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.

User Validation of VIIRS Satellite Imagery

“…The spatial and radiometric resolutions of VIIRS are determined at the SDR level. The SDR and EDR radiances are unchanged, except for the special processing that goes into the DNB/NCC pair [6]. Table 3 compares the VIIRS SDRs and Imagery EDRs.…”

“…To create the RGB composite image, the DNB lunar reflectance (following the model of Miller and Turner [31], or via NCC pseudo-albedo following Liang et al [6]) is assigned to the red and green components, and inverted I5 band brightness temperatures are assigned to the blue component. This results in a false-color image product where: open ocean and optically-thin low cloud appear black; optically-thick low cloud appears yellow; optically-thin high cloud appears blue; optically-thick high cloud appears white; and optically-thin high cloud overlaying optically-thick low cloud also appears white, but can often be discerned from optically-thick high cloud by context.…”

“…NCC is capable of providing visible-wavelength images at night, even under no moon conditions [6]. While the quality varies with the amount of moonlight, NCC has proven useful at night in locating clouds, ice edge, snow cover, tropical cyclone centers (eyes), fires and gas flares, lightning, dust storms, and volcanic eruptions [5,12,13].…”

VIIRS Day/Night Band—Correcting Striping and Nonuniformity over a Very Large Dynamic Range

“…Also, the methods described so far involve selectively processing uniform parts of scenes, but by nature the twilight region is not uniform. The NCC imagery produced from DNB produces a pseudo-albedo for day, twilight and night in such a way as to eliminate the orders-of-magnitude changes in radiance over the twilight transition region [29]. Therefore, any striping in the MGS becomes much more visible in the NCC.…”

“…This algorithm divides the DNB radiance by a factor that is dependent on the computed direct or indirect solar and lunar irradiance. The solar and lunar illumination levels are estimated in the twilight region using a fit to the twilight radiances during new moon [29]. The NCC images here are not from the official version, but use a similar technique to minimize twilight contrast.…”