Radiometric calibration of the GERB instrument

The balance between the incoming solar radiation from the Sun and the outgo ing reflected and scattered solar radiation and thermal-infrared emission f rom the Earth, the so-called Earth Radiation Budget (ERB), provides informa tion on the fundamental energy source of the climate system [1]. To fulfil global coverage and sampling requirements, the ERB measurements have to be made from space. Broadband measurements are necessary because all spectral regions in both the solar and infrared contribute to the Earth's radiative fluxes. The Geostationary Earth Radiation Budget instrument (GERB) will be launched on the Meteosat Second Generation (MSG) geostationary satellite in the year 2000. Both short-wave (0.32 mu m to 4 mu m) and total (0.32 mu m to 30 mu m) radiance measurements will be made, with long-wave (4 mu m to 3 0 mu m) data obtained by subtraction. The accuracy requirements, 1% short-w ave and 0.5% long-wave, are challenging but consistent with previous radiat ion-budget measurements. The instrument will be characterized and calibrate d on the ground prior to launch. The radiometric calibration is performed u sing two black bodies and a lamp-illuminated, integrating-sphere source in the near field. In this paper we describe the calibration algorithm and pro cedure.