Optimized calibration methodology of VIIRS day-night band low-gain stage using a solar diffuser.

An enhanced methodology of the standard on-orbit calibration of the day-night band (DNB) of the visible infrared imaging radiometer suite on board the Suomi National Polar-orbiting Partnership satellite is presented, specifically for the low-gain stage. The range of the "sweet spot" for the fully illuminated scans in the calibration events is expanded from 4° to 7.8° in order to increase the number of fully illuminated scans to 72, which then permits a complete calibration of the DNB to be carried out within one orbit for all 36 aggregated modes and the two mirror sides. This improves over the current operational methodology, which is with a more restricted range for the sweet spot that requires multiple orbits to complete a calibration. The expansion necessarily requires the expansion of a key calibration input, the product of the bidirectional reflectance factor of the solar diffuser (SD) with the vignetting function describing the transmission of the attenuation screen in front of the SD port, or BVP for short. The expanded BVP is derived by fitting the function in three separate intervals and joining them smoothly to form a single function. Additional enhancements include the adaptation of the previously improved SD degradation and all other improvements developed for reflective solar bands from the previous work. The time-dependent relative spectral response is also characterized and implemented, and its impact and improvements are studied and discussed. The result shows that the improved DNB calibration coefficients are more stable and smooth, and less noisy.