SMAPL-bandMicrowaveRadiometer:InstrumentDesignandFirstYearonOrbit

Jeffrey R. Piepmeier, Senior Member, IEEE, Paolo Focardi, Senior Member, IEEE, Kevin Horgan, Member, IEEE, Joseph Knuble, Negar Ehsan, Member, IEEE, Jared Lucey, Clifford

Brambora, Paula R. Brown, Pamela J. Hoffman, Richard T. French, Rebecca L. Mikhaylov, Eug-Yun Kwack, Eric M. Slimko, Douglas E. Dawson, Derek Hudson, Jinzheng Peng, Priscilla

Mohammed, Member, IEEE, Giovanni De Amici, Adam P. Freedman, James Medeiros, Fred Sacks, Robert Estep, Michael W. Spencer, Curtis W. Chen, Kevin B. Wheeler, Wendy N.

Edelstein, Peggy O’Neill, Fellow, IEEE, Eni G. Njoku, Fellow, IEEE

ABSTRACT

Abstract–TheSoilMoistureActivePassive(SMAP)L-bandmicrowaveradiometer

isaconicalscanninginstrumentdesignedtomeasuresoilmoisturewith4%volumetric

accuracyat40-kmspatialresolution.SMAPisNASA’sfirstEarthSystematicMission

developedinresponsetoitsfirstEarthsciencedecadalsurvey.Here,thedesignisreviewed

andtheresultsofitsfirstyearonorbitarepresented.Uniquefeaturesoftheradiometer

includealarge6-meterrotatingreflector,fullypolarimetricradiometerreceiverwith

internalcalibration,andradio-frequencyinterferencedetectionandfilteringhardware.

Theradiometerelectronicsarethermallycontrolledtoachievegoodradiometricstability.

Analysesofon-orbitresultsindicatetheelectricalandthermalcharacteristicsofthe

electronicsandinternalcalibrationsourcesareverystableandpromoteexcellentgain

stability.RadiometerNEDT<1Kfor17-mssamples.Thegainspectrumexhibitslownoise

atfrequencies>1mHzand1/fnoiserisingatlongertimescalesfullycapturedbythe

internalcalibrationscheme.Resultsfromskyobservationsandglobalswathimageryofall

fourStokesantennatemperaturesindicatetheinstrumentisoperatingasexpected.

I. Introduction

NationalAeronauticsandSpaceAdministration’s(NASA’s)SoilMoistureActive

Passive(SMAP)satellitewaslaunchedintoa685-kmnearsun-synchronous6AM/PMorbit

onJanuary31,2015.SMAP’sL-bandmicrowaveradiometerwascommissionedinFebruary

andMarchandhasnowreacheditsmilestoneofoneyearofsuccessfuloperation.Athree-

dimensionalrenderingoftheSMAPobservatoryinitsfullydeployedconfigurationis

showninFig.1.SMAPisthethirdinaseriesofmodernL-bandradiometersafterthe

EuropeanSpaceAgency’s(ESA)SoilMoistureOceanSalinity(SMOS)satellitelaunchedin

2009andNASA’sAquariusinstrumentaboardArgentina’sComisiónNacionalde

ActividadesEspaciales(CONAE)SatelitedeAplicacionesCientificas(SAC)-Dsatellite

launchedin2011[1]-[3].SMAP’sprimaryscienceobjectiveistoprovidesoilmoisture

measurementswithanuncertainty<0.04m3m-3forterrainhavingvegetationwater

contentsupto5kg/m2.Fortheradiometer,thisobjectiverequiresradiometricuncertainty

<1.3K(withforeandaftaveraging)with<40-kmspatialresolution.

SMAPisNASA’sfirstEarthSystematicMissiondevelopedinresponsetoitsfirst

Earthsciencedecadalsurvey[4].TheSMAPmissionhigh-levelsciencerequirementsand

derivedinstrumentrequirementsrelatedtotheradiometerareshowninTable1(adapted

from[5]).Earlyinitsmission,SMAPprovided10-kmresolutionsoilmoistureglobally

everythreedaysusingacombinedactive-passivemicrowaveinstrument.Theinstrument

comprisesaradiometerandasyntheticapertureradar(SAR),whichbothshareasingle

antenna.InJuly,theradarceasedtransmissionsandhasremainedinreceive-onlymode

since[6].Byitselftheradiometerenables40-kmresolutionsoilmoisturewiththesame

three-dayglobalcoverage.Nonetheless,withitsconical-scanning(realaperture)antenna

andadvancedRFImitigationcapabilities,theSMAPradiometerisprovidinghigh-quality

brightnesstemperaturemeasurementsofEarth’sland,iceandoceansurfaces.

TABLE1.SMAPRADIOMETERSCIENCEANDINSTRUMENTREQUIREMENTS.

ScientificMeasurementRequirements InstrumentFunctionalRequirementsSoilMoisture:0.04m3m-3volumetricuncertainty(1-σ)inthetop5cmforvegetationwatercontent≤5kgm-2;Hydroclimatologyat40-kmresolution

L-BandRadiometer(1.41GHz):Polarization:V,H,T3andT4Project3-dBbeamwidth≤40kmRadiometricUncertainty≤1.3KConstant40°incidenceangle

Samplediurnalcycleatconsistenttimeofday(6am/6pmEquatorcrossing);Global,3-day(orbetter)revisit

SwathWidth:1000kmMinimizeFaradayrotation

Observationoverminimumofthreeannualcycles

Baselinethree-yearmissionlife

Thetwokeytechnologyinnovations–thelargescanning6-meterreflectorandthe

totalpowerradiometerreceiverwithadvancedRFIdetectionandfilteringcapabilities–

combinedmaketheSMAPradiometerunique.On-orbitresultsofSMAP’sRFImitigation

capabilitiesarediscussedin[7].Inotherways,theSMAPradiometerderivesits

requirementsand/ordesignfrompastradiometersystems.Thefront-endarchitecture

witha6-meterantennasharedwiththeradarwasinheritedfromthepreliminarydesignof

theHydrosphereState(Hydros)EarthSystemSciencePathfinder(ESSP)mission,which

wasselectedasanalternateESSPbyNASAbutdidnotproceedwithdevelopmentin2005

[5].SMAP’santennaisconicalscanningwithafull360-degreefieldofregard.However,

thereareseveralkeydifferences(someunique)frompreviouslow-frequencyconical

scanningradiometerslikeWindSatorAdvancedMicrowaveScanningRadiometerforthe

EarthObservingSystem(AMSR-E)[8][9].Mostobviousisthelackofexternalwarm-load

andcold-spacereflector,whichnormallyprovidesradiometriccalibrationthroughthefeed

horn.Rather,SMAP’sinternalcalibrationschemeisbasedontheJasonMicrowave

Radiometer,theAquariuspushbroomradiometersandtheSMOSradiometerusing

referenceloadswitchesandcouplednoisediodes[9]-[11].Switchingoftheinternal

calibrationssourcesissynchronizedwiththeradaroperation,asitisonAquarius,sothe

radiometeroversamplesthefootprint.BothSMAPandAquariusutilizethisoversampling

techniquefortime-domaindetectionandfilteringofradio-frequencyinterference[12][13].

