MODIS Level 1B Algorithm Theoretical Basis Document - NASA

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MODIS Level 1B Algorithm Theoretical Basis DocumentMODIS Characterization Support TeamJack Xiong, Gary Toller, Junqiang Sun, Brian Wenny, Amit Angal, and William BarnesVersion 4June 14, 2013(Appendices added after 6.1 reprocessing, April 24, 2017 and updated on May 8, 2020)Prepared For:National Aeronautics and Space Administration


7. REFERENCES318. APPENDIX A: MODIS SPECIFICATIONS AND DESIGN PARAMETERS9. APPENDIX B: L1B TEB SCALED INTEGERS10 APPENDIX C: L1B RSB SCALED INTEGERS11. APPENDIX D: UNCERTAINTY INDEX IN THE L1B PRODUCTS12. APPENDIX E: TERRA PV LWIR CROSSTALK CORRECTION13. APPENDIX F: EARTH-VIEW BASED RVS FOR AQUA MODIS14. APPENDIX G: TERRA MODIS SWIR IMPROVEMENTS15. APPENDIX H: ACRONYMS AND ABBREVIATIONS3637373737414142FiguresFigure 1 MODIS Scan Cavity and On-Board CalibratorsFigure 2 Schematic of the Optical SystemFigure 3 MODIS’ Focal Plane Assemblies:Figure 4a The Primary Mirror Scan Angles of the Ground Calibration Sources, Space,and Earth.Figure 4b AOIs Corresponding to Primary Mirror Scan AnglesFigure 5 MODIS RSB Calibrator: Solar DiffuserFigure 6 Solar Diffuser Stability MonitorFigure 7 On-Board Blackbody Calibration SourceFigure 8 Specto-Radiometric Calibration AssemblyFigure 9 Blackbody Calibration Source (BCS)Figure 10 Lunar Responses for Band 31 and 33Figure 11 Band 35 Images Before and After PC Crosstalk CorrectionFigure 12 Earth View Radiance Uncertainties at a Typical EV RadianceFigure 13 MODIS Reflective Solar Bands Pre-launch Calibration Source: SIS-100Figure 14 TEB L1B Flow DiagramFigure 15 RSB L1B Flow Diagram566789991011171720212930TablesTable 1 MODIS Measured CharacteristicsTable 2 L1B Uncertainty Index (UI) Mapped to TEB Uncertainty in Percent420Acknowledgement: Truman Wilson contributed to the Appendix E, Terra PV LWIR CrosstalkCorrectioniii

1. INTRODUCTION1.1. OverviewThe MODerate-resolution Imaging Spectroradiometer (MODIS) is a key instrument for theNASA’s Earth Observing System (EOS) and, subsequently, Mission To Planet Earth (MTPE)programs. The EOS was designed to provide global observations and scientific understanding ofland cover changes and global productivity, sea surface temperature, atmospheric and climatechanges, and natural hazards [Xiong et al., 2009].MODIS [Xiong et al., 2005a] is a passive imaging spectroradiometer with 490 detectors,arranged in 36 spectral bands that are sampled across the visible and infrared spectrum. It is ahigh signal-to-noise instrument designed to satisfy a diverse set of oceanographic, terrestrial, andatmospheric science observational needs. The near-daily global coverage of MODIS, combinedwith its continuous operation, broad spectral coverage, and relatively high spatial resolution,makes the MODIS instruments central to the objectives of NASA’s EOS and MTPE programs.MODIS observations and science data products are applied to many of the areas identified asEOS science topics, such as land surface composition, land surface biological activity, surfacetemperature, snow and sea-ice extent and character, ocean and lake physics and biogeochemicalactivity, aerosol properties, and cloud properties. The MODIS Proto-Flight Model (PFM) waslaunched December 18, 1999 on-board the Terra spacecraft in a 10:30 AM (local time,descending node) orbit. The Aqua Flight Model (FM-1) was launched onboard the Aquaspacecraft into a 1:30 PM (local time, ascending node) orbit on May 4, 2002.The MODIS development was managed by NASA’s Goddard Space Flight Center (GSFC) inGreenbelt, Maryland. The MODIS instruments were designed, built, and tested by Raytheon /Santa Barbara Remote Sensing (SBRS) in Goleta, California. The MODIS CharacterizationSupport Team (MCST), working under the direction of the MODIS Team Leader, is responsiblefor the characterization and radiometric calibration of the MODIS instruments [Xiong et al.,2006]. MCST developed the Level 1B (L1B) software that converts instrument response indigital numbers (DN) to calibrated, geo-located top of the atmosphere (TOA) radiances for allbands and Earth reflectance factors for the 20 reflective solar bands (RSB). The MODIS dataproducts provided by the MODIS Science Team support the Earth science community at large,interdisciplinary investigators, and the MODIS Science Team members' own investigations.1.2. Historical PerspectiveThe MODIS was designed to continue global monitoring similar to the observations initiatedwith the Nimbus 7 Coastal Zone Color Scanner (CZCS), the Advance Very High ResolutionRadiometer (AVHRR), the High Resolution Infrared Spectrometer (HIRS), the LandsatThematic Mapper (TM), and the Orbview-2 Sea-viewing Wide Field of View Sensor (SeaWiFS).The selection of MODIS spectral bands and the development of many of the MODIS sciencedata products rely on the experiences and lessons learned from predecessor missions. Thiscontinuity allows many previously existing data records to be extended with improved coverage1

