IR detectors for the GERB instrument on MSG.

The Geostationary Earth Radiation Budget (GERB) instrument is to be flown on ESA’s Meteosat Second Generation (MSG) satellite in 2000. The purpose of the instrument is to measure accurately the daily cycle of the reflected and emitted radiation of the Earth over at least a five year period. The measurements will be made from geostationary orbit and will complement those planned from instruments in low Earth polar orbits. The data from GERB will provide the first consistent measurements of the hour-by-hour variation of clouds and simultaneous measurements of the radiation balance, and will allow climate models to be further developed and validated. The instrument will accumulate images of the Earth disc every 15 minutes in wavebands of 0.32 - 4.0 m and 0.32 - 30 m with a nadir resolution of 50 km. The detector for this instrument consists of a 256 pixel linear array of thermoelectric (TE) elements. The TE array operates at room temperature and is blacked to give a flat spectral response over the 0.32 - 30 m band. The detector hybrid consists of the 256 pixel detector plus 4 Application Specific Integrated Circuits (ASICs), comprising 64 channels each, which perform front end analogue signal processing, A/D conversion and multiplexing. As the MSG platform is spin-stabilised, the Earth image is stabilised on the detector using a de-spin mirror and is only present on the detector for 40 ms. Integration of the signal over the 40 ms and taken over a 15 minute observation period enables the radiance in both long and short wavebands to be measured to an accuracy better than 1%. The detector concept is described and test results of a prototype system are presented.