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Update on the status of Aqua MODIS cold focal plane assembly temperature fluctuation

TitleUpdate on the status of Aqua MODIS cold focal plane assembly temperature fluctuation
Publication TypePresentation
Year of Publication2017
AuthorsWang, Z., X. Xiong, A. Wu, and T. Chang
ProceedingsProc. SPIE Earth Observing Systems XXII
MCST ConferenceSPIE - Earth Observing Systems XXII
KeywordsFocal Plane Array; MODIS; Radiometric calibration; Thermal Emissive Band

The 16 thermal emissive bands (TEB) of MODIS instruments on-board Terra and Aqua satellites are located at two cold focal plane assemblies (CFPA), which are cryogenically cooled by a passive radiative cooler. The CFPA temperatures are controlled at a nominal value of 83 K. For Aqua MODIS, the cooler margin has gradually decreased since launch, which deteriorates the CFPA temperature control. Since 2006, Aqua CFPA temperature has deviated from its 83 K baseline and been fluctuating with the instrument temperature seasonally, daily and per orbit. The magnitude of the fluctuation steadily increased until reached its peak at 0.7 K in 2013 and has been decreasing. The gains of TEB detectors, especially those of the photoconductive bands 31-36, change with the CFPA temperature by up to 8%, as is demonstrated by both pre-launch calibration and on-orbit monitoring. In this paper, the CFPA temperature and its related telemetries are provided. The impact of the fluctuation to TEB radiometric calibration is also assessed. Overall, because the calibration is normally performed on a scan-by-scan basis based on the observation of an onboard blackbody (BB), the detector gain change can be retrieved nearly real time. Therefore, the impact is not significant overall. However, for bands 33, 35 and 36, their detectors saturate when observing BB at BB temperature above certain saturation limits during quarterly held BB warmup-cooldown activities. Since there is no valid scan-by-scan calibration during these periods, a special treatment has to be applied to calibrate these bands to reflect the detector gain fluctuation.