Detector linearity assessment of MODIS reflective solar bands using on-board calibrators

The detectors in the reflective solar bands (RSB) of Terra and Aqua MODIS use a linear relationship to relate the instrument response to the observed top-of-atmosphere radiance. Recently, we reported that Aqua MODIS bands 1 (645 nm) and 2 (858 nm) have deviations from gain linearity that change on-orbit, leading to errors in the NASA Level 1B radiance products for low radiance scenes. In this paper, we expand on these findings to assess the linearity of detector responses in both Aqua and Terra MODIS for all RSB using data from the on-board solar diffuser (SD) and spectro-radiometric calibration assembly (SRCA). We use comparisons of SD observations taken at two different radiance levels: with and without an attenuation screen. The SRCA, operated in radiometric mode, takes observations of RSB detector response at multiple radiance levels using a set of lamps and a neutral density filter. While the lamps do not provide a stable enough source for an accurate radiometric calibration, we can use the relative response with and without the neutral density filter to track changes in gain linearity. Unlike the SD, the SRCA results can be used for nearly all RSB of both instruments. We show that most RSB for both Terra and Aqua MODIS continue to have very linear responses throughout the missions. Some notable exceptions are bands 1 and 2 for both Aqua and Terra, which are used primarily for land imaging applications, and band 26 for Terra, which is used for cirrus cloud identification. The results from the SD and SRCA are in reasonably good agreement with each other.