and quality.New features incorporated into MODIS include a thin-cirrus cloud detection channel, low gainbands to detect surface fires, and high gain bands for ocean chlorophyll fluorescence-line heightdiscrimination. Additional information about the MODIS instrument development, MODISrequirements, and program development is presented by Barnes et al [2003]. Key MODISspecifications and design parameters are listed in Appendix A.1.3. Document Context and ScopeThis ATBD describes how MODIS operates in space and provides the equations implemented bythe L1B software to generate the MODIS MOD02 (Terra) and MYD02 (Aqua) data products. Itis a summary document that presents the formulae and error budgets used to transform MODISDN to radiance and reflectance. It describes the current (Collection 6), post-launch MODIScalibration process and supersedes previous ATBDs [Barker et al. (Version 1), 1994] [MCST(Version 2), May 1997] [MCST (Version 3), December 14, 2005]. Analysis of instrumental onorbit performance by MCST and investigation of L1B products by the Science Team haveresulted in several L1B software updates and improvements. This ATBD corresponds to theVersion 6.0 Terra and Aqua software releases. Prior to 2006, the MCST and the Science DataSupport Team (SDST) provided software deliveries to the Goddard Distributed Archive andAnalysis Center (GDAAC). Subsequently, the MODIS Adaptive Processing System (MODAPS)assumed the data production role formerly undertaken by the GDAAC. Product files arecurrently distributed using the Level 1 and Atmosphere and Archive Distribution Systemavailable at MODIS calibrated data product results from the application of the formulae and thedetermination of corresponding uncertainties described in this document and the referencedsupport documents. The support documents present details of how the instrument data aretransformed from digital counts to (1) reflectance factors and radiances for the reflective solarbands and (2) radiances for the thermal emissive bands. Items (1) and (2) are the focus of thisdocument and of the on-line production processing efforts. Changes in the center wavelengthsfor the solar reflecting bands and relative spatial shifts for the pixels along scan and the bandsalong track are evaluated off-line by the MCST.The instrument is described in section 2 of this document. The key calibration equations appliedby the L1B algorithms to the thermal emissive band (TEB) and the reflective solar band (RSB)data are presented in sections 3 and 4, respectively. These sections also describe MODISinstrumental effects handled within L1B and discuss the associated uncertainties inTerra/MODIS and Aqua/MODIS processing. An overview of the L1B calibration algorithm isgiven in section 5 and a summary follows in section 6.1.4. Relevant DocumentsDocuments containing more complete derivations and explanations of the implementation of2

these algorithms include [EOS, 1994], [MCST (1997)], [Guenther et al., 1996], [Isaacman et al.,2003] and [Toller et al., 2008 and 2013]. Pre-launch sensor characterization publications include[GSFC, 1993], [SBRS, 1993], [SBRS, 1994], [Guenther et al., 1995], and [Barnes et al., 1998].The L1B approach to calibration is described in the MODIS Level 1B In-Granule CalibrationCode (MOD PR02) High-Level Design [MCST, 2012a]. Program file specifications andmetadata are described in the MODIS Level 1B Product Data Dictionary [MCST, 2012b]. Theproduct format and the scaling algorithms used to generate the calibrated products are presentedin the MODIS Level 1B Product User's Guide [MCST, 2012c]. The MCST web site:[MCSTHome Page, 2013] contains abundant Terra MODIS and Aqua MODIS mission information.Data flow diagrams for the reflective solar and thermal emissive band algorithms, L1B inputfiles, data products, and a discussion of the look-up tables (LUTs) that provide the parametersneeded to generate L1B are summarized in Isaacman, et al., 2003. Detailed LUT documentationis also available from the MODIS LUT Information Guide [MCST, 2012d]. Solar and lunarposition vectors together with the MODIS geolocation product are used within the L1Balgorithm. A separate ATBD exists for the MODIS geolocation algorithms [Nishihama et al.,1997].2. INSTRUMENT DESCRIPTION2.1. OverviewMODIS is a passive cross-track-scanning imaging radiometer designed to take measurements inspectral regions that have been included in a number of heritage sensors. MODIS uses a twosided beryllium paddle-wheel scan mirror that continuously rotates at 20.3 rpm (a scan period ofo1.478s per mirror side). The instrument field of view (FOV) is 55 from the nadir. Viewing theEarth from a sun-synchronous near polar orbit at an altitude of 705km, the two sides of the scanmirror alternately produce a swath of 2330km along scan by 10km (at nadir) along track. BothTerra and Aqua MODIS are able to provide near-global coverage in 2 days, enablingcomprehensive short- and long-term studies of the Earth’s land, oceans, and atmosphere.MODIS [Xiong et al., 2005a] has 36 spectral bands with wavelengths from 0.41 to 14.5μm. Thecenter wavelength and band-pass of each band were carefully selected to optimize measurementsof key features of the Earth’s land, ocean, and atmosphere. MODIS bands 1-19 and 26 are thereflective solar bands (RSB) that provide images from daylight reflected solar radiation andbands 20-25, and 27-36 are the thermal emissive bands (TEB) that provide day and night imagesof thermal emissions. The measured characteristics in Table 1 can be compared to the MODISdesign specifications provided in Appendix A.MODIS bands 1-2 have a nominal nadir resolution of 250m with 40 detectors per band alongtrack, bands 3-7 have a nadir resolution of 500m with 20 detectors per band along-track, and3

