New radiance-based method for AVHRR thermal channel nonlinearity corrections

Abstract

Thermal channels 4 and 5 of the Advanced Very High Resolution Radiometer (AVHRR) on National Oceanic and Atmospheric Administration (NOAA) polar-orbiting satellites have an onboard calibration process that provides data from which incoming scene radiance is linearly related to AVHRR count output. However, prelaunch calibration tests show that the radiance is more accurately modelled as a quadratic in count value and that the actual quadratic fit depends upon the operating temperature of the AVHRR itself. NOAA has developed a new method to provide prelaunch information to operational data users that is both concise and accurate. It corrects the linear radiance estimate instead of correcting equivalent blackbody temperature values. The nonlinear correction to the linear radiance estimate is provided by a single quadratic equation, independent of the AVHRR temperature. The new method was first applied to the NOAA-14 AVHRR. When corrected radiance estimates for the NOAA-14 AVHRR are compared to precise prelaunch data, the rms difference is 0.14K in channel 4 and 0.08K in channel 5, in temperature units. At present, users of NOAA-14 AVHRR 1b data have to apply the radiance correction themselves, but for the NOAA-15 and future AVHRRs the necessary information is included in the expanded 1b datastream. Direct readout High Resolution Picture Transmission (HRPT) users have to implement the correction process themselves for NOAA-14, NOAA-15 and future AVHRRs.