The Geostationary Earth Radiation Budget (GERB) instrument on MSG-1 (Meteosat-8), measures the outgoing longwave and reflected solar energy every 15 minutes for the whole of the MSG region. These data, which are the first high accuracy broadband measurements at such high temporal resolution, will be presented both as instantaneous radiances and fluxes and averaged into monthly means for all data points and for each 15 minutes time period. In order to use these fluxes to study cloud forcing and to aid in climate model validation studies, the calculation of associated clear sky fluxes is required. Here we compare various strategies for determining clear sky fluxes for the GERB data, and investigate if the unique temporal sampling of the GERB instrument warrants a different approach to that used to derive clear sky fluxes for low Earth orbit radiation budget instruments. The study includes consideration of the relative merits of spatial and temporal averaging and the optimum use of imager data. A comparison with clear sky model data is also shown.