Assessment of scan-angle dependent radiometric bias of Suomi-NPP VIIRS day/night band from night light point source observations

The low gain stage of VIIRS Day/Night Band (DNB) on Suomi-NPP is calibrated using onboard solar diffuser. The calibration is then transferred to the high gain stage of DNB based on the gain ratio determined from data collected along solar terminator region. The calibration transfer causes increase of uncertainties and affects the accuracy of the low light radiances observed by DNB at night. Since there are 32 aggregation zones from nadir to the edge of the scan and each zone has its own calibration, the calibration versus scan angle of DNB needs to be independently assessed. This study presents preliminary analysis of the scan-angle dependence of the light intensity from bridge lights, oil platforms, power plants, and flares observed by VIIRS DNB since 2014. Effects of atmospheric path length associated with scan angle are analyzed. In addition, other effects such as light changes at the time of observation are also discussed. The methodology developed will be especially useful for JPSS J1 VIIRS due to the nonlinearity effects at high scan angles, and the modification of geolocation software code for different aggregation modes. It is known that J1 VIIRS DNB has large nonlinearity across aggregation zones, and requires new aggregation modes, as well as more comprehensive validation.