Radar Essentials AConciseHandbookforRadarDesign andPerformanceAnalysis • Principles • Equations • Data G.RichardCurry Raleigh,NC scitechpub.com PublishedbySciTechPublishing,Inc. 911PaverstoneDrive,SuiteB Raleigh,NC27615 (919)847-2434,fax(919)847-2568 scitechpublishing.com Copyright©2011bySciTechPublishing,Raleigh,NC.Allrights reserved. Nopartofthispublicationmaybereproduced,storedinaretrieval systemortransmittedinanyformorbyanymeans,electronic, mechanical,photocopying,recording,scanningorotherwise,exceptas permittedunderSections107or108ofthe1976UnitedStated CopyrightAct,withouteitherthepriorwrittenpermissionofthe Publisher,orauthorizationthroughpaymentoftheappropriate per-copyfeetotheCopyrightClearanceCenter,222RosewoodDrive, Danvers,MA01923,(978)750-8400,fax(978)646-8600,oronthe webatcopyright.com.RequeststothePublisherforpermissionshould beaddressedtothePublisher,SciTechPublishing,Inc.,911Paverstone Drive,SuiteB,Raleigh,NC27615,(919)847-2434,fax(919) 847-2568,[email protected]. Thepublisherandtheauthormakenorepresentationsorwarranties withrespecttotheaccuracyorcompletenessofthecontentsofthis workandspecificallydisclaimallwarranties,includingwithout limitationwarrantiesoffitnessforaparticularpurpose. Editor:DudleyR.Kay ProductionManager:RobertLawless Typesetting:MPSLimited,aMacmillanCompany CoverDesign:BrentBeckley Printer:Docusource Thisbookisavailableatspecialquantitydiscountstouseaspremiums andsalespromotions,orforuseincorporatetrainingprograms.For moreinformationandquotes,pleasecontactthepublisher. PrintedintheUnitedStatesofAmerica 10 9 8 7 6 5 4 3 2 1 ISBN:978-1-61353-007-8 Preface Iclosemyradarcoursesandtutorialswitha35-pointsummary.Itellthe studentsthatiftheyknowthese“KeyRadarFacts”,orhavethemhandy, they can contribute productively to any radar discussion. These points, expandedbyaddingtablesandchartswithkeyradardata,werepublished asthePocketRadarGuide[1]. Thenextstepwastomeettheneedforacompendiumofradarinfor- mationessentialfordesignandperformanceanalysis.Itshouldcombine elementsofaradartextandahandbook.Itshouldbecomprehensiveto serveasacompleteradarreference,yetcompactenoughtofitonanen- gineer’sdeskortravelwithhim.Itshouldcontainthedataandequations mostusefultopracticingradarengineers,yetalsomaterialtohelpradar non-expertsunderstandandusetheinformation. Thisisthatbook.Itcontainsbasicprinciplesofradardesignandanal- ysis,characteristicsofthemajorradarcomponents,keyradarequations, and tables and charts with the most-used radar performance data. The readercangodirectlytothetopicofinterest,wherereferencesarepro- videdtootherhelpfulsections.Itisintendedasareferenceforradarand aerospaceengineersandsystemanalysts,andprovidesahandydesktop sourceaswellasanessentialtravelingcompanion. Detailed discussions, derivations, examples, and design details be- yondthescopeofthisbookcanbefoundinthenewPrinciplesofModern Radar[2],thelatesteditionofRadarHandbook[3],andAirborneRadar [4].UsefulanalysistechniquesaregiveninRadarSystemPerformance Modeling[5],andRadarSystemAnalysisandModeling[6].(Manyofthe figuresinthisbookwereproducedusingthecustomradarfunctionsfor Excelspreadsheetsprovidedwith[5]).RadartermsaredefinedinIEEE Standard Radar Definitions [7]. Additional references to specific areas aregiveninthetext. I would like to acknowledge helpful discussions, encouragement, andsupportofDudleyKay,PresidentofScitechPublishing,Inc.,andthe manyconstructivesuggestionsfromJohnMilanandtheotherreviewers ofthisbook. Contents Preface vii 1 Radar Basics 1 1.1 RadarConceptandOperation 1 1.2 RadarFunctions 2 1.3 TypesofRadars 3 1.4 FrequencyBands 6 1.5 MilitaryNomenclature 7 1.6 RadarConfigurations 9 2 Radar Subsystems 12 2.1 Antennas 12 2.2 Transmitters 19 2.3 Receivers 22 2.4 Transmit/ReceiveModules 26 2.5 SignalandDataProcessing 28 3 Radar Performance 31 3.1 RadarCrossSection(RCS) 31 3.2 Signal-to-NoiseRatio 37 3.3 Detection 42 3.4 Search 54 3.5 Measurement 59 3.6 Tracking 65 4 Radar Environment 69 4.1 AtmosphericLosses 69 4.2 RainLoss 73 vi Contents 4.3 AtmosphericRefraction 74 4.4 TerrainMaskingandMultipath 76 4.5 RadarClutter 79 4.6 IonosphericEffects 84 4.7 ElectronicCountermeasures(ECM) 88 5 Radar Techniques 95 