Tracking MODIS RVS for the reflective solar bands in drifting orbits using SNO with VIIRS

The MODIS instrument has been operating on both the Terra and Aqua spacecraft for more than 20 years. After the last orbital maintenance maneuvers performed for Terra and Aqua in early 2020 and 2021, respectively, their orbits have been drifting. The orbit drift brings a challenge of maintaining a consistent radiometric calibration for the MODIS reflective solar bands (RSB). In the current Collection 6.1 and upcoming Collection 7 Level 1B (L1B) data products, the calibration of the RSB relies on measurements from an on-board solar diffuser (SD) combined with ancillary data from scheduled lunar observations and pseudo-invariant Earth targets. The lunar and Earth view data is collected at various scan angles to track changes in the response versus scan angle (RVS). The orbit drift causes changes in the solar illumination and satellite view angles to the Earth targets. The simultaneous nadir overpass (SNO) approach provides a direct comparison of spectrally matching bands between MODIS and any one of the VIIRS sensors on the SNPP, NOAA-20 and NOAA-21 satellites, in which impacts of scene variability, solar illumination and sensor view angles are largely reduced. This study shows how the orbit drift affects frequency, location and angular match between MODIS and VIIRS crossovers. We extended the SNO approach from MODIS near-nadir views to off-nadir scan angles to examine the stability of MODIS to VIIRS reflectance ratios at various scan angles. Trending results starting from 2018 based on SNO with NOAA-20 VIIRS are presented for Terra and Aqua MODIS bands 1-4, 8 and 9.