Evaluation of the on-orbit response versus scan-angle (RVS) performance for the MODIS reflective solar bands using multiple ground targets

Abstract

The reflective solar bands (RSB) of the MODIS instruments on the Terra and Aqua spacecraft support a variety of science applications. The RSB are primarily calibrated using regularly scheduled solar diffuser (SD) and lunar observations. As the instruments continue to operate beyond their designed life, significant degradation of the instrument gain is observed, particularly at the short wavelengths. In recent years, the onboard calibrators are insufficient in accurately characterizing the response versus scan-angle (RVS) for the RSB. Therefore, the MODIS Characterization Support Team (MCST) has implemented an enhanced approach whereby the on-board measurements are supplemented by observations of the pseudo-invariant calibration sites in Northern Africa to better characterize the on-orbit RVS changes in Level 1B Collection 6 and Collection 6.1 for select short and near-infrared bands of both MODIS instruments. This paper explores the use of alternative Earth-scene targets to characterize the on-orbit RVS. In particular, an alternative version of the on-orbit RVS, with the desert data substituted with the measurements from deep convective clouds (DCC), has been formulated and implemented. Results from this version are compared with the operational C6.1 algorithm. The long-term reflectance trends from the Dome Concordia (Dome C) site in Antarctica are derived using both versions and evaluated for long-term stability for independent assessments. Due to the Earth-scene saturation from the high-gain bands, the paper only focusses on the comparisons for bands 1, 3 and 4 of both MODIS instruments. This study finds that DCC can be used as alternative invariant ground targets in MODIS RVS characterization for these bands.