Solar Occultation
Solar occultation measurements are performed during sun rise occuring once per orbit in the northern hemisphere. Usually, the sun disk is scanned permanently, while spectral measurements are performed every 62.5 msec. Only directly transmitted light contributes significantly in occultation geometry. Transmissions are calculated dividing atmospheric measurements by an appropriate measurement from above the atmosphere.
The differential structure of transmission spectra are fitted using the optimal estimation method. With measurement information from several tangent heights, height resolved vertical profiles of atmospheric trace gases can be retrieved. First results of ozone and nitrogendioxid profile retrievals are very promising. Since solar occultation has high theoretical precisions at low computational costs, it is interesting for many applications. Unfortunately, spatial coverage is limited compared to other viewing geometries.
Our work is funded by the DLR, State of Bremen and the AFO-2000 project SACADA (Synoptic Analysis of Chemical constituents by Advanced Data Assimilation). Within the SACADA project, solar occultation data products will be provided to verify synoptic data analysis schemes. This is done in cooperation with the Rhenish Institute for Environmental Research (University of Cologne).
Lunar Occultation
The moon is used as the light source for Lunar Occultation. Like Solar Occultation, atmospheric extinction due to trace gases absorption is measured by SCIAMACHY instrument on board of ENVISAT as the moon rises or sets. Lunar occultation measurements begins at lunar phase of 0.6-0.7 and ends shortly after full moon. The moon follower tracks the moon at 17.2 km and follows it up to the altitude of 100 km; above 100 km the moon is used as a target of calibration and monitoring. SCIAMACHY lunar occultation event presently is observed in the southern hemisphere between latitudes 30° and 90°, in the northern hemisphere, no lunar measurements are performed due to the instrument design of SCIAMACHY.
Lunar radiance transmitted through the atmosphere is determined by rationing the lunar radiance within lunar occultation atmosphere to lunar measurement above the atmosphere (extraterrestrial lunar radiance). First results of SCIAMACHY lunar transmission for channels three and four were observed, showing absorption of atmospheric trace gases such as O3, NO2, NO3, O2 and H2O. Differential structures of lunar transmission spectra are fitted using the optimal estimation method. Height resolved vertical profiles of O3 and NO2 are retrieved so far with the same method as employed in solar occultation.
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