Suomi NPP VIIRS Solar Diffuser BRDF Degradation Factor at Short-Wave Infrared Band Wavelengths

The Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi National Polar-orbiting Partnership (SNPP) satellite is an imaging radiometer, collecting data of the Earth's surface in wavelengths ranging from 0.41 to 12.5 μm. To maintain and improve data quality, the VIIRS calibrates its reflective solar bands using a sunlit onboard solar diffuser (SD) panel as the radiance source. Due to exposure to solar radiative energy and high-energy particles and perhaps on-orbit contamination, the surface property of the SD panel changes with time. An onboard SD stability monitor (SDSM) measures the change in the value of the bidirectional reflectance distribution function (BRDF) of the SD at the SDSM SD view direction. The SDSM measurements show that the BRDF value decreases over time and that the degradation is wavelength dependent. The degradation varies monotonically with the wavelength and becomes much smaller at a longer wavelength. However, the SDSM design band wavelengths (412-926 nm) are much shorter than the VIIRS short-wave infrared (SWIR) band wavelengths (1238-2257 nm). Consequently, for simplicity, for SNPP VIIRS, it has been assumed that the degradation is zero at the SWIR band wavelengths. Nevertheless, at the present time, the degradation at the shorter end of the SWIR band wavelengths may not be small enough to be negligible to satisfy the 0.1% (at least) radiometric calibration stability required by the Ocean Color products. We propose empirical models to calculate the BRDF degradation in the SWIR wavelength region.