LikeWindSat,SMAPmeasuresallfourStokesparameterswithforeandaftviewing;unlike

WindSat,SMAPusescoherentdetectionforthethirdandfourthStokesparameters

implementedinadigitalreceiverbackend[14].ThefirsttwomodifiedStokesparameters,

TVandTH,aretheprimarysciencechannelsusedbythesoilmoistureretrievalalgorithm

[15].TheT3channelmeasurementprovidescorrectionofFaradayrotationcausedbythe

ionosphere[16].TheT4channelismeasuredasaconsequenceofthereceiverdesignand

thedataareusedtodetectRFI.ThemostsignificantdifferenceSMAPhasfromallpast

spaceborneradiometersystemsisitsaggressivehardwareandalgorithmapproachtoRFI

detectionandfiltering[13][7].TherequirementtodetectandfilterRFIdrivesandexploits

featuresoftheSMAPdesign.

Here,themainfeaturesoftheSMAPradiometerdesign,resultsofearlyorbit

operations,andobservationsforthefirstyearupthroughMarch2016arepresented.

SectionIIcoverstheobservatory,swathandantennadesign.SectionIIIdescribesthe

radiometerelectronicsandtheircalibrationbeforelaunch.Activitiesandresultsofpre-

launchcalibrationandearlyoperationson-orbitarediscussedinSectionsIVandV.A

reviewofoneyearofdataisdiscussedinSectionsVI&VII.

II. ObservatoryandAntennaDesign

SMAPorbitsEarthat685-kmaltitudewiththespacecraftpointingtogeodeticnadir.

Theinstrumentantennapoints35.5°awayfromnadirandgeneratesa2.4°3-dB

beamwidthfortheradiometer.Thisgeometrycreatesaninstantaneousfield-of-view

(IFOV)of36-by-47kmwithanEarthincidenceangle(EIA)of40°.Thedistanceacrossthe

swathis1000kmfromIFOVcenter-to-center.Thisswathwidth,combinedwiththeorbit

parameters,allowsthewholeofEarth’ssurfacetobecoveredinthreedays(exceptfor

typicalpoleholes)withnogapsattheequator.

Imagingisaccomplishedbyconicalscanningtheantennabeamwithafull360°field

ofregard.Theantennarotatesat14.6rpmcompletingascanwith3200-kmcircumference

every4.1seconds.Along-scanaveragingoccursover14-ms,whichsmearsthebeamalong

scantocreatea39-by-47kmeffectivefield-of-view(EFOV).Along-scansamplingoccurs

every11km,whichisfasterthanthe20-kmNyquistcriterion.Withthespacecraftmoving

at6.8km/sspeedoverground,thealongtrack(oracrossscan)samplingatcenterofswath

is28km–slightlyslowerthanthe24-kmNyquistcriterion.Thespatialsamplinggeometry

nearcenterofswathisshowninFig.2.

Theinstrumentantenna,sharedbetweentheradiometerandtheSAR,iscomposed

ofa6-moffsetreflectorfedbyadualpolarized,dualbandfeed-horn.Withafocallengthof

4.2mandaprojecteddiameterof6m,aKevlarnetshapesthedeployablemeshreflector

intoatriangularlyfacetedsurfacewithanRMSerrorcomparedtoaperfectparaboloidless

than2mmorabout1%wavelength.Optimizedforboththeradiometer(1.4015-1.4255

GHz)andtheSAR(1.2168-1.2982GHz)frequencybands,thefeed-hornincludesanOrtho-

Mode-Transducer(OMT)toseparatehorizontalandverticalpolarizations.TheOMTis

partlymadeoftitaniumtothermallyisolatethehorn,whichisexposedtothethermal

radiationenvironment,fromtheradiometerelectronics.

Fig.3showsacomparisonbetweentheCADmodeloftheSMAPobservatorywith

thereflectorfullydeployedanditsRFmodelusedtopredicttheantennaperformance.All

significantdetailsrelativeto21-cmwavelengthwereincludedintothemodeltogetthe

bestpossibleaccuracyintheradiationpattern.Thereflectorantennaradiationpatternwas

notmeasuredbeforelaunch;therefore,averyaccurateantennapatternknowledgewas

requiredtoverifyperformancerequirements(e.g.,beamefficiency)andfortheinitial

radiometercalibration.A1/10thscalemodelreplicatingallmajoraspectsoftheflight

hardwarewasalsodesigned,builtandtestedtovalidatetheRFmodel.Finalrequirement

verificationwasthendonewithacombinationofflightfeedassemblymeasurements,scale

modelpredictionsandmeasurementsandflightmodelpredictions.Ahorizontal

polarizationpatterncutalongthealong-scandirection(horizontaldirection)isplottedin

Fig.4.Themainlobehasahalf-powerbeamwidthof2.4degrees.Thebacklobes,primarily

duetofeedpatternspilloverandedgediffraction,fallintothespaceregion.Thebeam

efficiencyforverticalandhorizontalpolarizationsis88%,withmostofthenon-mainlobe

contributiondirectedtowardsspace.

III. RadiometerElectronicsDesign

Theradiometerelectronicsconsistofantennafeednetwork,radiometerfrontend

(RFE),radiometerbackend(RBE),andradiometerdigitalelectronics(RDE).These

subsystemsareshowninFig.5withsignalflowfromrighttoleft.Theantennafeed

networkincludesanexternalnoisesourceanddiplexerstoseparateradarfrequencies

fromtheradiometerpath.Theexternalnoisesourcesignalisaddedtotheantennasignal,

butisnotusedasaprimarycalibrationsourceandispresentforredundancy.The

diplexersincludeadditionalfilteringtolimittheamountofRFIenteringtheRFE.TheRFE

containstheprimaryinternalcalibrationswitchesandnoisesource,RFamplificationand

additionalfiltering.Theinternalnoisesourcesignalisaddedtothereferencesignalsto

provideRFI-freegaincalibration.TheRBEdown-convertstheRFsignalstoalowerIF

frequencyusingacommonphase-lockedlocaloscillator(PLO).Thereferenceclockforthe

PLOalsoclockstheanalog-to-digitalconverters(ADCs)intheRDE.TheADCssampleand

quantizetheIFsignalsforprocessingbyaFieldProgrammableGateArray(FPGA)-based

DigitalSignalProcessor(DSP).TheDSPgeneratesdetectedpowerforthefullpassband

(fullbanddata)andfor16channelsspacedevenlyacrossthepassband(subbanddata).The

RDEgeneratestimingsignalsneededtocontroltheinternalcalibrationsourcesand

synchronizesradiometerintegrationwiththeradartiming.Finally,dataarepacketizedby

theRDEandsenttospacecraftmassstorageforlaterdownlinktotheground.

TheRFsystemisdesignedtomakelinearradiometricmeasurementsinthe

presenceofRFIuptoanInterference-to-NoiseRatio(INR)of6dBoraneffectiveadded

noisetemperatureof2000KduetoRFIoutofatypical500Ksystemtemperature.Above

2000K,theaccumulatorsintheintegratorlogicsaturate.Cascadedfilteringand

amplificationsufficienttooperateeachstageatamaximumpowerof24-dBbelow1-dB

compressionkeepstheerrorcontributionfromnonlinearityduetoRFIsignalsnegligible

overthisrange.ThetotalsystemfrequencyresponseisshowninFig.6.Note,theresponse

is30dBbelowpeakattheallocationedgesat1400MHzand1427MHz.Theroll-offto

>100dBismuchfasteronthelowsidebecauseofthepresenceofairsearchradarsand

lessstringentonthehighsidebecauseoffewerknownRFIsources.

