NOAA-20 VIIRS reflective solar bands on-orbit calibration using solar diffuser and solar diffuser stability monitor

The NOAA-20 Visible Infrared Imager Radiometer Suite (VIIRS) was launched on November 18, 2017 and started to collect Earth view visible/reflective imagery on December 13, 2017. VIIRS has 22 bands, among which 14 are reflective solar bands (RSBs) covering a spectral range from 0.410 to 2.250 mu m. The RSBs are calibrated on orbit using an onboard solar diffuser (SD), whose on-orbit degradation is tracked by an onboard SD stability monitor (SDSM). The SD and the SDSM are illuminated by the sunlight through the screens in front of the SD port and the SDSM sun view port only in a very short time period when the instrument passes the southern pole from the night side to day side of the Earth in each orbit. Thus the accuracies of the SD and SDSM calibration strongly depend on the selection of the data collection period, so called "sweet spots", the accuracies of transmittances of the screens, so called vignetting functions (VFs), and the bidirectional reflectance factors (BRFs) for both SDSM view and the RSB view. In this paper, the "sweet spots" are carefully selected and the BRFs and VFs are accurately derived from the on-orbit yaw and prelaunch measurements. The SD on-orbit degradation, so called H-factors, and the RSB on-orbit calibration coefficients, so called F-factors, are derived from the SDSM and SD calibrations. The results are presented and compared with those of the SNPP VIIRS. The challenge issues of the RSB calibration using the SD and SDSM are also addressed and discussed.