Calibrating the AVHRR thermal infrared channels: determining the temperature of the internal calibration target

Inflight calibration of the near and thermal infrared channels of the AVHRR (advanced very high resolution radiometer) proceeds by linearly interpolating between a pair of reference radiances: the zero-radiance of deep space, and the calculable radiance of a passive blackbody honeycomb target (internal calibration target, ICT) whose temperature is measured with a cluster of four embedded PRTs (platinum resistance thermometers). Traditionally, the ICT temperature has been taken to be the equally-weighted average of the four PRT readings. The authors present evidence to show that equal weighting is not optimal, and describe how to compute revised weightings which improve calibration accuracies by ~0.13 K during times of rapid temperature change