5.1 Waveforms 95 5.2 MovingTargetIndication(MTI)andDisplaced Phase-CenterArray(DPCA) 100 5.3 PulseDopplerandSpace-TimeAdaptive Processing(STAP) 102 5.4 Synthetic-ApertureRadar(SAR) 106 5.5 Classification,Discrimination, andTargetIdentification 108 6 Computation Aids 113 6.1 UnitsandConversionFactors 113 6.2 Constants 114 6.3 Decibels 115 Symbols 118 Glossary 123 References 125 Index 129 Radar Basics 1 1.1 Radar Concept and Operation ThewordRADARisanacronymforRAdioDetectionAndRanging. Radar employs electromagnetic propagation of energy directed to- ward and reflected from targets. Electromagnetic energy propagates in theatmosphereandfreespaceinastraightlinewithpropagationvelocity c=3×108m/s,(Sec.6.2),exceptfor: • Reflection from metallic objects and those having a dielectric constantdifferentthanthatoffreespace.Reflectionfromtargets isthebasisforradaroperation(Sec.3.1);reflectionfromterrain and other objects may interfere with radar operation (Sec. 4.5 and4.7). • Refraction,thebendingofthepropagationpathduetodifferences inthepropagationvelocity.Refractionduetotheatmosphereand theionospherearecommonradareffects(Sec.4.3and4.6). • Diffraction,thebendingofthepropagationpatharoundtheedgeof anobject.Thiseffectisusuallynotsignificantatradarfrequencies. Inthebasicradarconcept(Fig.1.1),electromagneticenergyisgen- erated by a transmitter and radiated by the transmitting antenna in the directionofthetarget.Someoftheenergyreflectedbythetargetiscol- lectedbythereceivingantennaandprocessedinthereceivertoproduce informationaboutthetarget.Thiscaninclude: • Targetpresence,indicatedbyasignalreturnlargerthantheback- ground. • Targetrange,foundfromtheround-trippropagationtime,t.For monostaticradars,(Sec.1.3),thetargetrange, Risgivenby ct R = (1.1) 2 1 2 Chapter1 RadarBasics Target Velocity α Transmitter Radial Velocity Receiver Range Target Data Fig.1.1 Basicradarconcept • TargetradialvelocityV ,thecomponentofthetargetvelocity,V, R inthedirectionoftheradarformonostaticradars(Sec.1.3). V = V cos α (1.2) R where α is the angle between the target velocity vector and the radarline-of-sight(LOS),(Fig.1.1).ItisfoundfromtheDoppler- frequencyshiftofthereceivedsignal, f ,by D f c f λ V = D = D (1.3) R 2f 2 where f istheradarfrequencyandλisthewavelength. • Target direction, found from the antenna beam orientation for maximumsignalreturn. • Targetcharacteristics,foundfromthemagnitudeandfeaturesof thesignalreturn,suchasfluctuationcharacteristics,duration,and spectrumfeatures. 1.2 Radar Functions Inmostapplications,radarsperformoneormoreofthefollowingfunc- tions. • Search(alsocalledsurveillance),theexaminationofavolumeof spaceforpotentialtargets(Sec.3.4). • Detection,determiningthatatargetispresent(Sec.3.3). • Position measurement of target range, angular coordinates, and sometimesradialvelocity(Sec.3.5). 1.3 TypesofRadars 3 • Tracking,processingsuccessivemeasurementstoestimatetarget path(Sec,3.6). • Imaging,generatingatwo(orthree)dimensionalimageofatarget orarea,frequentlyusingsynthetic-apertureprocessing(Sec.5.4). • Classification,discriminationandidentification,determiningthe characteristics,type,andidentityofatarget(Sec.5.5). While many radars perform two or more functions in their normal operatingmodes,thetermmulti-functionradarisappliedtoradarsthatare effectiveinawidevarietyoffunctions,andusuallycanhandlemultiple targets.Theseradarsusuallyhavethefollowingcharacteristics: • Phasedarrayantennas,forrapidlydirectingbeamstothedesired angularpositions(Sec.2.1). • Multiple waveform types, for performing the various functions (Sec.5.1). • Digitalsignalprocessing,tohandlethevarietyofwaveformsand performthedesiredfunctionsonmultipletargets(Sec.2.5). • Computercontrol,toscheduletransmissionandsignalreception responsivetotheinformationneedsandthetargetconfiguration (Sec.2.5). 