Thedigitalbackendreplacesconventionaldiodedetectorswithmoment

accumulatorsandcomplexcross-correlators.ThefirstfourmomentsoftheADCoutputs

areestimatedbytheRDE.Thescience-processingalgorithmcomputesthesecondcentral

momentusingthefirstandsecondmomentstoestimatethetotalpower[17].Thesedata

areusedtoestimateantennatemperature.Thekurtosis(usedbytheRFIdetectors)is

computedinasimilarmannerusingrawmoments.Thecomplexcross-correlation

coefficientisalsomeasuredbytheRDEandisusedinthescience-processingalgorithmto

estimatethirdandfourthStokesparameters.

ThirdandfourthStokesparametersaremeasuredbytheradiometertocompensate

forFaradaypolarizationbasisrotationcausedbytheionosphereandtobeusedasRFI

detectors.Thedrivingrequirementonthereceiverwastominimize(relativeto24-MHz

bandwidth)differentialgroupdelaybetweentheverticalandhorizontalchannels.Because

thecomplexcorrelationismeasured,anymeanphasedifferencebetweenverticaland

horizontalpolarizationscanbecompensatedbyacomplexcoefficientmultiplicationinthe

science-processingalgorithm.Phaseslopedifference,however,wouldattenuatethe

correlationbetweensignals,andisminimizedinthedesignbyusingsymmetryand

avoidingunnecessarilylongcablesofdifferentlengths.

Theradiometerintegratorlogicoperatessynchronouslywiththeradarwherebythe

radiometerintegratesreceivedpowerduringtheradarreceivewindowandblanksduring

thetransmitwindow.TimingisshowninFig.7.Thefundamentalunitofintegrationis300

μscontainedwithinapulse-repetitioninterval(PRI).Fullbanddataareintegratedatthis

rate.Subbanddata,however,areintegrated1.2msoverapacketdefinedasfourPRI’s.

Duringafootprint,eight(8)packetsareutilizedtoviewearthfor9.6msandfour(4)

packetstoviewtheinternalreferenceloadandnoisesourceorcalibrationfor4.8ms.

Becauseofasmallamountofblankingduringradartransmitandcalibrationswitching

setuptime,thiscycleoccursona17msperiod.

Theradiometertiming,antennabeamwidthandscanrate,andgroundsoftware

averagingarecoordinatedsotheequivalentlow-passprocessesworktogethertoproduce

Nyquistsampledfootprintsalongthescandirection.Thefrequencyresponsesofthe

processesareshowninFig.8.Theon-boardintegratorsarerepresentedbythetwo

rightmosttraces(dash-dotanddash)for1and4PRI’s,respectively.The8-packet(32-PRI)

integrationdoneingroundsoftwaretoformafootprinthasalow-passresponseindicated

bysolidtracelabeledL1B_TB.TheLevel1productfootprintprocesshasa3-dBpointat30

Hz,whichfullysamplesthelow-passprocesscreatedbytheantennapatternsweeping

acrosstheearth.Thebumpinthefootprintresponsenear100Hziscausedbythe

interleavingofantennalookswithcalibrationlooks,whichrobsintegrationtimefromthe

scene.AbalanceisstruckinthealgorithmdesignbetweenincreasedNEDTandvs.

decreased∆G/Gnoise.Thehigh-frequencyenergyat100Hzintheantennaaveraging

responseismerelywhitenoisealiasedintothefootprintsandequivalenttotheincreasein

NEDTduetolimitingantennaintegrationtime.Finally,thedottedlinemarked“antenna”

showstheequivalentlow-passresponseoftheantennaapproximatedbyaGaussianbeam

(sweepingalong-scaninazimuthat770km/secspeedoverground)tonaturallyoccurring

thermalradiation.

Thefourpacketsofcalibrationobservationsarepartitionedintotwopacketsfor

referenceloadandtwopacketsforreferenceplusnoisediode.Conventionally,noiseis

injectedpriortoareferenceswitch;however,onSMAPasonAquarius,RFIissucha

concernthatthenoiseinjectionisdonebehindtheswitchtoensuretheradiometercanbe

calibratedregardlessofRFIenteringtheantenna.ThenoisesourceonSMAP,unlike

Aquarius,isasinglenoisesourcesplitcoherentlybetweentheverticalandhorizontal

receiverchannels.Thismethodwaschosenbecausethedigitalreceiverhasnegligible

cross-polarizationcouplingandthetotalpowerandcross-correlationdetectorscanbe

calibratedessentiallyindependently.Thephaseofthecorrelatednoisesourcewassetat

40otoenablecalibrationoftherealandimaginaryoutputsofthecorrelatorusingthesame

calibrationstate.

Theradiometerscience-processingalgorithmusesthesepairsofreferenceand

noisediodecountstocomputegainandoffsetcoefficients,whicharethenfurtheraveraged

overalongertimeperiod(multiplefootprints)toreduceestimationnoise.Thegainand

offsetarecomputedtwiceperfootprintinthescience-processingalgorithmandthen

averagedwitha5000-tap,uniform,centerednon-causalfilter.Thefrequencyresponsesof

thesefiltersarediscussedintheon-orbitdatasectionbelow.Carefulattentionwaspaidto

thermalcontroltoallowgainandoffsetcoefficientaveragingover10’sofsecondsandto

helpmaintainradiometerstabilityonorbitalandseasonaltime-scales.Thecombinationof

passivethermaldesignwithanactiveproportionalcontrollerachieves0.1oC/orbitwithin

theRFE[22][23].Worst-caseorbitalresultsarealsodiscussedintheon-orbitresults

sectionbelow.

IV. Pre-LaunchCalibration

Thepre-launchcalibrationoftheradiometerincludesbotharadiometricanda

polarimetricexercisetocharacterizetemperaturedependenceofthereferenceandnoise

sourcelooksandreceiverphaseimbalancerelativetothefeedhorninput.Theradiometric

calibrationwasaccomplishedusingtechniquessimilarto[18]andpolarimetriccalibration

[19].Thegoaloftheradiometriccalibrationistocharacterizethelosses(orequivalent)

andnoisediodeaddednoisetemperature,representedbythesimplifiedlossmodelshown

inFig.9,foruseinthescience-processingalgorithm.Likewise,thegoalofthepolarimetric

calibrationistodeterminethepolarimetricefficiencyandphasedifferencesofthereceiver

channels.

Inthescience-processingalgorithm,theantennatemperaturereferencedtothe

feedhornoutput/OMTinputiscomputedfromradiometeroutputcountsusingatwo-point

calibrationmodel:

𝑇%& = 𝑇%()* −,-./0,1

,23,50,-./𝑇%67 (1)

where𝑇′,()*istheinternalreferenceloadnoisetemperatureand𝑇′,67thecouplednoise

sourcetemperaturereferredtothefeedhornoutput/OMTinput.Theradiometeroutput

countsarerepresentedby𝑐: ,wherexindicatesreference(ref),antenna(A),andnoise

diode+reference(ND,R)states.Theintermediateantennatemperature(1)isinput-

referredtothefeedhornaperturebycorrectingforfeedandradomelossesandphysical

temperatures:

𝑇& = 𝐿(<=>?)𝐿*))=𝑇′& − 𝐿(<=>?) 𝐿*))= − 1 𝑇*))= − 𝐿(<=>?) − 1 𝑇(<=>?) (2)

whereLxandTxarethelossfactorsandphysicaltemperaturesforxequaltotheradome

andfeed.Theinternalcalibrationtemperaturescanbeexpressedasfunctionsofthe

lumpedlossesandphysicaltemperaturesshowninthelossmodelFig.9:

𝑇%()* = 𝐿ABC𝐿,>DE𝐿=FE𝑇GHI − 𝐿ABC𝐿,>DE 𝐿=FE − 1 𝑇=FE (3a)

−𝐿ABC 𝐿,>DE − 1 𝑇,>DE − 𝐿ABC − 1 𝑇ABC

𝑇%67 = 𝐿ABC𝐿,>DE𝐿=FE𝐿JKFL,M𝑇67 (3b)

Alternately,(3a)and(3b)canbeapproximatedusingalinearmodel:

𝑇()*% = 𝑇NOP + 𝑐NOP,RS*∆𝑇NOP + 𝑐TUV,()*∆𝑇TUV + 𝑐WXYZ,()*∆𝑇WXYZ + 𝑐[\Z,()*∆𝑇[\Z + 𝑇>**J)L(4a)

𝑇]7% = 𝑇,67 + 𝑐NOP,67∆𝑇NOP + 𝑐TUV,67∆𝑇TUV + 𝑐WXYZ,67∆𝑇WXYZ + 𝑐[\Z,67∆𝑇[\Z (4b)

wherecoefficients𝑐:arederivedfrompre-launchthermalvacuum(TVAC)testingand∆𝑇:

indicatesphysicaltemperaturedeviationawayfromthereferencetemperatureusedinthe

linearmodelfitting.

TheTVACtestsconsistedofaseriesofdatacollectionswiththeradiometerat

differentcombinationsofcontrolledtemperaturesforeachzone.ThefirstTVACtestwas

limitedtotheradiometerelectronicsandthecoaxialcomponentsportionofthefeed

network.Eachmajorcomponentwasinstalledonanindividuallycontrolledheaterplate,

wereconnectedtogetherusingspaceflightcoaxialcables,andtemperatureswerethen

varied±10°Cabout20°Cafter[20].Acoaxialcalibrationsourcecomprisingatemperature

stabilizedmatchedterminationandcoldFETwasusedtoprovidetwo-pointcalibration.

ThecoldFETwascalibratedagainstaliquidnitrogencoaxialstandardload.AsecondTVAC

testwasperformedwiththeOMTandfeedhorninstalledviewingaflatferritetileabsorber

plate.Whiledatatakeninthefirstwereusedtoobtainthesensitivityofthecoupler,

diplexerandtheinternalcalibrationsourcestotheirphysicaltemperature,thesecondtest

yieldedthesensitivityofthecalibrationtoOMTandfeedhorntemperatures.Theresulting

calibrationcoefficientsareshowninTable2.Thereisarelativelackofsensitivityofthe

referenceloadantenna-referredtemperatureduetovariationsinfeednetwork

components;however,thereferenceloadtemperatureisquitesensitivetochangesinthe

RFEtemperatureasindicatedbythe20%valueofcRFE,likelyduetochangesinthermal

gradients.Thenoisesourcehasatemperaturesensitivityof𝑐GHI/𝑇67 =2.5and2.7ppt/oC

duetoRFEtemperaturechangesfortheverticalandhorizontalpolarizationchannels,

respectively.

Table 2 SMAP Radiometer Calibration Coefficients (Full Band)

𝑐GHI 𝑐ABC 𝑐,>DE 𝑐=FE Note

𝑇()*% V-pol 0.205 4.78´10-5 -0.052 -0.073 𝑇>**J)L =0.225 K

H-pol 0.208 5.23´10-5 -0.056 -0.064 𝑇>**J)L =0.741 K

𝑇67% V-pol 1.18 0.015 0.036 0.002 𝑇67 = 465 K

H-pol 1.24 0.012 0.053 0.048 𝑇67 = 452 K

Theradiometer’scomplexcrosscorrelatorisusedtomeasurethirdandfourth

Stokesparameters.Theradiometerhasasymmetricdesign,butthetwopolarization

channelsarenotnecessarilyphasebalancedorspectrallybalanced.Adigitallycontrolled

correlatednoisesourcewithadjustablephasewasusedtodeterminethatthepolarimetric

efficiencyofthesystemwaseffectivelyunity(0.999).Thetwochannelshavethesame

passbandresponseandnegligiblegroupdelaydifference.Nonetheless,asthereceived

signalspropagatealongthereceiverchannels,therelativecorrelationanglewillbe

changed,asthereceiver’schannelphaseimbalanceisnonzero.Scatteringparameter

measurementsversustemperatureindicatephaseimbalanceisquitestable.Forexample,

theRFEinter-channelphaseimbalancevaries0.03°/°C.Thisamountisnegligible

consideringperformanceofthethermalcontrolsystem.Thus,phasesweremeasuredat

roomtemperatureusingdifferenttechniquesduringintegrationandcalibrationofthe

radiometer.

First,networkanalyzermeasurementsshowtheinternalcalibrationnoisediode,

whichisimbeddedinsidetheRFE,hasaphaseimbalance(fromtheRFEinputtooutput)of

1.6°.Positivesignmeansthatthev-polchannelhaslongerequivalentelectricallength.The

pathlengthsbetweentheRFEandRDEareunequal,creatinganadditional-41°phaseshift,

whichwasmeasuredbynetworkanalyzerandverifiedusingtheradiometercorrelator.

Finally,tocalibratethechannelphaseimbalancesintheradiometerbeforetheRFEinputs,

apolarizinggridoverLN2calibrationtargetwasused.Theprincipleofthistestistocreate

linearlypolarizationradiationatthefeedhorninput.Rotatingthegridgeneratesathird

Stokesparameterwithacorrelationcoefficientrotatedbythereceiver’stotalphase

imbalanceinthecomplexplane.ExcludingthechannelphaseimbalanceaftertheRFE

inputs,thechannelphaseimbalancefromthefeedhorntotheRFEinputsis39°.These

phasedifferencesareusedinthescience-processingalgorithmtoproducethirdandfourth

Stokesantennatemperatures.

V. EarlyOn-OrbitActivities

ThestowedconfigurationoftheSMAPantennaoffersanunobstructedviewofdeep

spacetothefeedhorn(seeFig.10).TheradiometerwaspoweredonFeb.12,2015totake

advantageofthestowedconfiguration.Usingthiswellknowncalibrationpoint,one

calibrationparameterintheradiometercanbeadjusted.Thenoisesourcewaschosen

becauseithasthelargestuncertaintyremainingfrompre-launchcalibrationtesting.

Estimatedcoldspaceantennatemperatureswereexpectedtobeapproximately4K±9K

(3-σ)basedonpre-launchcalibrationparametersandtheiruncertainties.Thenoisesource

pre-launchuncertaintyof1%islargelyresponsibleforerrorsinthisconfiguration.Other

errorsourcesincludeinternalreferenceloadtemperature,front-endcomponentloss,and

antennafeedpatternuncertainties.Atypicalorbitofantennatemperaturemeasurements

isshowninFig.11.Theresultsbasedonpre-launchcalibrationforV-polare5Ktoolow;

however,afternoisesourcecorrection,themeasuredantennatemperaturematchesthe

expectedcold-spaceantennatemperaturesquitewell.TheH-polresultsshowedonly1K

initialdiscrepancyandsimilaragreementaftercorrection.Theradiometerwaspowered

offonFeb.13toprepareforreflectordeployment.

