MODIS Lunar Calibration Data Preparation and Results for

MODIS Lunar Calibration Data Preparation and Results for GIRO Testing Zhipeng (Ben) Wanga and Xiaoxiong (Jack) Xiongb a: Science Systems and Applications Inc. , Greenbelt, MD 20706 USA b: NASA - Goddard Space Flight Center, Greenbelt, MD 20771 USA GSICS Lunar Calibration Workshop 2014, Darmstadt, Germany

Outline • Introduction – MODIS instrument overview – MODIS lunar calibration scheduling – lunar image acquisition • Lunar data processing – – background and instrument temperature correction Imagette integration method oversampling factor satellite position • GIRO: data preparation and results

MODIS Instrument • MODIS is a whisk-broom scanning spectroradiometer onboard NASA’s EOS Terra and Aqua satellites • MODIS has 36 spectral bands covering a spectral range of 0. 4 -14. 4 μm; 20 are reflective solar bands (RSB) • The spatial resolutions at nadir are: - 250 m for bands 1 -2; 500 m for bands 3 -7; 1 km for bands 8 -36 - Sample periods: 333/4 µs, 333/2 µs, and 333 µs, respectively

MODIS Calibration • MODIS has a set of on-board calibrators including a solar diffuser (SD), a blackbody (BB), a spectro-radiometric calibration assembly (SRCA) and a space view (SV) port • A scan mirror rotates, alternately directing light from SD, SRCA, BB, SV and Earth view (EV) to the detectors • The SD BRF is characterized pre-launch with NIST traceable reflectance standard

Scheduled Lunar Observation • Lunar calibration event is scheduled nearly monthly • Lunar phases are within the range of [54º, 56º] for Terra MODIS and [-56º, -54º] for Aqua MODIS • A satellite roll maneuver is needed for MODIS to observe the Moon through its SV port at the phase range • 139/119 calibrations for Terra/Aqua so far, respectively • Unscheduled observations through SV are also available

Lunar Image Acquisition • Lunar images are acquired over multiple scans • The radius of lunar image: spatial resolution orientation

Background Correction • The background and instrument correction is applied each scan • The background DN is extracted from both sides of the peripheral region of the Moon

Image Integration • The radiance is retrieved for each pixel with the detector gain coefficients derived from other calibrators • Integrate the radiance to get total lunar irradiance IMoon - Integration over detectors for each scan, then take average Ω: IFOV in solid angle N: number of “center” scans m 1: gain coefficient from SD f: frame/sample - Integration over scans for each detector s: scan number d: detector number f. OS: oversampling factor

Oversampling Factor • The oversampling factor f. OS is the number of scans it takes for a detector’s field of view to move one IFOV Event 1 Event 2 • f. OS is determined by the angular movement speed of the object in reference to FPA • f. OS, 250 m = 0. 5 f. OS, 500 m = 0. 25 f. OS, 1 km Terra Aqua

Satellite Position • The satellite ephemeris data are predicted by NASA instrument operation team to schedule a calibration • The geometric factors at the time of lunar calibration are calculated with the ephemeris • TLE data can be used to verify these factors

GIRO Input Files • The instrument spectral response function (srf) files and lunar data files in net. CDF format are generated using IDL • GIRO release 1 to 4 have been tested • The GIRO ROLO results look good (4 th release) • Resolved technical issues: - the Net. CDF-4 support, which is required to write channel_id in string type, is only available since IDL 8. 0 - scaling factor attribute for variables dc_obs, dc_obs_offset to account for non-integer temperature corrected dn* - SWIR bands ROLO results (when srf is beyond 2. 1 µm) • Data processing: GIRO vs MCST - the setting of moon_pix_thld, which is actually detector dependent - the integration range for lunar irradiance calculation - detector dependent srf

SRF File • MODIS srf is measured pre-launch at detector level • The srf of mid-detector is inserted for GIRO testing Terra Aqua

GIRO ROLO Results • 23 calibrations have been processed for Aqua

Summary • GIRO testing is successfully performed with MODIS lunar calibration data • The Aqua MODIS lunar irradiance trending, in reference to GIRO’s ROLO result, has been stable within its calibration requirement • The method to interpolate ROLO reflectance to sensor’s SRF could impact the absolute accuracy • The lunar processing algorithm of MODIS is briefly introduced and its difference with GIRO is presented