A successful integration of solar energy into the existing energy structure highly depends on a detailed knowledge of the solar resource. The recently started European project HELIOSAT-3 will supply high-quality solar radiation data and taking advantage of the enhanced capabilities of the new Meteosat Second Generation (MSG) satellites. The expected quality represents a substantial improvement with respect to the available methods and will better match the needs of the solar energy community. These goals will be achieved by an improvement of the current semi-empirical Heliosat calculation schemes (see section 2) as well as by the development and establishment of a new type of calculation scheme. This new type will be based on radiative transfer models (RTM) using the information of atmospheric parameters retrieved from the MSG satellite (clouds, ozone, water vapor) and the GOME/ATSR-2 satellites (aerosols). Within this paper, the new type of the solar irradiance calculation scheme, including the functional treatment of the diurnal variation of the solar irradiance, is described.