Improving NOAA 20 VIIRS screen transmittance and solar diffuser BRF estimation from both Yaw maneuver and regular on-orbit data

Abstract

The Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the NOAA 20 satellite performs radiometric calibration based on the Solar Diffuser (SD) collections for the Reflective Solar Bands (RSBs). The SD Bidirectional Reflectance Function (BRF) degradation (or H-factor) is measured by the Solar Diffuser Stability Monitor (SDSM) which uses Digital Count (DC) ratios between the signals of sunlit to SD and direct Sun illumination through a pinhole screen. The H-factor trends derived using the prelaunch version of the Sun view SDSM transmittance LUT show abnormal oscillations. This problem was not resolved even after applying the updated LUTs from the yaw maneuvers conducted on January 25th and 26th, 2018. As an alternative approach, the NOAA VIIRS team developed a methodology to update the noisy SDSM Sun transmittance function from the regular on-orbit SDSM collections. Initially, the on-orbit SDSM collections were performed each time VIIRS was approaching the night-to-day terminator. The frequency of SDSM collects was reduced to every other orbit starting from Dec. 14th, 2017. It was further reduced to once per day starting from Jan. 5th, 2018. From all the on-orbit SDSM collects, the SDSM Sun view transmittance function is calculated from the DC of the SDSM sun view at the time of SDSM collection, time dependent gain changes of the 8 SDSM detectors, incident Sun angle, Earth-satellite distance and solid angle of the SDSM Sun view. A new Sun transmittance LUT is derived using a combined data set from the yaw maneuver data and the on-orbit SDSM collection data with SDSM detector degradation correction. The new LUT significantly reduces uncertainties of SD degradation estimates (H-factor) from 0.8 percent to 0.2 percent level. Further improvements will be performed once a one-year cycle of SDSM solar azimuth changes will be completed.