Reanalysis of MetOp AVHRR/HIRS IR channel prelaunch calibration

AVHRR and HIRS on MetOp satellites have been observing the Earth for several years. However, despite meeting the instrument performance specifications, they fall short of user expectations in absolute accuracy especially in the context of climate change detection. In order to achieve further improvements, resolving inconsistencies in the prelaunch calibration data is of vital importance. Prelaunch calibration techniques for IR channels utilize three calibration sources (internal/external calibration target, or ICT/ECT, and Space View) to simulate the in-orbit calibration system. A number of inconsistencies, including the nonlinearity, ICT-ECT discrepancies, and configuration effects in the prelaunch calibration have been identified but unfortunately, these issues have not been fully resolved despite previous efforts. We report on the progress of the investigation of AVHRR/HIRS prelaunch data using a calibration model consisting of a collection of algorithms that takes into account the effects of the non-blackness of the blackbody, view factor differences between the ECT, ICT, and the cold target, and nonlinearity effects. We show that taking these effects into account removes much of the inconsistencies in the data, thus substantially improving the AVHRR/HIRS calibration accuracy.