NONZERO SATURATION OF AVHRR THERMAL CHANNELS OVER HIGH-TEMPERATURE TARGETS - EVIDENCE FROM VOLCANO DATA AND A POSSIBLE EXPLANATION

Our AVHRR Volcano and fire data are in agreement with Setter and Verstraete, showing that when channel 3 saturates the digital number (DN) recorded is greater than zero. The radiance at which this occurs varies within a single radiometer, and from platform to platform. It appears that non-zero saturation has affected all the thermal channels on all AVHRR instruments to date and may be due to the design of the analogue-to-digital converter circuits. Since the exact relation between non-zero DN and spectral radiance is not known, we suggest that all DN over high temperature targets that are less than or equal to the non-zero saturation DN cannot be trusted to be reliable measurements of spectral radiance.