Afteraccomplishingthespaceview,thereflectorwasdeployedinastatic

configuration.TheradiometerwaspoweredonagainFeb.27-28,2015toprovide

informationtoaidreflectordeploymentverification.Thereflectorwaspointedaftofthe

spacecraftandthenadiranglewaspredictedtobeslightlysmallerthanthatwhenspinning

becauseofboomdeflectionduetocentrifugalforce.Theresultsofthistestwerefavorable.

BothbrightnesstemperatureresponseandNEDTmeasurementsareasexpected.As

showninFig.12,brightnesstemperatureresponseoverland,ice,andoceanarereasonable

(notethecolorscaleislimitedtoemphasizevariationsoverland).Highbrightness

temperaturesareseenoverrainforestinSouthAmericaandWestAfricaanddesertin

NorthAfricaandMiddleEast.NEDTvaluesestimatedbythescience-processingalgorithm

are0.8Kand1.1Koveroceanandland,respectively.Thesevaluesareconsistentwith

instrumentdesignspecifications.

OnMarch31,2015,theradiometerreflectorwasrotatingatoperationalspeedand

theinstrumentelectronicswerepoweredbackon.Afteracheckoutperiod,theradiometer

wasdeclaredoperational.Ithasbeenoperatingsuccessfullysince.

VI. FirstYearonOrbit

TheradiometererrorbudgetisdominatedbyNEDTbecauseofthenarrowtime-

bandwidthproduct(9.6msby24MHz)availabletothesystem.TheorbitaverageNEDTfor

allfourStokesantennatemperaturesisshowninFig.13duringthefirstyearofoperation.

TheorbitaverageNEDTisconsistentwithanaverageoverland,oceanandicescenesand

isstablethroughouttheyearwithmeanvalue0.90Kand0.96Kforhorizontalandvertical

polarization,respectively.TheNEDTforthirdandfourthStokesparametersis1.34K,

whichisapproximatelytheexpectedfactorof√2largerthanNEDTforverticaland

horizontalpolarizations.

Transienttemperatures,particularthosethatcausechangesinthermalgradients

withintheRFE,cancausedriftinsystematiccalibrationbiases(inscaleand/oroffset).

Whilethescience-processingalgorithmcompensatesfortemperatureeffects,themodel

hasresidualuncertainty.Theradiometeristhermallystabilizedbypassivethermaldesign

andactivethermalcontroltominimizetheimpactsofthechangingthermalenvironment.

Severalplatinumresistancethermometers(PRTs)areusedtomeasurethetemperatures

andthermalstabilityoftheradiometercomponents.ThePRTmeasurementsofthefront-

endoverthefirstyearareshowninFig.14.EachPRTisreadevery20secondsandhas

0.01°Cresolutionand±0.5°Caccuracy.Duringmostofthepastyearandwhenunder

normaloperatingconditionsthetemperaturesarequitestable.Thediplexers,couplers,

andOMTareseentovary<1°Cseasonallyincludingduringthesouth-polesolareclipse

season(MaythroughJuly)inpanel(a).Evenifleftuncompensated,thevariationin

calibrationbiasduetothesefront-endthermalvariationsis<0.1K.Thefeedhornvariesin

temperaturequiteabitmore,about14°Cpeak-to-peak,butitsohmiclossisnearly

negligibleandvaries2ppm/°C.Thetemperaturesoftheinternalcalibrationsourceshas

0.4°Cpeak-to-peakvariationshowninpanel(b).Theinternalnoisesourcehasa

temperaturecoefficientof3ppt/°C(referredtothefeedhorn),whichresultsina

calibrationdependenceof0.3Kthatiscompensatedinthescience-processingalgorithm.

Thestepsintemperatureearlyintheoperationyearareduetovariousactivitiesduring

thefirsttwoweeksofinstrumentcommissioning.Theoccasional1°Cimpulsesaredueto

instrumentpowercyclingasaconsequenceofsatelliteoperations.Thesepowercyclesdo

interruptthecalibrationtemporarily;however,theinstrumentrecoverswithinafeworbits

andreturnstosteadystatethereafter.

Theshort-termorbitalvariationsintemperatureareshowninFig.15fortheworst-

casepeakoftheeclipseseason.Duringthisorbitthefeedhornvaried2°Candtheother

feednetworkcomponents0.2°C.Theinternalcalibrationsourcesvaried0.1°Coverthe

orbit.Thesevariationshavenegligibleimpactontheintra-orbitalstabilityofthereceiver

calibration.

Noisediodebiascurrentandavalanchebreakdownvoltagearemeasuredevery8

daysduringnormaloperationsandplottedinFig.16.Thebiascurrentisstableto2μA

peak-to-peakandbreakdownvoltage1.4mVpeak-to-peak.Basedonlaboratory

measurementsofdiodecomponents,thecalibrationdependenceontheseDCbias

variationsis250ppm.

Thecombinationofnoisediodebiasstabilityandthermalstabilityleadtoexcellent

radiometergainstabilityonorbitaltimescales.Theradiometerswitchingprovidesnoise

diodeandreferenceloadcountsevery8.4ms.Theseareusedtocomputegainandoffset

coefficientswithsomecorrectionsappliedfortemperature.Thecalibrationcoefficientsare

thenaveragedwitha5000-tapmovingaveragewindowspanning42secondsofelapsed

time.Thus,itisnecessaryforthehardwaregaintobestablewithperiods<84secondsso

theprocessNyquistsamplesthehardwarebehavior.Thegainspectrumandaveraging

filterresponseareshowninFig.17.Thegainisstable(<<10-5)above1mHz(1000-second

period)sothe42-secondfilterisaveragingoversmallfluctuations.Theorbitperiodof

5900secondsismarkedforreference.Thetemperaturecorrectionalgorithmcompensates

fororbital-scaledependenceingain.Below1mHz,thegainspectrumbeginstorisewithf-α

typebehavior;however,thischaracteristicisfullycapturedbythecalibrationscheme.

Noisediodecalibrationsourcesareknowntodriftonlongtimescaleswhileonorbit

(e.g.,[24])andthisbehaviorwasnoexceptiononSMAP’sprecursorAquarius,which

exhibitedanexponentialdriftwith0.5%amplitudeand100-daytimeconstant[25].Noise

sourcedriftof0.5%isequivalentto1Kdriftovertheocean.Thecalibrationdriftoverland

duetonoisesourcedriftislessbecausethelandantennatemperaturesareclosertothe

internalreferenceloadtemperature.SMAPusesthesamebasicdesignasAquariusforthe

noisesources,solong-termdriftisexpectedandisbeingmonitored.Becauseofthe

potentiallylongtimeconstantof100days,itistooearlytodetermineifSMAPisexhibiting

exactlysimilarbehavior.Nonetheless,SMAPstabilityismonitoredagainstaglobally

averagedoceanmodel(basedonthatdescribedin[26])withresultsreportedin[27]-[29].

CalibrationdriftcastasarelativechangeinnoisediodeintensityisplottedinFig.18.The

lowercurveshowstheestimateddriftincludingseveralstepsdownwardandupwarddue

tointentionalandunintentionalpowercyclesoftheradartransmitter.Theuppercurveis

anestimateofthedriftduetochangesinradiometerhardwareovertheyearwiththesteps

removed.Theverticaldashedlinesindicatethestartandfinishofsolareclipseseasonfor

thespacecraftduringthesouthernhemispherewinter.Thebumpdownwardsin

calibrationimmediatelyaftereclipseendsislikelyduetosomeuncompensatedfront-end

thermaleffect.Asdiscussedin[27]-[29],thereremainsuncertaintyinradomeandreflector

emissivitythatisconfoundedwithnoisesourcedrift.Theseparationoftheerrorsand

correctionthereofaretopicsofon-goingcalibrationactivities.Nonetheless,thestabilityof

SMAPisconsistentwithAquarius,althoughthephysicalmechanismsandtemporal

characteristicsarenotyetfullyresolved.

