Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), on the NASA Terra satellite, has three radiometers, the VNIR, SWIR and TIR. The TIR radiometer has five bands (10 to 14) in the thermal infrared region with a spatial resolution of 90 m. These TIR bands are radiometrically calibrated by a single onboard blackbody whose temperature can be changed between 270 K and 340 K. In the normal operation mode the blackbody is kept at 270 K, and a constant coefficient in a quadratic radiometric calibration equation for each detector is adjusted at that temperature before each Earth observation. Once in 33 days the gain term can be updated by a long term calibration in which the blackbody is measured at 270, 300, 320, and 340 K. The sensor response of all bands (particularly band 12) has been degrading since the launch, and periodical updating of the gain coefficient does not fully follow the degradation, so that the calibration error on level-1 products is sometimes unacceptable. We therefore have developed approximation equations for the coefficients to predict the most reasonable radiometric calibration coefficients (RCC) at the time of the observation. This will be implemented soon in the Level-1 data processing.
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