NOAA 20 VIIRS solar diffuser stability monitor (SDSM) sun transmittance function update from Yaw maneuver and 3 year on-orbit SDSM data

Abstract

The Visible Infrared Imaging Radiometer Suite (VIIRS) instrument on the National Oceanic and Atmospheric Administration (NOAA) NOAA-20 satellite was launched on November 18, 2017. Its on-orbit radiometric calibration is based on the Solar Diffuser (SD) observations for the Reflective Solar Bands (RSB). Due to the ultra-violet portion of the Sun light, the roughness of the SD surface is gradually increased which degraded the reflectance property of the SD surface. The SD degradation needs to be accurately measured as on-orbit radiometric calibration reference. For this purpose, Solar Diffuser Stability Monitor (SDSM) frequently measures the SD surface reflectance changes from the 8 SDSM detectors. The SDSM has two view ports called Sun and SD view ports. The Sun view with attenuation screen provides the reference Sun illumination at the time of SD view from the same detectors. The ratio between the Sun and SD observation provides the SD degradation (or called H-factor) at the 8 selected wavelengths. The prelaunch SDSM Sun view transmission function had significant problems that caused more than one percent oscillations in the H-factors. Previously, the SDSM Sun view transmittance function was updated by using the 15 yaw maneuvers on January 25 and January 26, 2018 as a part of post-launch tests. In addition to the yaw data, the SDSM Sun view transmittance function was updated using on-year on-orbit SDSM data to fill up the non-yaw angles. In this study, the NOAA VIIRS radiance team updated the SDSM Sun view transmittance function using 3 year on-orbit SDSM data. The H-factor with the updated Sun transmittance function is stabilized especially in the short wavelengths providing better on-orbit calibration for the on-orbit NOAA-20 VIIRS RSB calibration.