bands 8-36 have a nadir resolution of 1km with 10 detectors per band along-track. Bands 13 and14 have 2 arrays of 10 along-track detectors each, providing observations with high gain and lowgain through time delay and integration (TDI). Each sample for bands 13 and 14 combines theresponses from a pair of TDI detectors with their signals amplified through both high and lowgain amplifiers. Therefore, MODIS has a total of 490 detectors. For a 1km (along scan) by 10km(along track) frame of data, each detector of the 250m resolution bands (1-2) takes 4 samples andeach detector of the 500m resolution bands (3-7) takes 2 samples. Thus there are a total of 830samples for each frame. MODIS digitizes each sample to12-bit resolution. The average data rateover an orbit is 6.1 megabits per second.Table 1: MODIS Measured Characteristics. Note: The Terra SNR and NEdT arefor the initial on-orbit operational configuration. Terra A1 denotes the first periodwhen Terra operated using electronics side 12223Central λTerra, Aquanm646.3, 645.8856,5, 856.9465.7, 466.1553.7, 553.91242.3, 1241.51629.4, 1628.12114.2, 2113.9411.8, 412.4442.1, 442.2487.0, 487.4529.7, 530.1546.9, 547.2665.6, 666.0677.0, 677.6746.6, 746.8866.3, 866.9904.2, 904.4935.7, 936.4936.2, 936.31382.3, 1382.3Central λTerra, AquanmBandwidthTerra, Aquanm47.8, 47.237.7, 37.818.6, 18.819.7, 19.623.5, 22.828.4, 26.952.4, 52.314.7, 14.39.6, 9.610.5, 10.611.9, 11.910.2, 10.310.0, 10.011.3, 11.29.9, 9.815.5 ,15.534.7, 34.613.5, 13.545.7, 46.134.6, 36.4BandwidthTerra, Aquanm3788.3, 3780.23992.2,3981.83972.0, 3972.04056.7, 4061.6187.5, 186.982.8, 83.386.1, 85.485.6, 85.3Terra SNRAt LtypAqua SNRAt 5181519140037976509230Terra NEdT (K)(Terra 711138106237391503282Aqua NEdT (K)(Aqua B)

2425272829303132333435364473.2, 4448.34545.4, 4526.36770.5, 6786.87342.9, 7349.38528.7, 8555.39734.1, 9723.711018.9, 11026.212032.1, 12042.313365.0, 13364.713683.3, 13685.913913.2, 13925.214195.6, 14215.290.2, 92.291.1, 90.4239.1, 187.9320.6, 314.9344.1, 359.2297.2, 301.1516.3, 531.1520.7, 521.5307.6, 310.9324.1, 341.7327.7, 332.7284.9, 3Figure 1 shows the MODIS scan cavity and the on-board calibrators. The on-board calibrators(OBCs) include a solar diffuser (SD) and solar diffuser stability monitor (SDSM), a blackbody(BB) and a space view (SV) port, and a spectro-radiometric calibration assembly (SRCA).Figure 1: MODIS scan cavity and on-board calibratorsThe optical system schematic is shown in Figure 2. The scan mirror reflects energy to the foldmirror. The aft optics consist of a two mirror off-axis telescope, and a series of dichroic beamsplitters and band pass filters that separate the radiation onto four focal plane assemblies (FPAs).These are designated, according to their spectral regions, as: visible (VIS), near infrared (NIR),short and middle wave infrared (SMIR), and long wave infrared (LWIR).5

