Using solar eclipse events to validate VIIRS reflective solar band calibration at multiple radiance levels

Kevin A. Twedt; Ning Lei; Xiaoxiong Xiong

doi:10.1117/12.2532855

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10 October 2019 Using solar eclipse events to validate VIIRS reflective solar band calibration at multiple radiance levels

Kevin A. Twedt; Ning Lei; Xiaoxiong Xiong

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Proceedings Volume 11151, Sensors, Systems, and Next-Generation Satellites XXIII; 111511M (2019) https://doi.org/10.1117/12.2532855
Event: SPIE Remote Sensing, 2019, Strasbourg, France

Abstract

The VIIRS instruments aboard the SNPP and NOAA-20 (N20) satellites have 14 reflective solar bands (RSB) covering a spectral range from 412 nm to 2250 nm. The radiance of each VIIRS RSB is calculated from a quadratic function of the background-subtracted digital count, 𝑑𝑛, with the quadratic coefficients determined during pre-launch testing. On orbit, calibration is maintained using observations of a sunlit solar diffuser (SD), which views the Sun every orbit through a fixed attenuation screen. The SD observations, which are at nearly the same radiance level every orbit, provide a timevarying overall calibration adjustment factor, the F-factor. But there is no designed on-orbit mechanism for calibration at multiple radiance levels, so the relative strengths of the quadratic coefficients continue to be fixed at the pre-launch values. On a few rare occasions, the VIIRS instruments have passed through a partial solar eclipse during the part of the satellite orbit when the SD is illuminated by the Sun (near the South Pole). As of August 2019, the SNPP and N20 VIIRS SDs have observed five and three partial solar eclipses, respectively. While these events are rare, they offer a unique opportunity to test the RSB calibration using the SD at different radiance levels. In this paper, we compare the reduction in the measured SD signals during an eclipse to the predicted radiance reduction based on the Sun-Moon geometry and a solar radiance model. We find good agreement between the data and model for all events, indicating that the VIIRS RSB gain linearity has remained fairly stable on-orbit. The most significant deviation is for the N20 short-wave infrared bands, which had non-linearity concerns during pre-launch testing. We also investigate the SNPP results using different versions of the prelaunch quadratic gain coefficients.

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Kevin A. Twedt, Ning Lei, and Xiaoxiong Xiong "Using solar eclipse events to validate VIIRS reflective solar band calibration at multiple radiance levels", Proc. SPIE 11151, Sensors, Systems, and Next-Generation Satellites XXIII, 111511M (10 October 2019); https://doi.org/10.1117/12.2532855

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