ASTER/TIR onboard calibration status and user-based recalibration

Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), one of five sensors on Terra, has five bands (10 to 14) in the thermal infrared (TIR) region. These TIR bands are radiometrically calibrated by one onboard blackbody with the function of changing temperature between 270 and 340 K. In normal operation the blackbody is set up 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, but the gain coefficient cannot be adjusted at this time, while it can periodically be updated by long term calibration in which the blackbody is measured at 270, 300, 320, and 340 K. On the other hand 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 (L1) products is often unacceptable. We therefore have developed a recalibration method which is easily applied to L1 products by a general user. By using this method, the calibration error will mostly be reduced below the level of NEDT.