SCIAMACHY solar irradiance validation using radiometric calibration of balloonborne spectrometers

Solar occultation measurements of balloonborne LPMA (Limb Profile Monitor of the Atmosphere, a Fourier-Transform IR spectrometer) and DOAS (Differential Optical Absorption Spectroscopy, a two channel UV-VIS grating spectrometer) instruments are used for the validation of the SCIAMACHY level-1 product solar irradiance in this "added value" approach. Their primary objective is to measure trace gas concentrations. LPMA [1] and DOAS [2] are mounted on the same gondola and use the same entrance optics to track the solar disc from a stratospheric observation position during sunrise or sunset. As a result of the high float altitude (32 km and more) of the payload, only few corrections (mainly ozone) are needed for atmospheric absorbers, to compare the spectra with the (extraterrestrial) spectra from SCIAMACHY. In order to get absolute measurements of the solar irradiance, the balloon instruments have to be radiometrically calibrated before each flight. This is done on site during the campaigns by using NIST- traceable calibration sources. New optical designs implemented in the calibration sources enable a repeatability of better than 1% even under field conditions.