DeepSpaceClimateObservatory,EarthScienceInstrumentOverview

Version1June28,2016

1.0 MISSIONINTRODUCTION........................................................................................................................31.1 ORBIT................................................................................................................................................................................4

2.0 EARTHPOLYCHROMATICIMAGINGCAMERA(EPIC)....................................................................62.1 TELESCOPE......................................................................................................................................................................72.2 FILTERWHEELANDSHUTTER......................................................................................................................................82.3 CHARGECOUPLEDDEVICE&INSTRUMENTSPECTRALSENSITIVITY.....................................................................92.4 EPICPRODUCTS..........................................................................................................................................................13

3.0 NISTADVANCEDRADIOMETER(NISTAR)......................................................................................143.1 RADIOMETERASSEMBLYOVERVIEW......................................................................................................................153.1.1 FilterWheel............................................................................................................................................................163.1.2 Shutters....................................................................................................................................................................18

3.2 NISTARPRODUCTS...................................................................................................................................................193.2.1 Level1A....................................................................................................................................................................193.2.2 Level1B....................................................................................................................................................................19

1.0 MISSION INTRODUCTION

TheDeepSpaceClimateObservatory(DSCOVR)isaspaceweatherandEarthsciencespacecraftsituatedattheEarth-SunLagrangepoint(L1).LaunchedonFebruary11th,2015,DSCOVRarrivedintoorbitonJune7th,2015.ThespacecraftismanagedbyNationalOceanicandAtmosphericAdministrationwithNOAAhandlingthespaceweatherinstrumentsandday-to-dayoperations,andtheNationalAeronauticsandSpaceAdministration(NASA)providingtheEarthsciencedataprocessingandspaceweatherinstrumentcalibrations.

TheprimarymissionofDSCOVRistomonitorthereal-timesolarwindandenhanceNOAA’sabilitytoprovidespaceweatheralertsandforecasts.ItssecondarymissionistoenableNASAtostudyEarth’sclimate.ThevantagepointfromL1,uniquetoEarthscience,enablesimagerywithahigherspatial-temporalcadence,andthemonitoringoftheEarth’sreflectedandemittedradiation.

Figure1-DSCOVRspacecraftlayout

TheEarthscienceinstrumentsconsistoftheEarthPolychromaticImagingCamera(EPIC)andtheNationalInstituteofScienceandTechnologyAdvancedRadiometer(NISTAR).Theyarelocated“ontop”ofthespacecraftonitsEarth-facingsideinFigure1.Thesolarinstruments,whichincludetheFaradaycup,ElectronSpectrometer,andMagnetometer,sitontheSun-facingsideofthespacecraft.Moreinformationregardingtheseinstrumentscanbefoundinseparatedocuments.

Earth-Side

Sun-Side

1.1 ORBIT

Figure2-SchematicofDSCOVRorbitatLagrange-1showingtheEarthviewinggeometry

AboveisafiguredescribingDSCOVR’sviewoftheEarthfromtheorbitaroundtheEarth-SunLagrange1point.TheL1pointislocatedatabout1.5millionkilometersfromtheEarth.Theorbitisanon-repeatingLissajousfigure,tiltedwithrespecttotheEarth’seclipticplane.Ittakes6monthsforonecircuit,duringwhichtheapparentsizeoftheEarthvaries,withanominalviewingsizeof.5degrees.Duringthelifeofthemission,theorbitalshapewillchangefromanapproximateellipse,toacircle,andthebacktoanellipseinabout5years.TheorbitatL1isquasi-stable,andrequiresperiodiccorrectionfromtheonboardhydrazinerocketmotors.BelowarefiguresrepresentingtheLissajousorbit.

Figure3-Sizeandanglesforanon-repeatingLissajousorbitatL1.Eachapproximatecircuittakes6months.TheactualorbitisirregularbecauseofperturbationsfromtheMoonandperiodicorbitmaintenanceadjustments.

