Using the moon and stars for VIIRS day/night band on-orbit calibration

Two VIIRS instruments are currently operated in space; one on-board the S-NPP satellite launched in October, 2011 and another on-board the NOAA-20 satellite launched in November, 2017. The VIIRS design has included a unique day/night band (DNB), which can make both daytime and nighttime observations in the reflective solar spectral region. It has a broad spectral response, covering wavelengths from 500 to 900 nm, and an extremely large dynamic range. The DNB collects data in three gain stages: LGS, MGS, and HGS for low, mid, and high gain stage, respectively. Its HGS has excellent low-light sensitivity and is capable of detecting bright stars through the instrument space view port. Similar to other reflective solar bands (RSB), the DNB LGS and MGS responses to the Moon can be used to monitor their on-orbit calibration stability. This paper describes various strategies that we have developed to use the moon and stars for the DNB on-orbit calibration and calibration inter-comparisons and presents the results for the S-NPP and NOAA-20 VIIRS. Key technical issues, such as identifying the stars and processing data collected at different gain stages and aggregation modes, are also discussed, along with the efforts planned for future improvements.