Radiometric calibration of the GERB instrument

Abstract

The balance between the incoming solar radiation from the Sun and the outgoing reflected and scattered solar radiation and thermal-infrared emission from the Earth, the so-called Earth Radiation Budget (ERB), provides information 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 µm to 4 µm) and total (0.32 µm to 30 µm) radiance measurements will be made, with long-wave (4 µm to 30 µm) data obtained by subtraction. The accuracy requirements, 1% short-wave and 0.5% long-wave, are challenging but consistent with previous radiation-budget measurements. The instrument will be characterized and calibrated on the ground prior to launch. The radiometric calibration is performed using two black bodies and a lamp-illuminated, integrating-sphere source in the near field. In this paper we describe the calibration algorithm and procedure.