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 Verst raete, showing that when channel 3 saturates the digital number (DN) r ecorded 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 analogu e-to-digital converter circuits. Since the exact relation between non- zero DN and spectral radiance is not known, we suggest that all DN ove r high temperature targets that are less than or equal to the non-zero saturation DN cannot be trusted to be reliable measurements of spectr al radiance.