Modeling the Detector Radiometric Gains of the Suomi NPP VIIRS Reflective Solar Bands

Right after the opening of the nadir door of the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi National Polar-orbiting Partnership satellite, the detector gains of the near-infrared bands had decreased much faster than expected, indicating large degradation of the VIIRS optical system. To help determine the root cause and to access the potential outcome of the degradation, we developed a mathematical model based on a physical hypothesis that the observed degradation was due to the sensor Rotating Telescope Assembly (RTA) mirror surface contamination. To date, the detector gains have been consistent with a physical model of a thin contaminant layer of material on each of the four RTA reflective mirrors. The contaminated material, after exposure to solar radiation, reduces the mirror reflectance over the reflective solar band (RSB) wavelength region. We describe the mathematical model and apply the model to predict the RSB detector gains at the end of seven-year mission operation. The model also projects that the signal-to-noise ratios of the RSB will all be larger than the design requirements with a margin of at least 25% at the end of seven years of mission operation. In addition, the detector relative spectral response (RSR) is modulated by the wavelength-dependent optical throughput degradation. We compute the modulated RSR and its impacts on sensor radiometric calibration and the computed top-of-the-atmosphere spectral reflectance at the Sensor Data Record level.