An overview of MODIS RSB calibration and look-up-table delivery process

Abstract

The Moderate Resolution Imaging Spectroradiometer (MODIS) is an Earth-observing sensor currently operational on the Earth Observing System (EOS) Terra and Aqua satellites. Each MODIS instrument has 36 spectral bands, with data acquired using 490 detectors in the reflective solar and thermal emissive spectrum. The 20 reflective solar bands (RSB), covering a wavelength range from 0.4 to 2.2 μm, are calibrated on-orbit using a solar diffuser (SD), solar diffuser stability monitor (SDSM) and regularly scheduled lunar measurements. The instrument gain (1/m1) derived from SD measurements and the response versus scan-angle (RVS) derived using the SD and lunar measurements are the primary look-up-tables (LUT) that are updated on a regular basis. The short wavelength bands of both MODIS instruments have experienced a gain change of up to 50% as observed from the SD calibration. Given the longevity of both MODIS sensors, the detectors have aged, and the sensor’s radiometric characteristics have changed since launch. A strong dependence of the RVS on mirror side, detector, wavelength and time is also observed in visible (VIS) channels. The MODIS Characterization Support Team (MCST) is responsible for deriving and updating the on-orbit calibration coefficients. An accurate and timely update of the LUT is vital for maintaining the quality of the calibrated Level 1B (L1B) product. This paper provides an overview of the MODIS RSB calibration along with the major procedures used to regularly update the LUT. In addition, the various lessons learned and future improvements are also discussed.