AVIRISRadiometricLaboratoryCalibration,InflightValidation,andaFocusedSensitivityAnalysisin1998Robert0.Green,BetinaPavri,JessicaFaust,andOrlesaWilliamsNASAJetPropulsionLaboratory,CaliforniaInstituteofTechnology,Pasadena,CA91109ABSTRACTAVIRISisacalibratedEarth-lookingimagingspectrometerthatmeasuresthetotalupwellingspectralradianceinthesolarreflectedspectrumfor400to2500nm.In1998AVIRISwascalibratedinthelaboratoryusinganabsoluteradiometriccalibrationstandard.TovalidatethecalibrationofAVIRISintheflightenvironmentonboardtheER-2aircraft,aninflightcalibrationexperimentwasorchestratedon15June1998.Thisexperimentrequiredfieldmeasurement,radiativetransfercodecalculations,andanalysisofAVIRISspectraldataforacalibrationtargetatRogersDryLake,CA.TheresultsofthisinflightcalibrationexperimentshowthatAVIRISwasradiometricallycalibratedatbetterthan96-percentaccuracyin1998.Atthetimeofthisexperiment,theAVIRISsignal-to-noiseratiowascalculatedtorangefrom1000to1inthevisibleportionofthespectrumandapproach450to1inthe2000-nmspectralregion.Inadditiontotheinflightcalibrationexperimentcalculations,afocusedsensitivityanalysisshowedthatthepredictedradianceismostsensitivetothemeasuredsurfacespectralreflectanceandtotalcolumnwatervapor.Thesemeasurements,calculations,analyses,andresultsarepresentedinthispaper.1.OINTRODUCTIONNASA’sAirborneVisiblehfraredImagingSpectrometer(AVIRIS)measuresthetotalupwellingspectralradiancefrom400to2500nmat10-nmresolution.WhenflyingonboardtheER-2aircraftat20-kmaltitude,theseradiancespectraaremeasuredasimagesof11byupto100km,with20-mspatialresolution.SpectrafromAVIRISareusedforawiderangeofscienceresearch,application,andcalibrationobjectives(Greenetal1998a).OneofthemostimportantcharacteristicsofAVIRISisthequalityofthecalibrationofthemeasuredspectra.Calibrationisrequiredto:(1)enablephysicallybasedderivationsofparametersfromthemeasuredradiancespectra,(2)comparespectraandresultsfromdifferentlocationsanddifferenttimes,(3)analyzeAVIRISspectraandcomparethemwithmeasurementsbyotherinstruments,and(4)extractparametersandanalyzeAVIRISspectrawithcomputermodels.AVIRISiscalibratedinthelaboratorybeforeandaftertheyearlyflightseason(Chrienetal1990,Chrienetal1995,Chrienetal1996).TherearealsoperiodicchecksofthelaboratorycalibrationduringtheAVIRISflightseason.However,allAVIRISspectralimagesacquiredforinvestigatorsareacquiredintheairborneenvironment,notinthelaboratory.AninflightcalibrationexperimentstrategyhasbeendevelopedtoassessandvalidatethecalibrationofAVIRISintheflightenvironment.TheseinflightcalibrationexperimentshavebeenimplementedforAVIRISeachyearsince1987(Greenetal.1988,Conel,etal.1988,Greenetal.1990,Greenetal1991,Greenetal.1992,Greenetal.1993,Greenetal.1995,Greeneta11996,Greenetal1998b).Thispaperpresentsthemeasurements,calculation,analyses,andresultsoftheprimaryAVIRISinflightcalibrationexperimentin1998.2.0LABORATORYCALIBRATIONAVIRISiscalibratedinthelaboratorywithrespecttoanabsoluteradiometriccalibrationstandardlamp.ThecalibrationofthelampistracedtotheUnitedStatesNationalInstituteofStandardsandTechnology(NIST).ForcalibrationofAVIRISthelampismountedtoilluminateareflectancepanel.TheilluminatedpanelisobserveddirectlybyAVIRIS.Thetotaluncertaintyiscalculatedbasedonuncertaintyofthestandardlamp,reflectancepanel,andtransferrepeatability.TheradianceofthepanelandthecalculateduncertaintyisgiveninFigure1.TheratiooftheilluminatedpaneltothedigitizednumbersoutputbyAVIRISgivestheradiometriccalibrationcoefficientsshowninFigure2.TheprocessgeneratesthebaselineradiometriccalibrationforAVIRIS.10.0020.009.00~18.008.00b7.00h16.00~p5.00I::I:12.00~1.00-~,~~7,,i,,~,,i,,,,,i~,~,~,~,~~400.00700.001000.001300.001600.001900.002200.002500.00Wavelength(nm)Figure1.OutputofAVIRIS1998radiometriccalibrationstandardandcalculated400.0700.01000.01300.01600.01900.02200.02500.0Wavelength(nm)Figure2.AVIRIS1998radiometriccalibrationcoefficientsinunitsofradianceperdigitizednumber.3.0FieldMeasurementsAradiativetransfercodeisusedtopredicttheradianceattheAVIRISsensoraperture.FieldmeasurementareacquiredatacalibrationtargetatthetimeoftheAVIRISoverflightinorderconstrainaradiativetransfercode.On15June1998a40by200mcalibrationtargetwasestablishedonthesurfaceofRogersDryLake,CA.Thisareacorrespondstoanarrayof2by10AVIRISspatialresolutionelements.Forlocationofthecalibrationtarget,twolargeblueplastictarpswereplacedneareachendofthetarget.TheuniquespectralsignatureofthesetarpsallowsexactlocationofthecalibrationtargetintheAVIRISspectralimagedata.TheprincipalfieldmeasurementacquiredatthecalibrationtargetduringAVIRISinflightcalibrationexperimentisthesurfacespectralreflectance.On15June1998thesurfacereflectanceofthe40-by200-mcalibrationtargetwasmeasuredwithaportablefieldspectrometerwithaspectralrangefrom350to2500nmandspectralsamplingofbetterthan5nm.Morethan250spectraofthecalibrationtargetweremeasured.Spectraofareflectancestandardweremeasuredperiodicallyduringthecharacterizationofthecalibrationtarget.Thetargetandstandardspectrawerethenanalyzedtogeneratethereflectanceaverageanduncertaintyforthecalib