VIIRS solar diffuser bidirectional reflectance distribution function (BRDF) degradation factor operational trending and update

The Visible-Infrared Imaging Radiometer Suite (VIIRS) was launched onboard the Suomi National Polar-orbiting Partnership (NPP) spacecraft on October 28, 2011. Among the bands on VIIRS are 14 reflective solar bands (RSBs). The RSBs are calibrated using the sun as a source, after attenuation and reflection of sunlight from a Solar Diffuser (SD). The reflectance of the SD is known to degrade over time, particularly at the blue end of the visible spectrum. VIIRS incorporates a separate instrument, a Solar Diffuser Stability Monitor (SDSM), in order to measure and trend the SD Bidirectional Reflectance Distribution Function BRDF changes over time. Inadequate knowledge of the SDSM screen transmission as a function of solar geometry and SDSM detector dependent modulation effects require a unique processing methodology to eliminate unphysical artifacts from the SD BRDF trending. The unique methodology is used to generate periodic updates to operational Look-up Tables (LUTs) used by the Sensor Data Record (SDR) operational code to maintain the calibration of the RSBs. This paper will discuss on-orbit SD BRDF behavior along with the processing methodology used to generate RSB LUT updates incorporating the trended SD BRDF behavior.