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Meteosat Second Generation (MSG) is the new generation of European geostationary meteorological satellites operated at EUMETSAT. SEVIRI, the MSG main radiometer, measures the reflected solar radiation within three spectral bands centered at 0.6, 0.8 and 1.6 mum, and within a broad band similar to the VIS channel of MVIRI, the radiometer on-board the first generation of METEOSAT satellites. The operational calibration of these channels relies on modelled 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 evaluates the uncertainties associated with the characterization of this "reference", i.e., the modelled radiances. A theoretical estimation is first assessed, based on the impact of the target surface and atmospheric parameter error on the simulated radiance. Top-of-atmosphere simulated radiances are next compared with several thousands of calibrated observations acquired by the ERS2/ATSR-2 and SeaStar/SeaWiFS instruments over the SEVIRI desert calibration sites. Results show that the relative bias between simulation and observation does not exceed +/-5%. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.