M.E.WeberandT.A.NoyesWindShearDetectionwithAirportSurveillanceRadarsAirportsurveillanceradars(ASR)utilize abroad,cosecant-squaredelevationbeampattern,rapidazimuthalantennascanning,andcoherentpulsed-Dopplerprocessingtodetect.andtrackapproachinganddepartingaircraft.Theseradars,becauseoflocation,rapidscanrate,anddirectairtrafficcontrol(ATe)datalink,canalsoprovideflightcontrollerswithtimelyinformationonweatherconditionsthatarehazardoustoaircraft.Withanaddedprocessingchannel,anupgradedASRcanautomaticallydetectregionsoflow-altitudewindshear.ThisupgradecanprovidewindshearwarningsatairportswherelowtrafficvolumeorinfrequentthunderstormactivityprecludesthedeploymentofadedicatedTerminalDopplerWeatherRadar(TDWR). Fieldmeasure-mentsandanalysisconductedbyLincolnLaboratoryindicatethattheprincipaltechnicalchallengesforlow-altitudewindsheardetectionwithanASR-ground-cluttersuppression,estimationofnear-surfaceradialvelocity,andautomaticwindshearhazardrecognition-eanbesuccessfullymetformicroburstsaccompaniedbyrainatthesurface.Thisarticledescribesradarmodificationsandprocessingtechniquesthatallowairportsurveillanceradars(ASR)todetectmicroburst-generatedlow-altitudewindshear.Microburstshavebeenidentifiedastheprimarycauseof12majoraircarrieraccidentssince1970,resultinginthelossof575lives.Forairportswithlowtrafficdensityorinfrequentthunderstormactiv-ity,anupgradetoASRsprovideswindshearwarningsatalowercostthanthatofdedicatedwindsheardetectionsensors.ModerndigitalsignalprocessingforthenewestASRs-theASR-9s-generallyelimi-natesclutterfromprecipitationandgroundscatterers[l,2].EarlyacceptancetestingoftheASR-9, however,indicatedthatworkingairtraf-ficcontrollersactuallymadeconsiderableuseoftheweather-echoinformationontheirdisplays.Toreinsertweatherdatainanoninterferingmanner,theASR-9'ssignalprocessorwasaug-mentedwithadedicatedchannelforprocessinganddisplayingsixquantitativelevelsofpreci-pitationreflectivity(Le.,rainrate)[2, 3].Thisprocessordoesnotutilizetheradar'sco-herence.otherthanforDopplerfilteringofTheLincolnLaboratoryJoumQ~Volume 2.Number3 (1989)stationaryground-clutterechoes.TechniquestoextendtheASR'sweathermeasurementcapabilitytoallowforthedetec-tionofthunderstorm-generatedlow-altitudewindshearmustincorporate(a)signalprocess-ingforsuppressinggroundclutterandestimat-ingthenear-surfaceradialwindcomponentineachradarresolutioncell.and(b)imagepro-cesssingforautomaticallydetectinghazardousshearintheresultingvelocity field.AlgorithmsthataccomplishthesefunctionshavebeenevaluatedextensivelywithsimulatedweathersignalsandmeasurementsfromanexperimentalASRinHuntsville,Ala.Ouranaly-sisindicatesthatasuitablymodifiedASRcouldwithhighconfidencedetectmicroburstsaccom-paniedbyrainatthesurface-thepredominantsafetyhazardforaircraftinmanypartsoftheUnitedStates.ThefollowingsectiondescribesthebackgroundandpotentialoperationalroleofanASR-basedwindsheardetectionsystem.Wethendiscusstheprimarytechnicalissuesforachievingthiscapability,anddeSCribeourevaluationofprocessingmethodsthataddresstheseissues.511WeberetaI.