VIIRS day/night band radiometric calibration stability monitoring using the Moon

Abstract

AbstractThe Visible Infrared Imaging Radiometer Suite (VIIRS) is a passive scanning spectroradiometer on board the Suomi‐NPP satellite. It has 22 spectral bands including the day/night band (DNB), which is a panchromatic reflective solar band (RSB) covering a wavelength range of 500–900 nm. Similar to other RSBs, the radiometric calibration of the DNB is in reference to the sunlight reflected from an onboard solar diffuser (SD). As an independent validation to the SD measurement, lunar calibration has been regularly scheduled at nearly constant lunar phase. In this paper, the lunar calibration strategies developed for RSB are extended to DNB. The on‐orbit gain coefficient, or the so‐called F factor, is derived for DNB low‐gain stage (LGS) from lunar data for each lunar calibration event. Its on‐orbit change is compared with the change of the LGS SD F factor. For more accurate comparison, the impact of the on‐orbit relative spectral responses (RSR) change, caused by the wavelength‐dependent degradation of the optical throughput of VIIRS telescope mirrors, must be considered. This impact is more significant for DNB than other RSBs because of its wider bandwidth, and the impact to the SD and lunar calibrations are different due to different scene spectra. Simulation results show a gradually increased deviation of 1% between SD and lunar trends since launch till now and 0.3% deviation since 2 April 2012 lunar calibration, when the lunar F factor was firstly calculated, till now. Taking this effect into account, the on‐orbit changes of the SD F factor and lunar F factor agree with each other in less than 0.3%. Our results validate the stability of the DNB SD calibration while demonstrating how the on‐orbit RSR change should be considered in the radiometric calibration and data usage.