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The NOAA-20 Visible Infrared Imager Radiometer Suite (VIIRS) includes several on-board calibration sources for on- orbit radiometric calibration. In the Visible/Near Infrared (VISNIR) and Short-Wave Infrared (SWIR) bands, the on- orbit radiometric calibration is carried out by using a reference called Solar Diffuser (SD). The degradation of the SD surface reflectance is measured by another independent radiometer called Solar Diffuser Stability Monitor (SDSM). These two on-board calibrators provide a degradation-free reference to monitor degradation of the Reflective Solar Band (RSB) detectors. In this study, we describe the characterization and evaluation procedures of SD degradation from the SDSM observations, related on-orbit VIIRS Post Launch Testing (PLT) activities and events within the 100 days of since launch (DSL). The prelaunch version of the SDSM Sun screen transmittance and SDSM SD Bidirectional Reflectance Function (BRF) showed abnormally high oscillations in the H-factors, yaw maneuver derive LUTs were used for version 1 H-factors. The version 2 H-factor was derived from the yaw maneuver data and on-orbit SDSM data sets to fill the gaps between the yaw maneuvers to update the SDSM Sun transmittance LUT. By adding SDSM data set, H-factor versions were mitigated but SDSM detector 6, 7 and 8 showed abnormally high degradation rate compared to the SNPP case. It was caused by the SDSM detector gain changes in these detectors and they were corrected by a ratio approach. Finally, the NOAA-20 VIIRS H-factor results are compared to performance of the near-identical Suomi-NPP VIIRS to identify the best practices for the NOAA-20 VIIRS RSB calibration.
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