHz(1.13m)*Seecreepingwavediscussionforexceptionwhen88Rangeand88rSphere=BBr2FFF=4BBw2h2/882SphereRCS=1m2IndependentofFrequency*FlatPlate4-11.1Figure1.ConceptofRadarCrossSectionFigure2.RCSvsPhysicalGeometryRADARCROSSSECTION(RCS)Radarcrosssectionisthemeasureofatarget'sabilitytoreflectradarsignalsinthedirectionoftheradarreceiver,i.e.itisameasureoftheratioofbackscatterpowerpersteradian(unitsolidangle)inthedirectionoftheradar(fromthetarget)tothepowerdensitythatisinterceptedbythetarget.TheRCSofatargetcanbeviewedasacomparisonofthestrengthofthereflectedsignalfromatargettothereflectedsignalfromaperfectlysmoothsphereofcrosssectionalareaof1masshowninFigure1.2TheconceptualdefinitionofRCSincludesthefactthatnotalloftheradiatedenergyfallsonthetarget.Atarget’sRCS(F)ismosteasilyvisualizedastheproductofthreefactors:FF=ProjectedcrosssectionxReflectivityxDirectivity.RCS(F)isusedinSection4-4foranequationrepresentingpowerreradiatedfromthetarget.Reflectivity:Thepercentofinterceptedpowerreradiated(scattered)bythetarget.Directivity:Theratioofthepowerscatteredbackintheradar'sdirectiontothepowerthatwouldhavebeenbackscatteredhadthescatteringbeenuniforminalldirections(i.e.isotropically).Figures2and3showthatRCSdoesnotequalgeometricarea.Forasphere,theRCS,FF=BBr,2whereristheradiusofthesphere.TheRCSofasphereisindependentoffrequencyifoperatingatsufficientlyhighfrequencieswhere88Range,and88radius(r).Experimentally,radarreturnreflectedfromatargetiscomparedtotheradarreturnreflectedfromaspherewhichhasafrontalorprojectedareaofonesquaremeter(i.e.diameterofabout44in).Usingthesphericalshapeaidsinfieldorlaboratorymeasurementssinceorientationorpositioningofthespherewillnotaffectradarreflectionintensitymeasurementsasaflatplatewould.Ifcalibrated,othersources(cylinder,flatplate,orcornerreflector,etc.)couldbeusedforcomparativemeasurements.Toreducedragduringtests,towedspheresof6,14or22diametermaybeusedinsteadofthelarger44sphere,andthereferencesizeis0.018,0.099or0.245mrespectivelyinsteadof1m.Whensmallersizedspheresareusedfortestsyou22maybeoperatingatornearwhere8-radius.Iftheresultsarethenscaledtoa1mreference,theremaybesome2perturbationsduetocreepingwaves.Seethediscussionattheendofthissectionforfurtherdetails.FLATPLATECYLINDERTILTEDPLATECORNERSPHEREFmax=Br2Fmax=4BL4238Fmax=4Bwh22288Fmax=2Brh288Fmax=12BL4288Fmax=15.6BL42388LLLSameasaboveforwhatreflectsawayfromtheplateandcouldbezeroreflectedtoradarFmax=8Bwh22288DihedralCornerReflectorSPHEREFLATPLATECORNER360EPattern±90EPattern±60EPatternRELATIVEMAGNITUDE(dBsm)4-11.2Figure3.BackscatterFromShapesFigure4.RCSPatternsInFigure4,RCSpatternsareshownasobjectsarerotatedabouttheirverticalaxes(thearrowsindicatethedirectionoftheradarreflections).Thesphereisessentiallythesameinalldirections.TheflatplatehasalmostnoRCSexceptwhenaligneddirectlytowardtheradar.ThecornerreflectorhasanRCSalmostashighastheflatplatebutoverawiderangle,i.e.,over±60E,thereturnfromacornerreflectorisanalogoustothatofaflatplatealwaysbeingperpendiculartoyourcollocatedtransmitterandreceiver.Targetssuchasshipsandaircraftoftenhavemanyeffectivecorners.Cornersaresometimesusedascalibrationtargetsorasdecoys,i.e.cornerreflectors.Anaircrafttargetisverycomplex.Ithasagreatmanyreflectingelementsandshapes.TheRCSofrealaircraftmustbemeasured.Itvariessignificantlydependinguponthedirectionoftheilluminatingradar.1000sqm100101BEAMBEAMNOSETAILEEEEEPr'PtGtGr82F(4B)3R4R2BT'PtGtFPjGj4BPr'PtGtGr82F(4B)3R4'PjGjGr82(4BR)288SJ4-11.3Figure5.TypicalAircraftRCSFigure5showsatypicalRCSplotofajetaircraft.Theplotisanazimuthcutmadeatzerodegreeselevation(ontheaircrafthorizon).Withinthenormalradarrangeof3-18GHz,theradarreturnofanaircraftinagivendirectionwillvarybyafewdBasfrequencyandpolarizationvary(theRCSmaychangebyafactorof2-5).Itdoesnotvaryasmuchastheflatplate.AsshowninFigure5,theRCSishighestattheaircraftbeamduetothelargephysicalareaobservedbytheradarandperpendicularaspect(increasingreflectivity).ThenexthighestRCSareaisthenose/tailarea,largelybecauseofreflectionsofftheenginesorpropellers.Mostself-protectionjammerscoverafieldofviewof+/-60degreesabouttheaircraftnoseandtail,thusthehighRCSonthebeamdoesnothavecoverage.Beamcoverageisfrequentlynotprovidedduetoinadequatepoweravailabletocoverallaircraftquadrants,andthesideofanaircraftistheoreticallyexposedtoathreat30%ofthetimeovertheaverageofallscenarios.Typicalradarcrosssectionsareasfollows:Missile0.5sqm;TacticalJet5to100sqm;Bomber10to1000sqm;andships3,000to1,000,000sqm.RCScanalsobeexpressedindecibelsreferencedtoasquaremeter(dBsm)whichequals10log(RCSinm).2Again,Figure5showsthatthesevaluescanvarydramatically.Thestrongestreturndepictedintheexampleis100min2thebeam,andtheweakestisslightlymorethan1minthe135E/225Epositions.TheseRCSvaluescanbeverymisleading2becauseotherfactorsmayaffecttheresults.Forexample,phasedifferences,polarization,surfaceimperfections,andmaterialtypeallgreatlyaffecttheresults.Intheabovetypicalbomberexample,themeasuredRCSmaybe