Field of View
Every 15mins after 3 complete Total and SW 250x256 arrays of the Earth disk are taken a synthetic LW result is obtained from the mean difference between the two. Such ERB results are then combined with a resolution enhancement and cloud retrievals using the Spinning Enhanced Visible and Infrared Imager (SEVIRI) also on the MSG platform. The combination of GERB and SEVIRI through data synergy also required detailed mapping of each of the 256 GERB detector/telescope Field of View response or Point Spread Function (PSF, see Matthews (2004) ). This was done using a He-Ne laser computer controlled to map each of the 256 thermopiles responses after being covered with gold blacking. Full details of the GERB ground calibration can be obtained at Matthews (2003). The spectral response or measure of the relative absorption at different wavelength of light for each GERB detector is required for the process of un-filtering each thermopile's raw signal. This uses radiative transfer models to estimate the spectral shape of a particular scene radiance to estimate an un-filtering ratio, or the factor needed to account for non-uniform spectral response. For each GERB device this relies on the multiplication of unit level laboratory measurements of detector, telescope, DSM and quartz filter spectral throughput/absorption. The accuracy of GERB SW results is directly dependent on the quality of such measurements as the SW gain is determined using the VISCS lamp, whose spectrum is significantly shifted to longer wavelengths compared to that of the Sun. Such GERB accuracy is currently estimated to be around the 2% level by ref. Such un-filtering is performed by the Royal Meteorological Institute Belgium (RMIB), along with the synergy with SEVIRI data and conversion from radiance to irradiance using Angular Dependency Models (ADMs).