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The calibration of space borne observations is required for converting the engineering unit (digital count) into the relevant physical unit (radiance). The Meteosat spacecraft series has an on-board black body calibration mechanism, in which black bodies with known temperatures can be viewed. However, an absolute on-board calibration of the optical system is not possible, as part of the system consisting of the front optics is excluded from the calibration path. Nevertheless, on-board black body calibration will be used operationally on Meteosat-7 and may prove useful for detecting trends in the sensor sensitivity. For absolute calibration of the infrared channels, vicarious techniques are presently used, comprising the use of external reference data like sea surface temperatures for the IR channel or radiosonde observations for the WV channel. In such vicarious calibration technique the external reference data are converted into expected radiances at the top of the atmosphere using a radiative transfer model. The expected radiances are then related to the observed counts, the relation being the vicarious calibration coefficient. Black body calibrations have been performed for a short period on Meteosat-4, until a failure of the mechanism forced the black body calibrations to be suspended. During commissioning of the Meteosat-7 spacecraft black body calibrations have been resumed, and the results show a relatively good relation between the black body and the vicarious calibration. During Meteosat's eclipse seasons in autumn the temperatures of the black bodies respond directly to the diurnal variation of the ambient spacecraft temperature. Black body calibrations may be used to crosscheck or estimate the vicarious calibration. The calibration of the IR channel may be validated using satellites with an on-board calibration, like polar orbiting satellites as the NOAA series. In a preliminary study NOAA-14 is used for a cross- satellite comparison with a few orbits in 1996 and 1997. The NOAA-14 channel 4 and 5 observations were used for collocation with the Meteosat-6 IR channel. First results indicate a good agreement of the Meteosat calibration with satellite cross calibrations for both channels. The next series of Meteosat spacecraft (Meteosat Second Generation) will also have a black body calibration mechanism on board, which has an improved functionality with respect to the present system. As with the operational Meteosat spacecraft the black body mechanism excludes the front optics in its optical path, yet it is envisaged that the black body calibration can be used for absolute calibration if a correction model is applied. The present vicarious calibration techniques will be available for monitoring the operational calibration, or for assisting it.
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