MODIS solar diffuser calibration strategies for the post orbital drift era

MODIS is an imaging spectroradiometer onboard NASA’s Terra and Aqua satellites with visible to short-wave infrared capability enabled through 20 reflective solar bands (RSB) in the wavelength range of 400 to 2200 nm. RSB gain changes are monitored by using observations of a fully sunlit solar diffuser (SD) for the on-orbit radiometric calibration. SD signal changes, due to degradation of the SD surface, are monitored by using the SD Stability Monitor (SDSM), which has 9 detectors in the wavelength range of 400 to 936 nm. These calibration approaches depend on the geometry of the spacecraft’s orbit. Both Terra and Aqua have recently started to exit their respective constellations and are (or soon will be) in the process of drifting from their nominal orbits. These orbital changes will cause changes in the solar diffuser (SD) and solar diffuser stability monitor (SDSM) viewing geometry and calibration conditions. This, in turn, will drive a variation in the calibration parameters used to calculate reference adjustments for the MODIS reflective solar bands (RSB). We examine the expected effect of orbital drift on the reflectance and transmittance functions, the evaluation of on-going SDSM detector response, the variation of SD surface incidence angle ranges, and the estimation of fully-illuminated observing condition for SD signal monitoring.