Calibration concept of scanning imaging absorption spectrometer for atmospheric cartography (SCIAMACHY)

Abstract

SCIAMACHY (SCanning Imaging Absorbtion spectroMeter for Atmospheric CHartographY) is a spectrometer in the UV, VIS and near IR region, with which the concentration of atmospheric trace gases both in the troposphere and in the stratosphere can be determined. It has been selected for ENVISAT of which the launch is scheduled in 1999. SCIAMACHY is a common German-Dutch project. The optical configuration consists of 2 scan mirrors (a nadir/elevation mirror and an azimuth mirror), a telescope and a complex spectrometer, with which a spectral range from 0.24 - 2.4 micrometers is covered in 8 channels with array detectors. Axillary optics consist of two on-board calibration light sources, a sun/moon follower and a polarization measurement device (PMD). The sun/moon follower enables SCIAMACHY to track the sun or moon during occultation and sun/moon calibration measurements. The PMD is a simple seven channel spectrometer with basically sensitivity for one polarization direction only. The PMD is used to correct for the difference in sensitivity for different polarization states of the regular spectrometer channels. The optical system is described in detail elsewhere. The operational success of an instrument like SCIAMACHY depends strongly on the accuracy of the calibration. Proper calibration enables comparison of measurements with those of other instruments, that measure atmospheric trace gases. Besides it gives information about degradation of the spectrometer during its lifetime, so correction of the measurements can be performed to make the real trends in detection of ozone (and other gases) visible. For on-ground calibration of the diffuser and of the spectral channels at room temperature a special facility has been developed at our institute. In the following chapters some essential aspects of the calibration of SCIAMACHY and of the calibration facility are described.