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13 September 2011 Further investigation on MODIS solar diffuser screen vignetting function and its implementation in RSB calibration
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Abstract
The MODIS high-gain ocean color bands (B8-B16) are calibrated with its solar diffuser screen (SDS) closed to avoid saturation so that the vignetting function (VF) of SDS is necessary for the calculation of the gain coefficients of these detectors. Since there was no pre-launch system level characterization of the VF, a series of yaw maneuvers were carried out at the mission beginning for both Terra and Aqua to enable its on-orbit characterization. Current VF was derived from the low-gain bands (B1-B7 & B17-B19) data and applied to high-gain ocean color bands calibration, with the assumption that all bands and detectors should share the same VF since it is wavelength independent. As expected, error exists and it was carried over into the calibrated gain coefficients of those bands that use the SDS for their on-orbit calibration. In this paper, an improved VF calculation approach, still using the yaw data as input, is presented. The new approach takes the frame-level mismatch between different detector's footprints on the solar diffuser (SD) into account so that a proper SD image frame adjustment is made when the VF of the low-gain bands is translated into high-gain bands VF. A new set of band-and-detector dependent VFs can be derived using this approach. The implementation of the new VF into calibration of high-gain bands gain coefficient has effectively reduced the undesired seasonal oscillations in its trending from up to Terra's 0.6% and Aqua's 1.0% to nearly 0.2%.
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Zhipeng Wang, Xiaoxiong Xiong, and Williams L. Barnes "Further investigation on MODIS solar diffuser screen vignetting function and its implementation in RSB calibration", Proc. SPIE 8153, Earth Observing Systems XVI, 815307 (13 September 2011); doi: 10.1117/12.892482; https://doi.org/10.1117/12.892482
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