1.3 Types of Radars Monostatic radars have the transmit and receive antennas at the same location.Theirseparationissmallcomparedwiththetargetrange.This allowsbothantennastoviewthesamevolumeofspace.Inmanycasesa singleantennaisswitchedbetweentransmitandreceivefunctions.The radar can be located at a single site allowing simple transmit-receive coordination. The round-trip signal transmission time provides a direct measureoftargetrange(Eq.1.1),andtheDoppler-frequencyshiftgives ameasureoftargetradialvelocity(Eq.1.3). Bistatic radars have transmit and receive antennas that are widely separated.Thisavoidsinterferencebetweenthetransmitterandreceiver andallowsmultiplepassivereceiverstooperatewithasingletransmitter. Thetargetreflectionforsomebistaticgeometriesmaybeadvantageous (Sec. 3.1). The signal path consists of the range between the transmit antenna and the target, R , and the range between the target and the T receiveantenna, R .Thissumdefinesanellipsoidwiththetransmitand R receive antennas as the foci (Fig. 1.2). It can be found from the time 4 Chapter1 RadarBasics Target Locus of R + R T R α α T R R R T R V Transmitter Receiver Fig.1.2 Bistaticradartarget-measurementgeometry betweensignaltransmissionandreception: R + R =ct (1.4) T R TheDopplerfrequencyshiftisgivenby: V(cos α +cos α ) f = T R (1.5) D λ whereα istheanglebetweenthetargetvelocityvectorandthetransmit T LOS,andα isheanglebetweenthetargetvelocityvectorandthereceive R LOS(Fig.1.2). Pulsedradarstransmitapulseandthenlistenforthereflectedpulse return.Thisavoidsinterferencebetweenthetransmitterandreceiver,and enablesuseofasingleantennaforbothtransmitandreceive.Thetime frompulsetransmissiontopulsereceptioncanreadilybemeasured,al- lowingthetargetrangetobefound(formonostaticradars).Sincemono- staticradarscannotreceiveuntilthepulsetransmissionisfinished,the minimumrange, R ,oftheseradarsislimitedto: M cτ R = (1.6) M 2 whereτisthepulseduration.Whenpulsesaretransmittedatafixedpulse repetition interval (PRI, which is the reciprocal of the pulse repetition frequency, PRF), signal returns arriving after the next pulse has been transmitted may be interpreted as a return from a later pulse, giving an 1.3 TypesofRadars 5 erroneous range. This will occur when the target range is greater than nc(PRI/2),andiscalledasecond-timearound-returnwhenn =1,anda multiple-timearound-returnwhenn >1. Continuous-wave(CW)radarstransmitacontinuoussignalandre- ceivesimultaneously.Thetargetradialvelocitycanbedirectlymeasured fromtheDoppler-frequencyshift(formonostaticradars).Ifthefrequency of the continuous wave is changed (frequency-modulated continuous wave,FMCW),thetargetrangecanalsobemeasured.Interferencebe- tween the transmitter and receiver may limit the power, and hence the sensitivity,ofmonostaticCWradars. Coherentradarstransmitwaveformsderivedfromastablefrequency source(oftencalledtheSTALO),whichisalsousedinprocessingthere- ceivedsignals.Thisallowsmeasurementoftargetradialvelocity(Sec.1.1), coherentpulseintegration(Sec.3.2),andrejectionofbackgroundclutter bymoving-targetindication(MTI),pulseDoppler,andspace-timeadap- tiveprocessing(STAP)techniques(Sec5.2and5.3).Noncoherentradars lackthesecapabilities. Over-the-horizon(OTH)radarsutilizeionosphericreflectiontodi- rectradarwaveswellbeyondthenormalradarhorizon(Sec.4.4).They operate in the high-frequency (HF) band (3–30 MHz), (Sec. 1.4). The frequencyisselectedtoaccommodateionosphericconditionsandtarget range. Very large antennas, high-power transmitters, and long process- ing times are required. The range and angle measurement accuracy is relativelypoor,butmaybesuitableforwarning. Secondary-surveillanceradars(SSR)transmitpulsesfromarotating antennawithsufficientpowertoreachacooperatingaircraft.Theseair- craftemployatransponderthatrepliestothegroundstationatadifferent frequency. The responses contain coded pulses that supply information suchastargetaltitudeandidentification.Thistechniqueprovideslong- range operation with the low power needed for one-way transmission, andavoidsradarclutterreturnbyusingdifferentup-anddown-linkfre- quencies. The military system, which uses classified response codes, is calledidentificationfriendorfoe(IFF),(Sec.5.5). Synthetic aperture radars (SAR) transmit a series of pulses as the radarplatform(aircraftorsatellite)movesalongitsflightpath.There- turnsignalsareprocessedtoproducetheeffectofaverylargeaperture havingverysmallbeamwidth.This,alongwithgoodrangeresolution,al- lowsgenerationoftwo-dimensionalimagesofterrainandgroundtargets (Sec.5.4).