Figure4-DiagramofSolar/Earth/Viewingangle(SEV)overtimeintheorbit

InFigure4,thechangesintheSun-Earth-Viewinganglearerepresented.TheeffectoftheSEVanglecanbeseenintheEPICimages,asthelocationofthesolarterminatorswitchesfromEasttoWestoverthecoarseofanorbit.AlthoughDSCOVRisorientednominallysoitispointedattheEarth,thespacecraftoccasionallyperformscalibrationmaneuversthatinvolveoffpointingatdifferentangles.

TheplannedmissionlifeofDSCOVRis5years.

2.0 EARTH POLYCHROMATIC IMAGING CAMERA (EPIC)

TheEarthPolychromaticImagingCamera(EPIC)isa10-channelspectroradiometeronboardDSCOVR.AfterthearrivalofDSCOVRatL1onJune7th,2015,EPIC’sdoorwasopenedandstartedimagingonJune9th.

Fromitsvantagepoint,EPIChasauniqueviewoftheEarth,takingimagesofthefullylitsurfaceat1-1.6hourintervalsperday.

Figure5-EPICexteriordesign.ThefilterwheelcanbeseeningreenandtheCCDiscontainedwithintheorangesectionlabeled“Detectorhousing”

TheEPICinstrumentconsistsofa2048x2048charge-coupleddevice(CCD)attachedtoa30cmf/9.6Cassegraintelescope(Figure2).Usingafilterwheel,itsamplesat10channelnarrow-bandrangesfrom317.5nmto780nm(Table1).Thesebandsprovidetheabilitytogenerateavarietyofscienceproducts,includingozone,sulfurdioxide,aerosols,vegetation,andcloudheight.

Table1-EPICWavelengthsandmaindataproducts

2.1 TELESCOPE

TheEarthviewedbyEPIChasanominalwidthofapproximately0.5degrees.Theinstrumentstotalfieldofview(FOV)is0.62degrees.BecauseoftheLissajousorbitaboutL1,theapparentsizeoftheEarthchangesslightlyduringthe6-monthorbitalperiod.Figure5showsthesizeoftheEarthrelativetoEPIC’sfieldofviewFOV.WhilethepixelsizesamplestheEarthneartheequatorat8x8km2,thespatialresolutionofanEPICimageisabout12x12km2.Theresolutionisreducedbythe

Table1EPICWavelengthsandmaindataproducts

Wavelength(nm)

317.5±0.1

325±0.1

340±0.3

388±0.3

443±1

551±1

680±0.2

687.75±0.2

764±0.2

779.5±0.3

FullWidth(nm)

1±0.2

2±0.2

3±0.6

3±0.6

3±0.6

3±0.6

2±0.4

0.8±0.2

1±0.2

2±0.4

PrimaryApplication

Ozone,SO2

Ozone

Ozone,Aerosols

Aerosols,Clouds

Aerosols

Aerosols,Vegetation

Aerosols,Vegetation,Clouds

CloudHeight

CloudHeight

Clouds,Vegetation

CCD

FOV

.62°

Earth

.45°to

.53°

Figure6-EPICimagedemonstratingtheCCDfieldofviewangleversustheangularsizeoftheEarth

cosineofthelatitudeorlongitude,sothatby60ONtheresolutionis24x24km2.

Theschematicbelowofthetelescope(Figure7)showstheprimaryandsecondarymirrors,focusingorfieldlensgroup(FLG),andthefilterwheellocationjustinfrontoftheCCD.ThepurposeoftheFLGistoreducetheaberrationsinherentinaCassegraindesign.IntheEPICsystem,theFLGisphysicallylocatedbetweentheprimarymirrorandthefilterwheels.Alongwithnewfiltersandanti-reflectioncoatings,theFLGwasoneoftheopticalelementsreplacedwithanimproveddesigntoreducestraylight.

Figure7-SchematicoftheEPICtelescope

TheDSCOVRinstrumentsandspacecraftareshowninFigure1,demonstratingtheirrelativesize.EPICwaslaunchedwiththeexteriorlensdoorclosed.ThedoorwasopenedoncewhenthespacecraftachievedorbitatL-1.

