Scan-angle dependent degradation correction with the scanner model approach

SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) is a grating spectrometer in the UV-Vis-NIR spectral range on-board EnviSat (2002-2012). SCIAMACHY observes Earth’s atmosphere in nadir, limb and occultation geometry. The throughput of the instrument is monitored end-to-end by regular observations of the sun and the internal white light source (WLS). A new (scan-angle dependent) degradation correction is implemented as a joint effort of SRON (scanner model and algorithm prototype), IUP Bremen (operational m-factors) and MF-ATP-DLR (implementation into the L0-1b processor). Described by this technical note is the setup of the operational m–factor calculation, implemented for the next L0-L1b processor version (Version 8).
For the degradation correction of Version 8, the radiometric (and the polarization) calibration approach has been changed. Both are split in a scanner unit part and an optical bench module (OBM, including the detectors) part. The scanner unit is described by a physical model of its components, which includes contamination layers on top of the optical components. The increase of these contamination layers accounts to a large extent for the observed degradation of the instrument.