Improved VIIRS Day/Night Band Imagery With Near-Constant Contrast

Abstract

The Suomi-NPP Visible Infrared Imager Radiometer Suite (VIIRS) instrument provides the next generation of visible/infrared imaging including the day/night band (DNB) with nominal bandwidth from 500 to 900 nm. Previous to VIIRS, the Defense Meteorological Satellite Program Operational Linescan System (OLS) measured radiances that spanned over seven orders of magnitude, using an onboard gain adjustment to provide the capability to image atmospheric features across the solar terminator, to observe nighttime light emissions over the globe, and to monitor the global distribution of clouds. The VIIRS DNB detects radiances that span over eight orders of magnitude, and because it has 13-14-b quantization (compared with 6 b for OLS) with three gain stages, the DNB has its full dynamic range at every part of the scan. One process that is applied to the VIIRS DNB radiances is a solar/lunar zenith angle dependent gain adjustment to create near-constant contrast (NCC) imagery. The at-launch NCC algorithm was designed to reproduce the OLS capability and, thus, was constrained to solar and lunar angles from 0° to 105°. This limitation has, in part, lead to suboptimal imagery due to the assumption that DNB radiances fall off exponentially beyond twilight. The VIIRS DNB ultrasensitivity in low-light conditions enables it to detect faint emissions from a phenomenon called airglow, thus invalidating the exponential fall-off assumption. Another complication to the NCC imagery algorithm is the stray light contamination that contaminates the DNB radiances in the astronomical twilight region. We address these issues and develop a solution that leads to high-quality imagery for all solar and lunar conditions.