VII. StokesImagery

SMAPprovidesglobalcoveragewitha3-dayrevisitonan8-dayrepeatorbitcycle.

AntennatemperaturedataaveragedoveronesuchcycleforallfourmodifiedStokes

parametersareshowninFig.19.ThesedataareduringthethirdweekofApril2016from

[20].TheimpactofmoistureinthesoilonantennatemperatureinFig.19(a)verticaland

(b)horizontalpolarizationsisquiteevidentacrossthenorthernhemisphere,includingthe

MidwestandtheStateofTexasintheUnitedStateswhereextremeprecipitationledto

floodingresultinginlowbrightnesstemperatures180-190Kshowninblueonthecoolend

ofthecolorscale.OtherphysicalfeaturesofnoteincludethehighemissivityoftheAmazon

rainforestandthedrySaharashowninredatthewarmendofthecolorscale.Note,the

colorscalewastruncatedat(168-282K)toemphasizecontrastinantennatemperatureof

land.Thistruncationnecessarilysaturatesatoceanantennatemperatures,where

dominanceofFresnelreflectivitywouldotherwisebeevidentincontrastbetweenvertical

andhorizontalpolarizations.Strongcontrast,however,isseeninregionsofseaicewith

190and230Kantennatemperaturesathorizontalandverticalpolarizations,respectively.

ThethirdStokesantennatemperatureFig.19(c)showsastrongdipolefeature

causedbyionosphericFaradayrotation.ThedipoleinthirdStokesarisesfromalignmentof

earth’smagneticfieldwithdirectionofpropagationofobservedmicrowaveemission.The

amplitudeofthirdStokesisstrongestoveroceanbecauseofthelowemissionandstrong

polarizationcausedbyitsFresnelreflectivityandweakeroverland(especiallyAmazonand

Congorainforests)duetohigheremissivityanddepolarizationduetoscattering.Thereare

alsoartifactsintheimageduetocombiningascendinganddescendingorbits.Thefourth

Stokesparameter,ontheotherhand,isnearlynon-existentinFig.19(d).Thecolorscaleis

slightlyoffsettoaccountforaglobalbias,likelyduetoantennacross-polarizationmixing.

Thecontinentsareslightlymorenegativethanoceanandtheiroutlinesareevident

becauseofantennacross-polmixing,whichcouplessomecombinationoffirstandsecond

intofourthStokes.Finally,thereareuniquepatternsoffourthStokesantennatemperature

overGreenlandandAntarctica,perhapsvestigesofthepolarimetricsignaturewitnessedby

WindSat[30].

VIII. Discussion

OneyearofnearlycontinuousoperationoftheSMAPL-bandmicrowaveradiometer

wasmarkedonMarch31,2016.Twokeytechnologies–the6-meterscanningreflectorand

theRFIdetectionandfilteringdigitalbackendwithpolarimetriccapabilities–combineto

maketheradiometerunique.Radiometerfootprintssampledat17-msprovideangular

NyquistsamplingandexhibitNEDT<1K.On-boardcalibrationcombinedwithgood

thermalstabilityyieldsexcellenton-orbitgainstability.Globalswathimageryofallfour

Stokesantennatemperaturesshowsgoodresults.Verticalandhorizontalpolarized

channelsdisplayexpectedbehaviorforland,oceanandicescenes.ThethirdStokeschannel

respondsstronglytoionosphericFaradayrotation.ThefourthStokeschannelindicates

littlecircularlypolarizedemission,exceptoverlargeicesheets.Theinstrumentcontinues

tooperateequallywellasofthiswriting.Thus,theradiometermeetsthekeyanddriving

missionrequirementsneededtomeasuresoilmoistureat40-kmspatialresolutionand

0.04volumetricuncertainty.

Acknowledgments

SMAPismanagedforNASA'sScienceMissionDirectorateinWashingtonbyJPL,

withinstrumenthardwareandsciencecontributionsmadebyNASA'sGoddardSpaceFlight

CenterinGreenbelt,Maryland.JPLbuiltthespacecraftandisresponsibleforproject

management,systemengineering,radarinstrumentation,missionoperationsandthe

grounddatasystem.NorthropGrummanAstroAerospaceDivisionmadethereflectorand

boomassembly.Goddardisresponsiblefortheradiometerinstrumentandsciencedata

products.

References

[1] D.Entekhabi,E.G.Njoku,P.E.O’Neill,etal.,“TheSoilMoistureActivePassive(SMAP)

Mission,”Proc.IEEE,Vol.98,No.5,pp.704-716,May2010.

[2] Kerr,Y.H.etal.,``TheSMOSMission:ANewToolforMonitoringKeyElementsofthe

GlobalWaterCycle,''Proc.IEEE,vol.98,pp.666-687,2010.

[3] G.Lagerloefetal.,“TheAquarius/SAC-DMission:DesignedtoMeettheSalinityRemote

SensingChallenge,”Oceanography,vol.21,no.1,pp.68–81,Mar.2008.

[4] NationalResearchCouncil.EarthScienceandApplicationsfromSpace:National

ImperativesfortheNextDecadeandBeyond.Washington,DC:TheNationalAcademies

Press,2007.[Online].Available:http://dx.doi.org/10.17226/11820.

[5] D.Entekhabietal.,"TheHydrosphereState(Hydros)SatelliteMission:anEarthSystem

PathfinderforGlobalMappingofSoilMoistureandLandFreeze/Thaw,"Trans.IEEE

Geosci.Rem.Sens.,vol.42,no.10,pp.2184-2195,Oct.2004.

[6] "NASASoilMoistureRadarEndsOperations,MissionScienceContinues",NASA/JPL,

2016.[Online].Available:http://www.jpl.nasa.gov/news/news.php?feature=4710.

[Accessed:28-Mar-2016].

[7] P.N.Mohammedetal.,"SMAPL-BandMicrowaveRadiometer:RFIMitigationPrelaunch

AnalysisandFirstYearOn-OrbitObservations,"inIEEETrans.Geosci.Rem.Sens.,vol.54,

no.10,pp.6035-6047,Oct.2016.

[8] P.W.Gaiseretal.,"TheWindSatSpacebornePolarimetricMicrowaveRadiometer:

SensorDescriptionandEarlyOrbitPerformance,"Trans.IEEEGeosci.Rem.Sens.,vol.42,

no.11,pp.2347-2361,Nov.2004.

[9] T.Kawanishietal.,"TheAdvancedMicrowaveScanningRadiometerfortheEarth

ObservingSystem(AMSR-E),NASDA'scontributiontotheEOSforglobalenergyand

watercyclestudies,"Trans.IEEEGeosci.Rem.Sens.,vol.41,no.2,pp.184-194,Feb.

2003.

[10] D.M.LeVine,G.S.E.Lagerloef,F.R.Colomb,S.H.YuehandF.A.Pellerano,"Aquarius:An

InstrumenttoMonitorSeaSurfaceSalinityfromSpace,"Trans.IEEEGeosci.Rem.Sens.,

vol.45,no.7,pp.2040-2050,July2007.

