NOAA-20 VIIRS Relative Spectral Response Effects on Solar Diffuser Degradation and On-Orbit Radiometric Calibration

Abstract

The Visible Infrared Imaging Radiometer Suite (VIIRS) on the National Oceanic and Atmospheric Administration-20 (NOAA-20) satellite performs on-orbit radiometric calibration based on regular solar diffuser (SD) observations illuminated by the Sun at the termination point near the South Pole. Due to exposure to the ultraviolet portion of the solar irradiance spectrum, the SD bidirectional reflectance distribution function (BRDF) has been degrading over time. The SD degradation (called H-factor) was measured by the on-board calibrator called the SD stability monitor (SDSM). Nevertheless, over two years of operation, there have been systematic on-orbit calibration differences between the SD-based and independent moon-based calibration results. In this study, the NOAA VIIRS team used a surface roughness Rayleigh scattering (SRRS) model as a baseline SD degradation, simulated on-orbit center wavelength-based approach of SD degradation, and a new SDSM relative spectral response (RSR)-dependent SD degradation estimation method to evaluate the degradation. There were time-dependent growing differences between the SDSM RSR-applied H-factors and center wavelength interpolated H-factors especially in the short wavelength detectors (SDSM detector 1-4). The NOAA-20 SD-based calibration coefficients (SD F-factors) were reprocessed using the RSR-applied H-factors, and the new SD F-factors show similar long-term trends compared with the independent monthly lunar F-factors. The newly processed SD F-factor suggested that the NOAA-20 VIIRS detectors in the reflective solar bands (M1-M11 and I1-I3) showed very stable responses within 0.5% level over the two years of on-orbit operation.