Evaluation of radiative transfer simulations over bright desert calibration sites

The Spinning Enhanced Visible and Infrared Imager (SEVIRI), the Meteosat Second Generation main radiometer, measures the reflected solar radiation within three spectral bands centered at 0.6, 0.8, and 1.6 mum, and within a broadband. This broadband is similar to the solar channel of the radiometer onboard the first generation of METEOSAT satellites. The operational absolute calibration of these channels relies on modeled radiances over bright desert sites, as no in-flight calibration device is available. These simulated radiances represent, therefore, the "reference" against which SEVIRI is calibrated. The present study describes the radiative properties of these targets and evaluates the uncertainties associated with the characterization of this "reference," i.e., the modeled radiances. To this end, top-of-atmosphere simulated radiances are compared with several thousands of calibrated observations acquired by the European Remote Sensing 2/Along-Track Scanning Radiometer 2 (ERS2/ATSR-2), SeaStar/Sea-viewing Wide Field-of-view Sensor (SeaWiFS), Systeme Pour l'Observation de la Terre 4 (SPOT-4VEGETATION), and the Environmental Research Satellite/Medium Resolution Imaging Spectrometer (ENVISAT/MERIS) instruments over the SEVIRI desert calibration sites. Results show that the mean relative bias between observation and simulation does not exceed 3% in the red and near-infrared spectral bands with respect to the first two instruments.