Chapter 5: Calibration and Monitoring

Spaceborne spectral measurements over long time periods require calibration and monitoring of the instrument as a crucial prerequisite for successful retrieval of atmospheric parameters. Calibration applies a sequence of steps to the measurement data while monitoring assesses the optical performance thus permitting degradation corrections. The parameters characterising the instrument were obtained in a sequence of on-ground calibration runs under different environmental conditions. They are stored as Key Data and serve as input when deriving calibrated spectra. Relevant calibration steps include the memory effect and non-linearity, wavelength calibration and both spectral and spatial stray light corrections. Since SCIAMACHY is sensitive to the polarisation state of the incoming light, po-larisation needs to be thoroughly taken into account. The final step performs the radiometric calibra-tion. Once in orbit, the optical performance monitoring establishes information concerning the channel and wavelength dependent degradation. From the combination of the results for the different light paths it is even possible to learn how individual optical components degrade