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SNPP and NOAA-20 VIIRS have been on orbit for more than 7.5 and 1.5 years, respectively. Since their nadir doors opened on 21 November 2011 and 13 December 2017, respectively, the two VIIRS instruments have been making observations of the moon on a near-monthly basis as a monitor of their reflective solar bands (RSBs) on-orbit gain changes. The VIIRS RSB are primarily calibrated by an onboard Solar Diffuser (SD) panel and an accompanying Solar Diffuser Stability Monitor (SDSM). However, due to a non-ideal behavior in the SD degradation, discovered recently, the SD/SDSM calibration contains non-negligible errors, especially for the short wavelength bands, that can significantly impact the accuracy of the sensor data and eventually the science products. On the other hand, the lunar surface is very stable and can be used to provide more reliable characterization of on-orbit gain changes of the RSB, especially for the long-term trend. The RSB calibration coefficients derived from the lunar calibration are generally consistent with those derived from the SD/SDSM calibration, but clear differences in trend are seen due to aforementioned SD calibration errors, especially for the short wavelength bands. Thus, the lunar calibration plays a key role in both SNPP and NOAA-20 VIIRS RSB calibration and critical for obtaining high accuracy and quality of the sensor data records (SDR) as well as the environmental data records (EDR), especially the ocean color EDR. In this paper, we comprehensively review the VIIRS lunar calibration methodology and the improvements we have made to refine the lunar calibration results, especially the removal of the seasonal oscillations in the derived calibration coefficients due to imperfection of the lunar view geometric effect correction. The lunar calibration results for the two instruments are shown and discussed.
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