SNPP VIIRS solar diffuser on-orbit change factor determination without the screens (Conference Presentation)

Abstract

The Visible Infrared Imaging Radiometer Suite (VIIRS) aboard Suomi National Polar-orbiting Partnership satellite performs radiometric calibrations of its reflective solar bands through an onboard sunlit solar diffuser (SD). On orbit, the bidirectional reflectance distribution function of the SD changes over time. The change factor, known as the H-factor, is determined by an onboard solar diffuser stability monitor (SDSM) using the signal strength ratio determined by observing the Sun through an attenuation screen and the sunlit SD. The sunlight goes through another attenuation screen before striking the SD. The screen relative transmittances can be accurately characterized with on-orbit data, except for the regions where the solar azimuth angles are at or close to the extremes. It is unfortunate that over the initial orbits (< orbit 154) of the SNPP, the solar azimuth angle is at one of the extreme regions. Since there are no SDSM data over the initial orbits, the measured H-factors from late orbits are extrapolated to orbit zero to find a scale factor that yields, at orbit zero, the final H-factor of a value of one. The accuracy of the extrapolation depends on the quality of the transmittances of the screens. Due to inaccuracy of the transmittances over the initial orbits, the scale factor may not be accurate. Here, we use our previously developed technique to determine the H-factor without using the screen transmittances and thus are able to examine the accuracy of the scale factor and obtain the H-factor time change rate at early satellite orbits.