The airborne LUnar Spectral Irradiance (air-LUSI) Mission

1.OVERVIEW The airborne LUnar Spectral Irradiance (air-LUSI)mission is a NASA

AirborneInstrumentTechnologyTransition(AITT)project.ThegoaloftheAITTprogram is tomature airborne instruments from the demonstration phase toscience-capableinstruments.

The USGS RObotic Lunar Observatory (ROLO)model represents themostpreciseknowledgeoflunarspectralirradianceandisusedfrequentlyasarelative calibration standard for Earth observation by space-borne sensors(KeifferandStone,2005). However,apparentphase-dependentbiasesinROLOlimitsitsapplicationforabsoluteradiometriccalibration.

3.ENGINEERINGFLIGHTS After a year of design anddevelopment, air-LUSI executed its first

testflightatAFRCinPalmdale,CAAug1stand2ndbetweenthehoursof0300and 0500 (UTC-7). The results exceeded expectations. During its test flights,the ARTEMIS subsystem kept the IRIS telescope locked onto the Moon towithin 0.1° on average and the IRIS enclosure kept the spectrometer andvalidation reference solidly at sea-level pressure and constant temperature.Themeasurements recordedduring these flights showedhighsignal-to-noiseratioandsensitivitytothelunarspectrum.Theair-LUSIteamisnowanalyzingtheseretultsandtheengineeringdataandpreparingfortheirdemonstrationflightslaterin2019.

KevinR.Turpie1,SteveBrown2,JohnWoodward2,SteveMaxwell2,ThomasLarason2,ClarenceZarobila2,SteveGrantham2,AndrewGadsden3,AndrewCataford3,TomStone4

1UniversityofMaryland,BalitmoreCounty(UMBC),2NationalInstituteofStandardsandTechnology(NIST),3UniversityofGuelph,

4USGeologicalSurvey(USGS)

OceanOpticsXXIV8-12October2018Dubrovnik,Croatia

REFERENCESCataford,A.,Gadsden,S.A.,Turpie,K.R.,Biglarbegian,M.,(2018),air-LUSI:Estimation,Filtering,andPIDTrackingSimulation.2018IEEECanadianConferenceonElectricalandComputerEngineering(CCECE),QuebecCity,Quebec.

Cramer,C.E., Lykke,K.R.,Woodward, J.T.,Smith,A.W. (2013),PrecisemeasurementofLunarspectral irradianceatvisiblewavelengths,JournalofResearchoftheNationalInstituteofStandardsandTechnology,118,396-402.

Kieffer,H.H.,Stone,T.C.,TheSpectralIrradianceoftheMoon,AstronomicalJournal,129,2887-2901(2005).Turpie, K.R., Eplee, R.E., Meister, G. (2015), Propagation of Visible Infrared Imaging Radiometer Suite (VIIRS) calibrationuncertaintytrendstooceancolordata,ProceedingsoftheSPIE9111,OceanSensingandMonitoringXX,9607-60.

Theobjectiveofair-LUSIistoprovideNASAacapabilitytoimproveROLO by measuring exo-atmospheric lunar spectral irradiance withunprecedented accuracy. Careful characterization of the Moon from abovethe atmosphere will make it a stable and consistent SI-traceable absolutecalibrationreference.ThiscouldrevolutionizelunarcalibrationforsomeEarthobserving satellites and would be especially beneficial to ocean colormissions. Because of the high sensitivity of aquatic remote sensing tocalibration (Turpie et al., 2015), improvement of lunar calibration coulddirectlyaffectupcomingPACEandJPSS (VIIRS)missions,andretrospectivelyfortheSeaWiFS,EOS(MODIS),andS-NPP(VIIRS)datarecords.

Situated in the ER-2 wing pod, the IRIS telescope sits on the ARTEMIS.

ControlCcomputer Actuators Telescope

TrackingCamera

2.DESIGNBuildingon themountain-basedworkofCrameretal. (2013), air-

LUSI approaches this characterization by taking lunar spectral irradiancemeasurementsathigh-altitudeontheER-2aircraft.Thisisaccomplishedwitha non-imaging telescope to collect moonlight, which is passed to a NIST-calibratedspectrometer (calledthe Irradiance InstrumentSubsystemor IRIS)viafiberoptics. Thespectrometerandanon-boardvalidationreferencearehermeticallysealedinanenclosuremadeofatwosolidblocksofaluminum,keeping the spectrometer and reference at constant sea-level pressure and20°C during the high altitude flight. The Autonomous, Robotic TElescopeMountInstrumentSubsystem(ARTEMIS)keepsthetelescopepointedat theMoonusingacameratotracktheskyinfrontofthetelescope(Catafordetal.,2018).

ARTEMISTRACKINGTheARTEMISfeedbackcontrolalgortihmfindsthecentroidofthelunarimageanddeterminesthedistancefromtheFOVcenter(i.e.,thetrackingerror).Thecomputerusesthatinformationtocommandtheactuatorstocorrectthetelescopedirection.Trackingerrorsduriingtheengineeringtestflightheldbetween0and1pixels,whichequatesto<0.1°accuracy.

a. telescope b. tracking camera (TC) c. IMU d. insulated cables e. NO2 purge line f. mount yoke w/ actuators

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g. back cover with integrating sphere h. umbilical with fiber optics i. bulkhead panel with ARTEMIS and IRIS umbilicals j. ARTEMIS computer assembly k. IRIS instrument enclosure