Proceedings Article | 15 October 2012
Proc. SPIE. 8510, Earth Observing Systems XVII
KEYWORDS: Staring arrays, Long wavelength infrared, Photovoltaics, Mercury cadmium telluride, MODIS, Detection and tracking algorithms, Sensors, Calibration, Algorithm development, Temperature metrology
The MODIS instruments on-board the Terra and Aqua spacecrafts have 16 thermal emissive bands (TEB), located on
two cold focal plane assemblies (CFPA). The CFPAs are cryogenically cooled by a passive radiative cooler, with their
temperatures further controlled at a nominal value of 83K. For Aqua MODIS, the cooler margin has gradually decreased
since launch, which deteriorates the CFPA temperature stability. Since 2006, Aqua CFPA temperature fluctuates with
the instrument temperature in both seasonal and orbital oscillation patterns. The magnitude of the fluctuation steadily
increases on yearly basis. The gains of TEB detectors change with the CFPA temperature in a nearly linear way, as is
demonstrated by both pre-launch calibration and on-orbit monitoring. As of mid 2012, the magnitude of the CFPA
temperature fluctuation reaches 0.65K, causing band-dependent detector gain fluctuation of up to 6%. In this paper, the
CFPA temperature and its related telemetries are monitored over both a short-term and long-term basis. The impact of
the fluctuation to TEB radiometric calibration is assessed, too. Because the calibration is normally performed on a scanby-
scan basis based on the observation of an onboard blackbody (BB), the detector gain change can be retrieved in
nearly real time. Therefore, the impact is insignificant in general. However, for bands 33, 35 and 36, their detectors
saturate when observing BB at BB temperature above certain saturation limits during quarterly held BB warmupcooldown
(WUCD) 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.