2.2 FILTER WHEEL AND SHUTTER

Insidethetelescopeisadoublefilterwheelwith12spacesfor10filtersplus2openholes(Figure8)arrangedwithsixholesineachwheel.Computercontrolledsteppermotorspositionthefiltersinapre-determinedsequencethatcanbealteredbyonboardorground

commands.

Figure8-TheEPICdoublefilterwheel

Infrontofthefilterwheelsthereisashuttermechanism(Figure9).Theshutterbladecontains3slots:thenarrowandmediumwidthslotsareusedtoproduce2to10msexposures,respectively;thewideslotisusedforexposures>46ms.For10to46msexposures,theshutterbladeismovedsothatthemiddlesizedslotcrossesthelightpathinasinglemotion.Forexposureslongerthan46ms,thewideslotisused.Whileintheopenposition,thebladeslowsdown,thenspeedsuptocompletetheexposure.Foranexposurelongerthanabout60ms,thebladecomestoacompletestopintheopenposition,priortoclosing.

Figure9-Shutterwheelassemblyshowing3shutterpositions;Narrow,Medium,andWide

2.3 CHARGE COUPLED DEVICE & INSTRUMENT SPECTRAL SENSITIVITY

Thefocalplaneisa2048x2048pixelCCD,backside-thinned,backside-illuminated

andanti-reflection(hafnium)coatedtooptimizequantumefficiencydownto300nm.TheCCDispassivelycooledtoapproximately-20°Conorbittoreducedarkcurrentandothernoiseeffects.InnormaloperationtheCCDwillbereadoutfromoppositecornersat500kHz,buttheentirearraycanbereadoutfromeitherside,thusprovidingsomemeasureofredundancy.TheCCDcharacteristicsaresummarizedinTable2.

Table2-EPICCCDcharacteristics

CCDcharacteristicsCCDformat 2048x2048pixels(3.072x3.072cmsquare)Pixelsize 15μmx15μm,100%fillfactorPixelFOV 1.078arc-secCCDtype Thinned,backsideilluminatedSpectralrange 200-950nm(QE>25%)Pixelfullwelldepth >95,000electronsReadout Singleordual(diagonalcorners)Pixelreadoutrate 500kHzCCDoperatingtemperature -40°C,bypassivecoolingSystemNoise:topanalogtrain sigma=1.6DNSystemNoise:bottomanalogtrain sigma=3DNDarkcurrent <5electronspersecondperpixel

TheCCDquantumefficiencyisshowninFigure10showingareasonableresponseatbothendsofthedesiredspectrum,317nm(80%)and800nm(50%).

Figure10-CCDquantumefficiencyshowingtheusefulrangeoftheEPICinstrumentwithinthebox

ThemeasuredtelescopetransmissionisshowninFigure11,whichvariesbetween70and80%oftheusefulrangeofEPIC

Figure11-Telescopetransmissioninpercent

ThemirrorsaremadefromZerodurwithanaluminumcoatingandaSiO2coatingontheprimarymirrorandaluminumovercoatedwithMgF2coatingonthesecondarymirror.Thestructuremaintainingtheopticalseparationbetweentheprimaryandsecondarymirrors(meteringtube)isagraphitecompositecylinderdesignedtoexhibit0CTE(coefficientofthermalexpansion).Themechanicalstructuresupportingtheprimaryandsecondarymirrors(betweenthemirrorsandthemeteringtube)isInvar36tominimizethermalexpansionproperties.

TheEPICfilterswererefurbishedtohaveimprovedantireflectioncoatingsand

betteroutofbandrejection(lightfromotherwavelengths).ThemeanfiltertransmissionfunctionsareshowninFigure12.Formostscienceproducts,themeasurementsfromeachfilterarecombinedinpairsasaratioordifference.Anexceptionisforestimatingcloudreflectivityusing340or388nm,whereasinglechannelisused.