[11] S.Brown,C.Ruf,S.Keihm,A.Kitiyakara,“JasonMicrowaveRadiometerPerformance

andOn-OrbitCalibration,”MarineGeodesy,vol.27,no.1-2,pp.199-220,2004.

[12] S.MisraandC.S.Ruf,``DetectionofradiofrequencyinterferencefortheAquarius

radiometer,''IEEETrans.Geosc.Rem.Sens.,vol.46,pp.3123--3128,2008.

[13] J.R.Piepmeier,J.T.Johnson,P.N.Mohammed,D.Bradley,C.Ruf,M.Aksoy,R.Garcia,D.

Hudson,L.Miles,M.Wong,"Radio-FrequencyInterferenceMitigationfortheSoil

MoistureActivePassiveMicrowaveRadiometer,"inTrans.IEEEGeosci.Rem.Sens.,

vol.52,no.1,pp.761-775,Jan.2014.

[14] J.R.PiepmeierandGasiewski,A.J.,"DigitalCorrelationMicrowavePolarimetry:

AnalysisandDemonstration,"Trans.IEEEGeosci.Rem.Sens.,vol.39,no.11,pp.2392-

2410,Nov.2001.

[15] P.E.O'Neill,E.G.Njoku,T.J.Jackson,S.Chan,andR.Bindlish,“SMAPAlgorithm

TheoreticalBasisDocument:Level2&3SoilMoisture(Passive)DataProducts,”Jet

PropulsionLaboratory,CaliforniaInstituteofTechnology,Pasadena,CA,JPLD-66480,

2015.[Online.]Availableat

http://nsidc.org/data/docs/daac/smap/sp_l2_smp/index.html

[16] S.H.Yueh,"EstimatesofFaradayrotationwithpassivemicrowavepolarimetryfor

microwaveremotesensingofEarthsurfaces,"Trans.IEEEGeosci.Rem.Sens.,vol.38,

no.5,pp.2434-2438,Sep2000.

[17] Piepmeier,J.R.etal.SMAPAlgorithmTheoreticalBasisDocument:L1BRadiometer

Product.SMAPProject,NASAGSFCSMAP-ALGMS-RPT-006Rev.B,NASAGoddardSpace

FlightCenter,Greenbelt,MD.Feb.2016.[Online.]Availableat

http://nsidc.org/data/docs/daac/smap/sp_l1b_tb/index.html

[18] C.S.Ruf,S.J.KeihmandM.A.Janssen,“TOPEX/POSEIDONmicrowaveradiometer

(TMR):I.Instrumentdescriptionandantennatemperaturecalibration,”IEEETrans.

Geosci.RemoteSens.,vol.33,pp.125–137,Jan.1995.

[19] J.PengandC.S.Ruf,"CalibrationMethodforFullyPolarimetricMicrowave

RadiometersUsingtheCorrelatedNoiseCalibrationStandard,"inIEEETrans.Geosci.

Rem.Sens.,vol.46,no.10,pp.3087-3097,Oct.2008.

[20] Johnson,Chris,“TemperatureStabilityandControlRequirementsforThermal

Vacuum/ThermalBalanceTestingoftheAquariusRadiometer,”25thSpaceSimulation

Conference,Annapolis,MD,October20-23,2008.

[21] Piepmeier,J.R.,P.Mohammed,J.Peng,E.J.Kim,G.DeAmici,andC.Ruf.2015.SMAP

L1BRadiometerHalf-OrbitTime-OrderedBrightnessTemperatures,Version3.[March31-

April3,2015].Boulder,ColoradoUSA.NASANationalSnowandIceDataCenter

DistributedActiveArchiveCenter.http://dx.doi.org/10.5067/VIQYQV0AJATI.

[22] Mastropietro,A.J.,etal.,"PreliminaryEvaluationofPassiveThermalControlforthe

SoilMoistureActivePassive(SMAP)Radiometer,"41stInternationalConferenceon

EnvironmentalSystems(ICES),2011,AIAA2011-5070.

[23] Mikhaylov,R.,E.Kwack,R.French,D.Dawson,andP.J.Hoffman,"Implementationof

ActiveThermalControl(ATC)fortheSoilMoistureActiveandPassive(SMAP)

Radiometer,"44thInternationalConferenceonEnvironmentalSystems(ICES),2014,

ICES-2014-060.

[24] Brown,S.T.,S.Desai,W.LuandA.Tanner,"OntheLong-TermStabilityofMicrowave

RadiometersUsingNoiseDiodesforCalibration,"inIEEETrans.Geosci.Rem.Sens.,vol.

45,no.7,pp.1908-1920,May2007.

[25] Piepmeier,J.R.,L.HongandF.A.Pellerano,“AquariusL-BandMicrowaveRadiometer:

3YearsofRadiometricPerformanceandSystematicEffects,”IEEETrans.Geosci.Rem.

Sens.,vol.8,no.12,pp.5416-5423,Dec2015.

[26] F.J.WentzandD.M.LeVine,“Algorithmtheoreticalbasisdocument:Aquariussalinity

retrievalalgorithm,”RemoteSensingSystems,SantaRosa,CA,USA,RSSTech.Rep.

082912,2012[Online].AvailableFTP:ftp://podaac-

ftp.jpl.nasa.gov/allData/aquarius/docs/v2/AQ-014-PS-0017_AquariusATBD_Level2.pdf

[27] J.R.Piepmeieretal.,SMAPRadiometerBrightnessTemperatureCalibrationforthe

L1B_TBandL1C_TBBeta-LevelDataProducts,NASA’sSMAPProject,JetPropulsion

Laboratory,Pasadena,CA.Jul.2015.[Online.]

https://nsidc.org/data/smap/data_versions.

[28] J.R.Piepmeieretal.,SMAPRadiometerBrightnessTemperatureCalibrationforthe

L1B_TBandL1C_TBVersion2DataProducts,NASA’sSMAPProject,JetPropulsion

Laboratory,Pasadena,CA.Nov.2015.[Online.]

https://nsidc.org/data/smap/data_versions.

[29] J.R.Piepmeieretal.,SMAPRadiometerBrightnessTemperatureCalibrationforthe

L1B_TBandL1C_TBVersion3DataProducts,NASA’sSMAPProject,JetPropulsion

Laboratory,Pasadena,CA.Apr.2016.[Online.]

https://nsidc.org/data/smap/data_versions.

[30] Li,L.,P.Gaiser,M.R.Albert,D.G.LongandE.M.Twarog,"WindSatPassiveMicrowave

PolarimetricSignaturesoftheGreenlandIceSheet,"IEEETrans.Geosci.Rem.Sens.,vol.

46,no.9,pp.2622-2631,Sept.2008.

Fig.1.SoilMoistureActivePassive(SMAP)observatoryinfullydeployconfiguration.ImageCredit:NASA/JPL-Caltech.

Fig.2.Footprintspacingnearsub-satellitetrack.

11 km x 28 kmspacing near

sub-satellite track

Sub-satellite trackEFOV

39 km x 47 km3-dB contours

(a) (b)

Fig.3.SMAPobservatory(a)solidmodelfrommechanicaldesignsoftwareand(b)meshmodelfor

3Dantennaanalysissoftware.

(a)

(b)

Fig.4.Antennapatterncutofhorizontalpolarizationalongtheantennascanningdirectionfor(a)

thefullpatternand(b)themainlobe.Comparisonisbetweencalculatedandmeasureddataforthe

1/10SMAPscalemodel.

