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NOAA-20 was successfully launched on Nov. 18, 2017. Intensive radiometric calibration and validation activities for VIIRS onboard NOAA-20 were carried out immediately after the sensor data become available. Several bands of NOAA-20 VIIRS such as M6, and M8 have maximum dynamic range requirements or waivers on the saturation value and require post-launch assessment of the impacts on the L1B (or EDR) products. Since the saturation threshold in data processing is defined as the measured Digital Number (DN) count, the corresponding saturation radiance value in the L1B data product can vary due to detector difference and degradation in the sensor optical throughput. On the other hand, users of L1B data care more about the accuracy of scene-dependent radiance value and the impact of detector saturation on the radiance data. In order to validate the detector saturation level and assess their impacts on the radiance data products, histogram-based Probability Distribution Function (PDF) is derived both at detector level and at band level from the daily global radiance data. With such distribution function, the saturation level in radiance can be identified from the sharp fold-over and cut-off at the high radiance value in the distribution. In addition, the percentage of the affected pixels in the global data can be quantified and the detector performance can be compared. Since NOAA- 20 and SNPP are 50 minutes apart in the same orbital plane and have the same equator-crossing time, the probability distribution function method also enables comparison of the radiometric calibration performance between the two sensors. For example, the saturation radiance value for VIIRS M6 on NOAA-20 is ~44.2 (W/m2-sr-um) on 01/11/2018, which is ~26% lower than that of SNPP VIIRS. Such difference can be traced to the more rapid degradation of rotating telescope assembly mirror reflectance in SNPP VIIRS. In addition to saturation analysis, the PDF method allows us to analyze the VIIRS Day/Night Band (DNB) according to the solar zenith angle range, and the performance of stray light correction can be compared between SNPP and NOAA-20 as well.
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