NOAA-20 VIIRS reflective solar bands on-orbit calibration

The NOAA-20 Visible Infrared Imager Radiometer Suite (VIIRS) has been in orbit for more than one and a half years. 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). NOAA-20 VIIRS is also scheduled to view the Moon approximately monthly and the lunar observations are used to track the RSB on-orbit changes as well. Both SD and lunar calibration results for the RSBs are shown and it is demonstrated that the two sets of the calibration coefficients diverge with time, especially for short wavelength bands. The divergence is due to the nonuniformity of the SD degradation, a phenomenon observed in SNPP VIIRS and the two MODIS instruments. Because of this divergence, a hybrid method, which properly combines the SD and lunar calibration results, need to be applied to generate the RSB calibration coefficients look-up tables (LUTs) as has been done for SNPP VIIR RSBs. The challenging issues of the RSB calibration using the SD/SDSM and the Moon are also addressed and discussed.