Figure12-Meanfiltertransmissionfunctionsinpercent

Inordertobeuseful,certaincorrectionsmustbeappliedbeforeeitherimagesorscienceproductscanbeobtained.Themajorcorrectionsarefor“flat-fielding”andstraylight.“Flat-fielding”isbasedonmeasurementswithauniformlightsourcetomeasurethedifferencesinsensitivityforeachofthe4millionpixels.TheresultingcorrectionmapisappliedtothemeasuredcountsfromtheCCD.StraylightwasmeasuredinthelaboratoryusingaseriesofsmalldiameterlightsourcesenteringthetelescopeandbeingdetectedbytheCCD.Theilluminationbypixelsoutsidethemaindiameterofthelightsourcewasmeasured.Adetailedmatrixmapwasmadeoftheentirestraylightfunction(theeffectoflightdirectedateachpixelaffectingeveryotherpixel).Thestraylightcorrectionisappliedtoeveryimage.Othercorrectionsarealsoappliedbasedonlaboratorymeasurements,suchastheetaloningatlongerwavelengthsduetointerferenceeffectsbetweenthefrontandbackofthesiliconCCDwhichissemitransparentforwavelengthslongerthan600nm(redandnear-IR).Outofbandleakageisverysmall(0.04percentfor325nm).

2.4 EPIC PRODUCTS

EPICdatasetsproducedandsenttotheAtmosphericScienceDataCenterconsistofalevel1A,alevel1Bproduct,andacolorimage.Alevel1Aandlevel1Bproducteachconsistsof1setofimages.ForEPIC,an“image”isdefinedasasingleimagetakenofoneband.A“set”isdefinedas10images(1imageperband)takenduringasingleviewingperiod.Aviewingperiodistypicallywithin8minuteslengthandthenumberofviewingperiodsisbetween13-22perday.

Inlevel1A,theEPICimageshavebeenprocessedfromthespacecrafttelemetry,decompressed,andcorrectedtoremoveartifacts.Thisincludestheflatfieldingcorrection,searchingforbrightenedor“enhanced”pixels,readwavecorrections,offsets,latency,slope,andnon-linearitycorrections,andcalibrationfortemperature.Theimageshavebeennormalizedintounitsof“countspersecond”;thenumberofphotosthatthedetectormeasuredovertime.Thelevel1Aproductsareintheoriginalpixelgridconfigurationandorientationasmeasuredbythesensor.

Forlevel1A,thereisappendedgeolocationinformation.Thisincludeslatitudesandlongitudes,aswellasper-pixelsunandinstrumentviewingangles.Metadataisincludedthatdescribesthesensorconfigurationatthetimetheimagewastaken,aswellasancillarygeolocationandspacecraftorientationinformation.

Forlevel1B,thegeolocationisapplied.Thelevel1Aimagesarereprojectedintoonefixedgridfortheset.ThiscorrectsforthelocationdriftthatoccursduetotherotationoftheEarthandthemotionofthespacecraftovertheviewingperiod.Afterthisprocess,thepixelsinthedifferentbandswillhavethesamephysicallocationacrosstheset.

Thesameancillarydataandmetadataisincludedforthelevel1Baswasinthelevel1A.

3.0 NIST ADVANCED RADIOMETER (NISTAR)

TheNationalInstituteofStandardsandTechnologyAdvancedRadiometer(NISTAR)instrumentiscomposedofthreecavityradiometersandonephotodiodechannel.TheinstrumentisdesignedtotakeadvantageofDSCOVR’slocationattheL1pointtomeasuretheenergyemittedandreflectedbytheEarth.BymeasuringtheEarth'senergybalance,thesolarenergystrikingEarthminuswhatisreflectedandradiatedintospace,NISTARwillimproveourunderstandingoftheeffectsofchangescausedbyhumanactivitiesandnaturalphenomena.NISTARwillprovidethefirstdirectmeasurementsoftheradiantpowerreflectedbythefullEarthdiskwithagoalof0.1%accuracy.NISTARwillmakesimultaneousmeasurementsinthreebandsplusonereference.

