Advances in the SLIM Lunar Spectral Irradiance Model; Many Observations, One Moon

Work on the Spacecraft and Earth-based Lunar Irradiance Model (SLIM) system has concentrated on the SLIMED model (pronounced as if you lost weight), which uses a weighted least-squares fit to many instrument data sets and is continuous in all dimensions. Total Solar Irradiance (TSI) and Solar Spectral Irradiance (SSI) variations are now included. A libration correction to the observations based on integration of reflectance maps made by Lunar orbiters can be included. Current fits involve 2 observatory sets (ROLO and NIST:Cramer) and 8 LEO instruments (SeaWiFS, L8-OLI, Hyperion, Terra and Aqua MODIS, SNPP-VIIRS, PLEIADES A and B) for a total of 99,000 points with mean-absolute-residual of 0.6% . Data from GEO instruments thus far show too much variation in irradiance to be included in the fits.All the above instruments, plus GOES-8,9,10,11,12,14,15, SEVIRI 1 to 4, GOES-16 and 17 ABI (and NOAA20-VIIRS) have been calibrated with a SLIMED model and the average gain bias of each instrument band derived, as well as asymptotic trends and in some cases periodic variations. There is general agreement at the few percent level for some operational instruments yet significant differences between others. The magnitude of differences between instruments suggests that the methodology of extracting lunar irradiance from lunar images is the culprit in many cases. As lunar models improve, more detailed image-processing will be warranted. Understanding the hardware and processing differences between nadir and lunar imaging can require special sequences, preferably planned during commissioning.There is still a lot of work to be done to approach the potential utility of lunar calibration, ppt or better! All spacecraft teams with lunar observations are invited to participate.