Images taken by geostationary satellites may be used to estimate solar irradiance fluxes at the earth＇s surface. The Heliosat method is a widely applied procedure for this task. It is based on the empirical correlation between a satellite derived cloud index and the irradiance at the ground. Modifications to this procedure that may reduce the temporal variability of the correlation are presented. The modified method may open the way to the use of a generic relation of cloud index and global irradiance.