Fig.5.RadiometerBlockDiagramincludingfeednetwork,RadiometerFront-End(RFE),Back-End

(RBE),andDigitalElectronics(RDE).Signalflowisfromrighttoleft.

Fig.6.FrequencyresponseofSMAPmicrowaveradiometer.ThisresponsecombinesRF,IFand

digitalfilters.Verticaldottedlinesindicatethespectrumallocationat1400to1427MHz.

frequency (MHz)1380 1400 1420 1440

resp

onse

(dBc

)

-140

-120

-100

-80

-60

-40

-20

0

Fig.7.TimingofradiometersamplingforaPRI,apacket,andafootprint.

Radiometer Integration Window ~300 μs

Pulse Repetition Interval (PRI) ~350 μs

Radar Transmit

PRI 1 PRI 2 PRI 3 PRI 4

Radiometer Packet1.4 ms elapsed time

1.2 ms integration time

PRI

Packet

Footprint 1 2 3 4 5 6 7 8 9 10 11 12

4 Packets of Scene ObservationCal

Counts4 Packets of Scene ObservationCal

Counts

Fig.8.Representativefrequencyresponseofaveragingperformedinradiometeroperationand

processing.Thetworightmosttraces(dash-dotanddash)showthelow-passresponseoftheon-

boardboxcarintegratoroveroneandfourPRI,respectively.Thesolidtraceshowsthefrequency

responseofthecalibratedantennatemperaturesfoundintheLevel1B_TBdataproduct.Finally,the

dottedlinemarked“antenna”showstheequivalentlow-passresponseoftheantennabeam

(sweepingalong-scaninazimuth)tonaturallyoccurringthermalradiation.

Frequency (Hz)100 101 102 103 104

Nor

mal

ized

PSD

(Hz-1

)

0

0.2

0.4

0.6

0.8

1 51 17 1.4 0.35 ms 39 13 km

PRI4PRIL1B_TBantenna

Frequency (Hz)

Fig.9.Simplifiedcalibrationmodelshowinglumpedlossesandphysicaltemperatures.

Fig.10.Spacecraftmodelshowsfeedhornviewingcoldspacewithstowedreflector.

Lreflector Treflector

Reflector

Lradome Tradome

Radome

Lfeed Tfeed

Feed

LOMT TOMT

OMT

Lcoup Tcoup

Coupler

Ldip Tdip

Diplexer

TA T ’A

TRFE TND Trx

Lswitch

+ +

ref. noise receiver switch source noise

Fig.11.Antennatemperature(V-pol)measuredinstowedconfigurationshowingpre-andpost-

launchcalibrationresultscomparedtomodeledcold-spaceantennatemperature.Theinitialresult

isbiased5Klowconsistentwithpre-launchcalibrationuncertainty.H-polmeasurementsshoweda

smaller1-Kdifference.

Time (hour/minute)22:30 22:45 23:00 23:15 23:30

TA (K

)-2

0

2

4

6

8

post-launch

pre-launch

modeled

Fig.12.Horizontallypolarizedbrightnesstemperature(K)measuredwithstatic(nonrotating)

antenna.Widthoftheswathis40-kmandisexaggeratedhereforclarity.

Fig.13.Radiometricresolution(NEDT)dailyaveraged(overland,ocean,andice)duringfirstyear

ofoperationsforallfourStokesbrightnesstemperatures.ThethirdandfourthStokesparameters

(topcurves)haveNEDT√2largerthanverticalandhorizontalpolarizationsasexpected.

Apr'15 Jul'15 Oct'15 Jan'16 Apr'16

NEDT

(K)

0.8

0.9

1

1.1

1.2

1.3

1.4

TA,3, TA,4

TA,v

TA,h

(a)

(b)

Fig.14.Temperaturesof(a)feednetworkand(b)internalreferenceloadandnoisesourcefor

horizontal-polarizationduringfirstyearofoperations.

Apr'15 Jul'15 Oct'15 Jan'16 Apr'16°C

10

15

20

25

30H-pol Feed Network

Feedhorn

OMT

Coupler

Diplexer

Tem

pera

ture

(oC

)

Apr'15 Jul'15 Oct'15 Jan'16 Apr'16

°C

19.9

20

20.1

20.2

20.3

20.4

20.5RFE

o C

Noise diode

Reference

Tem

pera

ture

(oC

)

F

(a)

(b)

Fig.15.Temperaturesof(a)feednetworkand(b)internalreferenceloadandnoisesourcefor

horizontal-polarizationoveronorbitonJune23,2015nearthepeakthermaleffectofeclipse

season.

00:30 01:00 01:30

°C

10

12

14

16

18

20

22H-pol Feed Network

CouplerOMT

Diplexer

FeedhornTe

mpe

ratu

re (o

C)

00:30 01:00 01:30

°C

20

20.05

20.1

20.15

20.2

20.25RFE

Noise diode

Reference

Tem

pera

ture

(oC

)

(a)

(b)

Fig.16.Noisesourcebiashistorymeasuredonan8-dayperiodduringfirstyearofoperations.(a)

Currentbiasvariationiswithin±1.1μAor±180ppmand(c)avalanchevoltagevaries±0.7mVor

±80ppm.

Apr'15 Jul'15 Oct'15 Jan'16 Apr'16"

i b (7A)

-2

-1

0

1

2

Apr'15 Jul'15 Oct'15 Jan'16 Apr'16

" V

Br (m

V)

-1.5

-1

-0.5

0

0.5

1

1.5

Fig.17.Radiometergainstabilityandgainaveragingfilterresponses.Therightmosttraceisthe

responseoftheon-boardsamplingofnoise-diodeandreferencecountsevery8.4ms.Thescience

processingsoftwareaverages5000gainestimatestogether,whichspan42seconds.Theorbitcycle

ismarkedat5900seconds.Thespectrumofgaincoefficientfluctuation,theleftmosttrace,reveals

anincreasingspectrumbelow1mHz.

Frequency (Hz)10-4 10-2 100 102

Nor

mal

ized

PSD

(Hz-1

)

0

0.2

0.4

0.6

0.8

15900 42 0.008 s

L1B averaginggain spectrum

10-4

5x 10-5

0

(ΔG

/G)2

(Hz-

1 )

Frequency (Hz)

on-board averaging

Fig.18.Radiometercalibrationstabilitycastasgaindriftduringfirstyearofoperationdetermined

fromcomparisontoglobally-averagedoceanmodel(lowercurve).ThetwostepsinAprilaredueto

intentionalchangeinphysicaltemperatureduringearlycommissioningactivities.Thestepnear

June14wasduetoanintentionalpowercycle.ThefinallargestepinJulyisduetoterminationof

radartransmission.Theuppercurveisthesamenoisesourcedriftwiththeradar-inducedsteps

removed.Theverticaldashedlinesindicatedthestartandfinishofsolareclipseseasonexperienced

bySMAPinthesouthernhemispherewinter.

Apr'15 Jul'15 Oct'15 Jan'16 Apr'16

"T N

D/TND(%)

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

Fig.19.SMAPradiometermodifiedStokesantennatemperaturesgriddedandaveragedduring1-7

September2015.Thefourpanelsshow(a)verticallypolarized,(b)horizontallypolarized,(c)third

and(d)fourthStokesparameters,respectively.

80 100 120 140 160 180 200 220 240 260 280 80 100 120 140 160 180 200 220 240 260 280

-4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3

(a) (b)

(c) (d)