Update on the performance of Suomi-NPP VIIRS lunar calibration

Abstract

Lunar observations have been regularly scheduled for the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument aboard the Suomi National Polar-orbiting Partnership (S-NPP) satellite since its launch on October 28th, 2011. In reference to the ROLO irradiance model, the detector gain coefficient or F-factor can be derived from these lunar observations for the reflective solar bands (RSB). Unlike its predecessor Moderate Resolution Imaging Spectroradiometer (MODIS), the Moon and the on-board solar diffuser (SD) are viewed by VIIRS detectors at the same angle of incidence (AOI) to the half angle mirror (HAM). Eliminating the impact from the variation in the instrument response to the HAM AOI, this design allows the detector gain changes tracked by the Moon and the SD to be directly compared. In this paper, we update the lunar F-factors from the scheduled lunar calibration. The long-term trends of the lunar F-factor trending and the SD F-factor trending still agree in general for all RSBs. We also calculate the lunar F-factor at detector level and compare the detector dependency of the lunar F-factor and the SD F-factor. For a few RSBs at shorter wavelengths, a bias of up to 1% between the two has been identified. Using the detector-dependent lunar F-factors will decrease the retrieved Earth view radiance of lower-number detectors in relative to higher-number detectors than the SD F-factors. The inconsistency indicates systematic bias between the lunar and SD calibration approaches.