Unscheduled Lunar Observations for Radiometric Characterization of VIIRS Reflective Solar Bands

Regularly scheduled lunar observations are a major component of the on-orbit calibration of the Suomi National Polar-orbiting Partnership (SNPP) Visible Infrared Imaging Radiometer Suite (VIIRS) reflective solar bands (RSBs). With a few exceptions, these scheduled lunar observations have been performed with the same phase angles ranging from −51.5° to −50.5°. In addition to these observations that require a roll maneuver, the VIIRS instruments also view the Moon via its space-view (SV) port without a roll maneuver, covering a wide range of phase angles. In this article, we present techniques used to derive the radiometric gain for the VIIRS RSB using unscheduled lunar observations that are made over a range of phase angles. An empirical correction to account for residual phase angle dependencies is derived from on-orbit SNPP VIIRS unscheduled lunar events and is successfully applied to estimate the radiometric gain that shows good agreement with the gains derived from the scheduled lunar events and those derived from the solar diffuser (SD). In addition to the follow-on VIIRS instrument on the NOAA-20 satellite, three more VIIRS instruments are scheduled to be launched in the next decade. The methodologies developed in this work are expected to be applied to unscheduled lunar events from other VIIRS instruments to support their on-orbit RSB calibration.