• Band-A(0.2to100µ)-visibleplusfarinfraredchanneltomeasuretotalradiantpowercomingfromtheEarth

• Band-B(0.2to4µ)-solarchanneltomeasurereflectedsolarradiation• Band-C(0.7to4µ)-nearinfraredchanneltomeasurereflectedinfraredsolar

radiation• Photodiode(.3to1µ)–channeltobeusedasanon-boardcalibrationreference

NISTAR,picturedinFigure14,consistsoftheRadiometerAssembly(RA)andtheInterfaceandControlElectronics(ICE)module.TheRAhousesthreeelectricalsubstitutionradiometerswithseparateopticalshutter/contaminationdoorsforeach,asiliconphotodiode(PD)filterdetector,aheatsink(HS),andatwelve-positionfilterwheel(FW).TheICEconsistsoftheflightcomputer,motordriver,interfaceandhousekeepingelectronics.

RadiometerAssembly

InterfaceControlElectronic

Figure13-NISTARinstrument

Figure14-NISTARexteriorschematic

NISTARhasaradiometerFieldofView(FOV)of1°andFieldofRegard(FOR)of~6.7°.BecauseoftheFOR,theNISTARboresightmustbe3.5°awayfromalightsourcetopreventlightfromthatsourceenteringtheinstrument.Thisisimportantforoperationssuchasdarkspacecalibration.NISTARisalignedwiththeEarthPolychromaticImagingCamera(EPIC)cameraboresight,anotherscientificinstrumentmountedontheDSCOVRobservatory.

3.1 RADIOMETER ASSEMBLY OVERVIEW

TheRAdepictedinFigure16consistsofthreereceivercavitiesthataremechanicallysupportedfromasurroundingHS.Thereceiversareshapedintoacavityforenhancedabsorption.ThreeprecisionaperturesaremountedontheHSdirectlyinfrontofeachofthethreereceivercavities.ThePhotodiodeAssembly,whichconsistsofthephotodiodetelescopeandphotodiode,isalsomechanicallysupportedfromtheHS.Themechanicalenclosureforthereceivercavities,photodiode,andheatsinkconsistsofthebasicmechanicalstructure,fourshuttersandonefilterwheel(includingmotors,drivers,limitdetectorsandassociatedelectronics).

Baffle

ReceiverCavity

Heat Sink

Filter WheelMechanicalEnclosure Shutters

Photodiode

PhotodiodeTelescope

PrimaryAperture

Figure15-Radiometercrosssection

ThedifferentialtemperaturebetweentheReceiverCavities(RC)andtheHSisactivelycontrolledatafixedoperatingpointbyanelectronicservoloop.Also,theHSisactivelytemperaturecontrolledatafixedabsolutetemperaturebyaseparateelectronicservoloop.TheopticalpowerenteringthecavitywhentheshutterisopenedismeasuredbymonitoringtheappliedelectricalheaterpowerrequiredtomaintaintheRCsatafixedoperatingpoint.

3.1.1 Filter Wheel

Thefilterwheelisaduallevelwheelwithtwelveopticalpositionsnumbered1through12eachcapableofholdingtwofiltersinseries.Thefiltersare25millimeters(mm)indiameterandaremadeoffusedsilicawithmulti-layercoatings.Thereareatotalofsixlocationswithfilters(threeeachforBand-BandC),andsixlocationsthatareopen(Band-A)asillustratedinFigure16.Typically,thefiltertransmissioninspacewilldegradewithexposure.Ultravioletradiation,oxygenoroutgassingcontaminantswillchangethefilter.Havingmultiplefiltersforeachbandandusingoneofthefiltersineachband99%ofthetimeandthenswitchingtotheothersforshortperiodsoftimewillallowdegradationofthefilterstobetracked.

Table3andTable4isakeythatcorrelatesthefilterwheelpositionnumbertothepositionsofthevariousfiltersovereachcavity.Thenormalpositionforthewheelisatafilterwheelstepnumberof3.