-WindShearDetectionwithAirportSurveillanceRadarsBackgroundandOperationalMissionFigure1illustratesthetwoprincipalcausesoflow-altitudewindshear.InFig. l(a),anin-tensethunderstormdowndraftencounterstheearth'ssurfaceandproducesabriefoutburstofhighlydivergenthorizontalwinds,ormicro-burst(4).Aircraftthatpenetrateamicroburstontakeofforlandingexperiencehead-wind-to-tail-windvelocityshearcompoundedbyadowndraftinthemicroburstcore.Gustfrontsarethunderstormoutflowswhoseleadingedgespropagateawayfromthegeneratingprecipita-tion,asshowninFig. l(b) (5).Becausethewindshearencounteredbyanaircraftthatpenetratesagustfrontincreasestheplane'slift,agustfrontisconsideredlesshazardousthanthewindshearassociatedwithamicroburst.Thewindsbehindthegustfront,however,areturbulent,andthelong-termchangeofwinddirectionfollowingthepassageofagustfrontaffectsrunwayoperations.Trackingandpredictinggustfrontarrivalsatmajorairports~111l'2000"',CompensatingConvergence.s~F1500...J•Downdraft...CJ~............~......E~,...Ol1000O'i ,...'Qj2>"...I,&,,<v?3,C,~,,500&<:',,vro'?i,'-:-Outflow:\,II-2000-100001000 2000Distance Relative to Downdraft Center (m)(a)2015Turbulent AreaGust Front EnvelopeMultiple Surges10Distance from Gust Front (km)(b)3000 E...JCJ2000~EOl'Qj1000 I5.Turbulent AreaFig.1-(a)Vertical cross sectionofmicroburst wind field. (b) Vertical cross sectionofgust front (redrawn from Goff, Ref.5).512The Lincoln LaboratoryJournal.Volume2.Number3 (1989)WeberetaI.-WindShearDetectionwithAiTportSW"VeillanceRadarswillallowmoreefficientuseofrunways.Inresponsetothehazardsofwindshear,theFederalAviationAdministration(FAA)initiatedatwo-partenhancementtoitsterminal-areaweatherinformationsystem.Theairportnet-workofsurfacewind-speedandwind-directionsensors-theLowLevel WindShearAlertSys-tem(LLWAS)-isbeingimprovedbyareworkeddetectionalgorithmand,atmajorairports,anincreasednumberofsensors[6).Inaddition,adedicatedmicrowaveTerminalDopplerWeatherRadar(TDWR)willbedeployedat45airportstomeasuretheradar-reflectivityandradial-velocitysignaturesassociatedwithlow-alti-tudewindshear[7).ASRswereinitiallyrejectedascandidatewindsheardetectionsensorsbecauseoftheirperceiveddeficienciesinsensitivityandground-clutterrejection,aswellasaninabilitytoresolvenear-surfacethunderstormoutflowswiththebroadelevationbeams.Totheextentthattheseproblemscanbeovercome, however, ASR-9swillcomplementthededicatedwindshearde-tectionsensorsinthreeareas:(1)Airportswithlow trafficvolumeorinregionswithinfrequentthunderstormactivitymaynotwarrantadedicatedTDWRorenhancedLLWAS.A modifiedASRcouldprovidewindshearprotectionattheseairportsatasmallercostthanthededicatedsystems.(2)AtairportsequippedwithenhancedLLWASbutlackingTDWR,datafromanASRcouldreinforceLLWASwindshearreportsanddetectwindshearinopera-tionallysignificantareasnotcoveredbythesurfacestationnetwork.(3)Atairportsslatedto receive a TDWR,ad-ditionalradarwindmeasurementsfromanASRcouldhelptoreducehead-wind-tail-windshear-estimateinaccura-ciesthatresultwhenamicroburstout-flowisasymmetric.ThesitingoftheASRwill often provide abetterviewinganglethantheTDWR forhead-wind-tail-windshearmeasurementsalongsomerun-ways. Alternately,datafromthetwora-darsmaybecombinedtocomputethetotalhorizontalcomponentofthewindTheLincolnLaboratoryJournal.Volume 2.Number3 (1989)vectoroverareaswhereradialsfromthetworadarsintersectatapproximatelyrightangles.Inaddition,therapidscanrateofanASR