Visible Infrared Imaging Radiometer Suite reflective solar bands on-orbit calibration using solar diffuser illuminated by scattered light through the nadir port.

A new variant to the standard on-orbit calibration of the reflective solar bands (RSBs) using a solar diffuser (SD) is formulated. Instead of direct solar exposure through the SD port in the front of the instrument as originally designed, the variant method uses light reflecting off Earth's surface coming through the nadir port as the light source to illuminate the built-in onboard SD. The methodology is applied to the Visible Infrared Imaging Radiometer Suite on board the Suomi National Polar-orbiting Partnership satellite, and is shown to be viable and useful. This approach effectively preserves the standard calibration pipeline other than using a different set of illumination data, corresponding to a different illumination source, for computing the radiance emanating from the SD. It has the added advantages of not dealing with operational needs for the standard calibration activities and completely bypassing the characterization of the transmission function of the attenuation screen in the front of the SD port. The RSB calibration coefficients are computed from the data of scattered light from the SD sector per each orbit, and a 16-day average is taken. The variant calibration coefficients are shown to well match the standard solar-based RSB calibration coefficients for Bands M5 to M8, but diverging results emerge for Bands M1 to M4, highlighting the known non-ideal behavior in the degradation of SD that contributes to the worsening error in RSB calibration. The result also shows a consistent 2% variation mission-long for all RSBs, showing the overall consistency of this first analysis of the new method but also the level of the uncertainty. The result and the implications of this study are discussed.