Figure 2: Schematic of the optical systemFigure 3: MODIS’ focal plane assemblies6

The locations of the MODIS’ 36 spectral bands (490 detectors) on the four FPAs are shown inFigure 3. The detector numbering convention in Figure 3 corresponds to that of the MCST Level1B processing and is called “product order”. Product order is the inverse of the instrumentbuilder’s “SBRS order” detector numbering convention. The SMIR and LWIR FPA arecontrolled by a radiative cooler to 83K during on-orbit operation.The MODIS detectors view the on-board calibrators through the same optical path as the Earthobservations, but at different viewing angles or at different angles of incidence (AOIs) to thescan mirror. As the MODIS scan mirror rotates, each side scans the Solar Diffuser (SD), theSpectro-Radiometric Calibration Assembly (SRCA), the Blackbody (BB), the space view (SV),and the Earth (EV). Figures 4a and 4b illustrate the scan angles and their corresponding AOIs.MODIS calibration corrects for a response versus scan angle (RVS) effect.The VIS and NIR detector arrays are photovoltaic (PV) silicon hybrids that are operated atinstrument ambient temperature. The SMIR FPA uses PV HgCdTe hybrid arrays. The LWIRFPA consists of PV HgCdTe detector arrays for bands with wavelengths less than 10μm andphotoconductive (PC) HgCdTe detectors for bands beyond 10μm.The analog output signals produced by the PV FPAs are buffered and digitized in the space viewanalog module (SAM). The signals produced by the PC detectors on the LWIR FPA are preamplified by the cooler located amplifier module (CLAM) and then post-amplified and digitizedby the forward viewing analog module (FAM). The digital outputs from the SAM and FAM areformatted into science data packets by a formatter/processor in the main electronics module(MEM). They are then buffered and sent to the spacecraft through a first-in first-out (FIFO)buffer and fiber distributed data interface (FDDI) circuits.Figure 4a: The primary mirror scan angles of the ground calibration sources, space, and Earth.7

Figure 4b: The angles of incidence (AOIs) corresponding to primary mirror scan angles for theobserved elements.2.2. Solar Diffuser (SD) and Solar Diffuser Stability Monitor (SDSM)The SD and SDSM shown in Figures 5 and 6 operate together as a system for calibrating thereflective solar bands (RSB) with wavelengths from 0.41 to 2.2 μm. The diffuser is made ofspace-grade Spectralon , a proprietary thermoplastic formulation of polytetrafluoroethylene(PTFE). The SD bi-directional reflectance factor (BRF) was characterized pre-launch withNational Institute of Standards and Technology (NIST) traceable reflectance standards. The SDon-orbit degradation is tracked by the SDSM during each periodic (initially weekly, currentlyevery 3 weeks) calibration sequence. The SDSM itself is a ratioing radiometer which monitorson-orbit SD BRF variation (degradation) by alternatively viewing diffusely reflected Sun lightfrom the SD panel and direct Sun light through an attenuation screen with a nominal 1.44%transmission. The screen is used to keep the signals from the SD and sun view at nearly the samelevel. The SDSM has nine filtered detectors embedded in a small solar integrating sphere (SIS)that monitors the SD degradation in the wavelength range from 0.41 to 0.94 μm . The specifiedMODIS RSB uncertainties are 2% reflectance and 5% absolute radiance. The reflectancecalibration of the RSB from the SD measurements can be converted to a radiance calibrationbased on published values for the solar spectral irradiance. Two illumination levels of SDcalibration are provided via a deployable 8.5% transmission screen. The screen must be in placefor calibrating the high gain bands (B8-16 for ocean color observations) since they saturate whenviewing direct sun exposure of the SD.8

Figure 5: MODIS Solar DiffuserFigure 6: Solar Diffuser Stability Monitor2.3. On-board Blackbody (BB)The thermal emissive bands (TEB) are calibrated by viewing the on-board blackbody (BB)which provides a known radiance source, and, subsequently, cold space through the Space View(SV) port providing measures of the instrument thermal background and electronic offset. Thiscalibration is performed on a scan by scan basis. The on-board blackbody, shown in Figure 7, isa “V-groove” device with known emissivity (approximately 0.992) determined from pre-launchradiometric calibration and characterization. Twelve thermistors embedded beneath the BB frontsurface, measure the temperature of the BB each scan. The thermistors were calibrated prelaunch using NIST temperature standards. The BB temperature can be varied from the MODISscan cavity ambient of about 270K up to 315 K by means of electrical heating elements attachedto the back of the BB.

The MODIS development was managed by NASA’s Goddard Space Flight Center (GSFC) in Greenbelt, Maryland. The MODIS instruments were designed, built, and tested by Raytheon / Santa Barbara Remote Sensing (SBRS) in Goleta, California. The MODIS Characterization

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