Examination of the angular dependence of the SNPP VIIRS solar diffuser BRDF degradation factor

Modern Earth-orbiting Earth-observing remote sensing sensors often use onboard solar diffusers (SD) to calibrate their reflective solar bands (RSB), such as the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi National Polar-orbiting Partnership (SNPP) satellite. The SD’s optical scattering property is measured by a bidirectional reflectance distribution function (BRDF). Once on orbit, the BRDF is found to degrade over time and the degradation factor is determined by, in the case of the SNPP VIIRS, an onboard solar diffuser stability monitor (SDSM) which observes the SD at a fixed view angle (relative to the SD surface). The SNPP VIIRS RSB however, observes the SD at a different angle. It is conventionally assumed (for cases like VIIRS) that the angular dependence of the BRDF degradation factor is negligible and hence the degradation factor determined by the SDSM is used to calibrate the RSB. We examine the angular dependence of the SNPP VIIRS SD BRDF degradation factor. We find that the degradation factor is clearly angle dependent when the SD BRDF degrades enough (< 0.98), and the larger the degradation the stronger the dependency.