Alternative approach to characterize response versus scan-angle (RVS) for MODIS reflective solar bands

Abstract

MODIS has 20 Reflective Solar Bands (RSB), covering a spectral range from 0.41 μm to 2.2 μm. Accurate on-orbit calibration of MODIS RSB observations is required for generating high-quality data products. MODIS RSB calibration is performed on-orbit using its on-board calibrators (OBCs), including a Solar Diffuser (SD), a Solar Diffuser Stability Monitor (SDSM), and a Spectroradiometric Calibration Assembly (SRCA). Since MODIS views the Earth's surface over a wide range of angles of incidence (AOI) to the scan mirror, on-orbit changes in its response versus scan-angle (RVS) also need to be accurately characterized and corrected. Current algorithms that describe on-orbit changes in the RSB RVS are based on the SD/SDSM, Moon and SRCA observations as well as the Earth view (EV) responses in terms of mirror side (MS) ratios. Time dependent RVS look-up tables (LUT) are produced and updated on a regular basis for the Level 1B (L1B) data production. In support of upcoming MODIS collection 6 data reprocessing, several new approaches have also been developed and examined by the MODIS Characterization Support Team (MCST), either by using different combinations of data sets, treating each mirror side independently, or normalizing to a different AOI. All these approaches fit the data at each AOI over time and then derive the relative changes at different AOIs. In this paper, we propose an alternative approach for the RSB onorbit RVS characterization and present its preliminary results. This approach fits the data first over AOI and then over time. For comparison purposes, data sets from a single EV target and that from multiple targets are used to derive the RSB RVS. Results from this study have demonstrated that this alternative approach can also be used effectively for the on-orbit characterization of MODIS RSB RVS.