Continuing impact of satellite orbit drift on MODIS solar diffuser calibrations

The MODIS instruments on NASA’s Terra and Aqua satellites each use an on-board solar diffuser (SD) as the primary calibration source for the reflective solar bands (RSB). The reflectance properties of the MODIS solar diffusers were extensively characterized prior to launch and those measurements form the basis for the absolute reflectance and radiance calibration of the MODIS Level-1B data products. On orbit, additional characterization was done during early mission yaw maneuvers to verify the angular dependence of the SD reflectance and measure the transmission of an optional attenuation screen that can be placed in front of the SD. To account for degradation of the SD reflectance from cumulative solar radiation exposure, MODIS is also equipped with an on-board solar diffuser stability monitor (SDSM). Regular SD and SDSM calibrations have been made throughout the duration of the Terra and Aqua missions, for more than 20 years now. In the past few years, both Terra and Aqua have been drifting away from their historically maintained orbital planes. Observations of the SD continue to be made, but with incident solar angles that are outside the limits of the pre-launch and early mission characterizations, which presents a problem for accurate calibration. Recently, in August 2024, an additional set of yaw maneuvers was performed for Aqua to help characterize the upcoming drift-induced changes in SD measurements. We discuss the current state of Terra and Aqua MODIS SD calibration, the results of the Aqua yaw maneuvers, and the various options available to extend the SD measurements and RSB calibration through the end of the Terra and Aqua missions.