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.