Table3-Filterwheelpositionversusfilterwheelnumber,part1

FWSteps

Detector 3 102 202 302 402 502

RC1 1C 12A 11B 10A 9A 8C

RC2 4A 3A 2B 1C 12A 11B

RC3 7B 6A 5C 4A 3A 2B

Photodiode 10A 9A 8C 7B 6A 5C

Table4-Filterwheelpositionversusfilterwheelnumber,part2

FWSteps

Detector 602 702 802 902 1002 1101

Figure16-Filterwheelpositiondiagrams

RC1 7B 6A 5C 4A 3A 2B

RC2 10A 9A 8C 7B 6A 5C

RC3 1C 12A 11B 10A 9A 8C

Photodiode 4A 3A 2B 1C 12A 11B

Thefilterwheelhastwomechanicallimitsdetectorslocatedatpositions0and1103.Theseareinplacetokeepthefilterwheelfrombeingabletomakemorethanonerevolutionineitherdirection.Ifthefilterwheelismovedsothatthelimitdetectorishitduringitsmotion,thefilterwheelwillbere-calibratedautomaticallyandwillreturntoposition3ifthe”closed”(position0)limitswitchwashitoritwillbereturnedtoposition1101ifthe“open”(position1103)limitswitchwashit.Thelimitdetectorshavealifetimelimitof25,000hits.

Figure17-Filterwheelandshutterassembly

3.1.2 Shutters

MechanicalshuttersarelocatedatthetopofeachRCandthePDtopreventcontamination.

Eachshutterhastwomechanicallimitsdetectorslocatedatsoftwareshutterpositionnumbers0and200.Theseareinplacetokeeptheshuttersfrommovingtoofarintothemechanicalenclosure.Ifashutterismovedsothatthelimitdetectorishitduringitsmotion,theshutterwillbere-calibratedautomaticallyandwillreturntoposition2ifthe“closed”(position0)limitswitchwashitoritwillbereturnedtoposition198ifthe“open”(position200)limitswitchwashit.Thelimitdetectorshavealifetimelimitof25,000hits.

TheNISTARflightsoftwarehastheabilitytoautocycleselectedshutters.Thisautocyclingopensandclosesonlytheselectedshuttersataselectedperiod.

3.2 NISTAR PRODUCTS

NISTARdatasetsproducedandsenttotheAtmosphericScienceDataCenterconsistofalevel1A,andalevel1Bproduct.Level1Aconsistsofthescience,engineering,andcalibrationdatafortheinstrument.Level1Baresummaryproducts.

3.2.1 Level 1A

EachNISTARinstrumentlevel1Asciencedataproductconsistsofonefull(24hour)day’sworthofdatafromfoursensors,threeactivecavityradiometersandaphotodiodewhichwillservebothasacalibrationreferencefortheradiometersandfilters,andasadetectorintherangefrom320nmto1100nm.Onefulldayisdefinedastheintervaloftimefrom12:00:00.00hUTCto11:59:59.99hUTCthefollowingday(ie,“Noon”to“Noon”).ThedataareprimarilyfromthenearlyfullEarth,butcanalsocontainlunarandstarfielddata.Ancillarydataassociatedwiththesciencedataincludedatacollectiontime,Earthcentroidcoordinates,andspacecraftattitudeandephemeris.

3.2.2 Level 1B

TheNISTARlevel1Bdataproductsaregeneratedfromthelevel1Aproducts.Thelevel1BdataconsistsofirradiancevaluesfornearlytheentireEarth.Irradiancevaluesareseparatedbyfilterband(A,B,C)andtheviewobject(Earthorstarfield).Thedataarecollectedandstoredonfourshuttercycle,fourhours,andonedaytimescales.Thesmallestusefulresolutionforaveragingisequaltofourtimestheshuttercycleperiod,whenshutterautocycleisactivated.Thisisduetothe4-boxcarwidedemodulationalgorithm,whichisusedtoextractanirradiancemeasurementfromtheradiometerpowersignal.ThegroundprocessingsoftwaredetermineswhattheshutterperiodisforeachJulianday.