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62168-6831/13/$31.00©2013IEEEieeeGeoscienceandremotesensinGmaGazinemarch2013Abstract—SyntheticApertureRadar(SAR)hasbeenwidelyusedforEarthremotesensingformorethan30years.Itprovideshigh-resolution,day-and-nightandweather-independentimagesforamultitudeofapplica-tionsrangingfromgeoscienceandclimatechangeresearch,environmentalandEarthsystemmonitoring,2-Dand3-Dmapping,changedetection,4-Dmapping(spaceandtime),security-relatedapplicationsuptoplanetaryexploration.Withtheadvancesinradartechnologyandgeo/bio-phys-icalparameterinversionmodelinginthe90s,usingdatafromseveralairborneandspacebornesystems,aparadigmshiftoccurredfromthedevelopmentdrivenbythetech-nologypushtotheuserdemandpull.Today,morethan15spaceborneSARsystemsarebeingoperatedforinnumer-ousapplications.ThispaperprovidesfirstatutorialabouttheSARprinciplesandtheory,followedbyanoverviewofestablishedtechniqueslikepolarimetry,interferometryanddifferentialinterferometryaswellasofemergingtech-niques(e.g.,polarimetricSARinterferometry,tomographyandholographictomography).Severalapplicationexam-plesincludingtheassociatedparameterinversionmodelingareprovidedforeachcase.Thepaperalsodescribesinno-vativetechnologiesandconceptslikedigitalbeamform-ing,Multiple-InputMultiple-Output(MIMO)andbi-andmulti-staticconfigurationswhicharesuitablemeanstoful-filltheincreasinguserrequirements.ThepaperconcludeswithavisionforSARremotesensing.I.IntroductIonSyntheticApertureRadar(SAR)hasenteredintoagoldenage.Morethan15spaceborneSARsensorsarebeingoperatedtodayand10newSARsystemswillbelaunchedwithinthenext5years.SARisuniqueinitsimagingcapability:Itprovideshigh-resolutiontwo-dimensionalimagesindependentfromdaylight,cloudATutorialonSyntheticApertureRadaralbertomoreira,PauPratS-iraola,marwanYouniS,GerhardKrieGer,irenahajnSeK,andKonStantinoSP.PaPathanaSSiouMicrowavesandRadarInstituteoftheGermanAerospaceCenter(DLR),GermanyDigitalObjectIdentifier10.1109/MGRS.2013.2248301Dateofpublication:17April2013©ŠDLR7march2013ieeeGeoscienceandremotesensinGmaGazinecoverageandweatherconditions[1]–[9].ItispredestinedtomonitordynamicprocessesontheEarthsurfaceinareliable,continuousandglobalway.SARsystemshaveaside-lookingimaginggeometryandarebasedonapulsedradarinstalledonaplatformwithaforwardmovement.Theradarsystemtransmitselectromagneticpulseswithhighpowerandreceivestheechoesofthebackscatteredsignalinasequentialway.TypicalvaluesforthepulserepetitionfrequencyrangefromafewhundredtoafewthousandHertzforairborneandspacebornesystems,re-spectively.Theswathwidthvariestypicallyfromafewkilometersto20kmintheairbornecaseandfrom30to500kminthespacebornecase.ThetransmittedpulseinteractswiththeEarthsurfaceandonlyaportionofitisbackscatteredtothereceivingantennawhichcanbethesameasthetransmitantenna(foramonostaticra-dar)oradifferentone(forabi-ormulti-staticradar).Theamplitudeandphaseofthebackscatteredsignaldependsonthephysical(i.e.,geometry,roughness)andelectri-calproperties(i.e.,permittivity)oftheimagedobject.Dependingonthefrequencyband,considerablepen-etrationcanoccursothattheimagedobjectsandmediamustbemodeledasavolume(e.g.,vegetation,iceandsnow,drysoil).Morepenetrationoftheelectromagneticpulsesinmediawilloccurforradarsystemsusinglongerwavelengthswhichusuallyhaveanaccentuatedvolumecontributioninthebackscatteredsignal.CommonlyusedfrequencybandsinSARsystemsandtheassociatedwave-lengthrangesareshowninTable1.Thesimplestradarsystemprovidesa2-Dreflectivitymapoftheimagedarea,i.e.,targetswithhighbackscatteredsignalareidentifiedasbrightspotsintheradarimagesandflatsmoothsurfacesasdarkareas.Theflightdirectionisdenotedasazimuthandtheline-of-sightasslantrangedirection.Untilthe50simagingradarsweredenotedasSLAR(side-lookingairborneradar)anddidnotusetheprincipleofthesyntheticaperture.Thisledtoamoder-ateazimuthresolutionwhichdeterioratesastherangeincreases.Forexample,anX-bandSLARsystemwitha3-meterantennadahasanazimuthantennabeamwidthof..d3003001mmrad,aamH===(1)wheremisthewavelength.Theazimuthresolutionadisgivenbythesmallestseparationbetweentwopointtargetsthatcanbedetectedbytheradar.IntheSLARcasethisisgivenbytheilluminationlengthoftheazimuthantennabeamontheground.Assumingarangedistance,,r0fromtheantennatothetargetsof5kmyields..drr001500050mmaaa00$$$dmH====(2)ThemoderatetolowazimuthresolutionhasbeenthemaindrawbackoftheSLARsystem.WithaninventionbyCarlWileyin1951followedbyapatentapplicationin1954[10],thislimitationhasbeenovercomebytheuseofacoherentradarandtheprincipleofDopplerbeamsharpen-ing[11],leadingtoanimprovementoftheazimuthresolu-tion.Inthefollowingyears,thisconceptwasextendedtotheprincipleofthesyntheticapertureasitisknowntoday[12].Theresultingazimuthreso-lutionbecomesequaltohalftheazimuthantennalength(/)d2aad=andisindepen-dentoftherangedistance.Thismeansthattheazimuthresolu-tioninthepreviousexampleisequalto1.5m,i.e.,morethan30timesbetterthantheresolu-tionoftherealapertureSLARsystem.ForaspaceborneSARsystemwithequalazimuthantennalength,theazimuthresolutionwillbethesame(i.e.,1.5m).TheinventionoftheSARprinciplerepresentedamajormilestoneforthedevelopmento