Vicarious satellite calibration in the solar spectral range by means of calculated radiances and its application to Meteosat.

The method of vicarious calibration by means of calculated radiances allows absolute calibration of satellite radiometers in orbit. It works by comparing counts from the radiometer to be calibrated with corresponding absolute radiances, calculated from actual values of the relevant optically acting parameters of the atmosphere and the earth's surface. The method is applied to the VIS-channel (it measures in the visible and near IR) of the European geostationary satellite Meteosat-1. To minimize uncertainties, the procedure is carried out over different surfaces, at different atmospheric conditions, and at different sun and satellite angles. The ratio between the effective radiances (the radiances at the satellite weighted with its spectral response) and the measured 6-bit counts of the Meteosat-1-VIS-channel is the calibration constant c(sat) = 2.66 W . m(-2) . sr(-1)/count. The accuracy of the calibration is +/-6%. The inaccuracy is mainly due to the broad digitization steps of the channel. Conversion factors are presented which allow one to calculate from the effective radiance the radiance at the satellite (the radiance leaving the atmosphere).