S-NPP VIIRS DNB Dark Offset and Detector Dark Current Trending Study

The Visible Infrared Imaging Radiometer Suite (VIIRS) Day-Night Band (DNB) is a panchromatic band in the VisNIR spectral range from 0.5 to 0.9 μm with a dynamic range from 3x10-9 to about 0.02 W cm-2 sr-1. DNB achieves this large dynamic range by having three gain stages: low gain (LGS), mid gain (MGS), and high gain (HGS). HGS is the average of two redundant detector arrays, HGA and HGB. The HGS offset determination is critically important to improve the imagery capability and calibration accuracy and stability at novel low radiances. Currently, the dark offset is determined on a monthly basis by observing new moon data in the dark regions in the Pacific Ocean. The data is mainly comprised of detector dark current, electronic/clock offsets, artificial illumination sources, and nighttime airglow contamination. The first two are instrument phenomena/characterizations and the last two are scene contaminations.In this presentation, we discuss the long-term growth in offset with short-term fluctuations we have captured since the beginning of the mission. We associate the long-term growth with dark current increase. We show that the offset rate of change over time is proportional to the number of Charge Coupled Device (CCD) elements aggregated to comprise a DNB pixel. We compare offset growth rate from new moon data in the dark ocean and that from calibration sector data at the same time; which contain very limited scene contaminations. We associate the short-term variability in offset growth with airglow effects that survive the filtering process used to derive the offsets from dark ocean data. These spurious offset fluctuations are removed from the offset LUTs via long time scale smoothing of the offsets. The remaining persistent time average contribution due to airglow can be estimated by comparison of the pitch maneuver propagated offsets determined from deep space scans early in the mission and the offsets determined from the dark regions in the Pacific Ocean.