Estimation of the accuracy of the SNPP VIIRS SD BRDF degradation factor determined by the solar diffuser stability monitor

Abstract

To radiometrically calibrate its reflective solar bands, the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi National Polar-orbiting Partnership satellite observes a sunlit onboard solar diffuser (SD). The degradation factor of the SD bidirectional reflectance distribution function (BRDF) is determined by an onboard solar diffuser stability monitor (SDSM). We improve the accuracy of the measured SD BRDF degradation factor in four ways. First, we remove the bias in the previously computed relative product of the SD screen transmittance and the BRDF at t 0 (when the BRDF degradation just started) τ R/SD, eff BRDF( t 0 ). The bias is introduced by the angular dependence of the SD BRDF degradation factor. Second, to computeτ R/SD, eff BRDF( t 0 ), we use both the yaw maneuver and a small portion (~ 3 months) of regular on-orbit data. Third, we average the computed degradation factors over a large angular range of an orbit, reducing the impact of the solar power and detector gain noise. And fourth, we center the average of computed degradation factor at a fixed angle relative to the SD surface normal vector to remove the variation due to the dependence of the degradation factor on solar radiation energy incident angle. We fit the degradation factor to a smooth function of time and use the fitting residuals to estimate the accuracy of the degradation factors measured by the SDSM on a per orbit basis.