Modeling the detector radiometric response gains of the Suomi NPP VIIRS reflective solar bands

Right after the opening of the NADIR door of the Visible/Infrared Imager/Radiometer Suite (VIIRS) aboard the Suomi National Polar-orbiting Partnership (NPP) satellite, the detector gains of the Near InfraRed (NIR) bands have decreased much faster than expected, indicating large degradation of the VIIRS sensor optical system. To determine the root cause and to access the potential outcome of the degradation, we developed a mathematical model based on physical hypotheses on the optical system, especially the Rotating Telescope Assembly mirror surfaces. Current detector gains are consistent with a model of a four-mirror system, with each mirror having a thin contaminant layer of material that reduces the mirror transmission and the reduction is activated by solar radiation. The ratios of the reduced transmission coefficients are in good agreement with those from the reflectance measurement of the Traveling Witness Mirror (after exposure of ultraviolet light from a test lamp) independently carried out as part of the degradation anomaly investigation. In this Proceeding, we describe the mathematical model and discuss the results.