Calibration of NOAA-20 VIIRS reflective solar bands with combined solar diffuser and lunar data for NASA Collection 2 L1B

The VIIRS instrument on the NOAA-20 satellite continues to have excellent performance in its reflective solar bands (RSB), with detector gains changing by less than 0.5% over the first four and a half years of the mission. The calibration of the RSB for NASA's Collection 2 Level 1B product relies primarily on data from the on-board solar diffuser (SD). Recently, we updated the calibration algorithm to include corrections to the long-term gain trends based on regularly scheduled lunar observations. Earlier in the NOAA-20 mission, due to insufficient lunar data, the long-term correction applied to the SD trends was based on scaled results from the first VIIRS instrument on the SNPP satellite. In this paper, we show updated NOAA-20 data trends of the solar diffuser degradation and the RSB gains derived from every-orbit SD observations. We also show updated lunar data and describe our algorithm for using the lunar data to correct the long-term gain trends. The gain differences between the previous algorithm, using scaled SNPP lunar data, and new algorithm, directly using NOAA-20 lunar data, are largest for bands M2 (0.4%), M3 ( 0.4%), M4 (0.5%), and I1 (0.6%), and negligible (<0.2%) for the other RSB. The updated algorithm began to be implemented in forward production of the NASA VIIRS Level 1B product starting in September 2021 and will allow for more accurate tracking of the long-term RSB gain in the future.