Comparisons of GERB and CERES measurements

Geostationary Earth Radiation Budget (GERB) instruments are aboard Meteosat Second Generation (MSG) satellites, providing the first measurements of the Earth radiation budget at high temporal resolution so as to determine the variation of the radiation budget during the day. These data will permit investigations of the energetics of weather processes with 15-minute resolution, opening a new range of research. There are now two of these instruments in orbit. The GERB aboard the MSG-1 began operating in 2002 and continues to function well. The GERB aboard MSG-2 began operations in January 2006. Each instrument measures the Earth's outgoing longwave radiation and the reflected solar radiation over the disc of the Earth that is visible from geosynchronous altitude (36000 km).

The measurements are made by an array of 256 detectors that operate in a spin-scan mode. Every 15 minutes the detectors measure the total radiation from the Earth and the reflected solar radiation at a spatial resolution of 1 degree in latitude and longitude at the equator. In-flight calibration is performed by use of a blackbody for the total measurements and an integrating sphere that is illuminated by the Sun for the reflected solar measurements.

The CERES (Clouds and the Earth's Radiant Energy System) flight model 2 (FM-2) scanning radiometer aboard the Terra spacecraft is used as a transfer radiometer to compare the 256 GERB detector measurements among themselves and to provide a comparison with CERES and other earlier measurements of Earth radiation. In order to observe the same radiances as the GERB detectors, the FM-2 is operated in a mode so as to give CERES measurements that are coaligned with those of GERB as the Terra spacecraft underflies the MSG spacecraft. For the first GERB to fly, CERES has operated in four campaigns to measure radiances that are coaligned with GERB for two Northern Hemisphere winters and two Northern Hemisphere summers.

Comparisons are reproducible within +/- 1% among campaigns. For shortwave radiances, GERB detectors vary by +/-2% among themselves. Between adjacent detectors there is a random uncorrelated change of ~1/2% and a long-range structure (over a score or more of detectors) of +/-2% superimposed. For longwave radiances, the ratio of GERB to CERES is in the range of 1.00 +/- 0.01. These results apply to the measurements that are not at high latitudes. For high latitudes, the geometry of coaligned measurements does not yield good sampling or footprint coverage.