Characterization of MIPAS elevation pointing

Sufficient knowledge of the pointing is essential for analyses of limb emission measurements. The scientific retrieval processor for MIPAS operated at IMK allows to retrieve pointing information in terms of tangent altitudes along with temperature. The retrieved tangent altitudes are independent of the engineering Line-Of-Sight (LOS) information delivered with the ESA Level 1b product. The difference of pointing retrieved from the reprocessed high resolution MIPAS spectra and the engineering pointing information was examined with respect to spatial/temporal behaviour. Among others the following characteristics of MIPAS pointing could be identified: Generally the engineering tangent altitudes are too high by 0?1.8 km with conspicuous variations in this range over time. Prior to December of 2003 there was a drift of about 50?100 m/h, which was due to a slow change in the satellite attitude. A correction of this attitude is done twice a day, which led to discontinuities in the order of up to 2 km in the tangent altitudes. There is a systematic difference in the mispointing between the poles which amounts to 1.5?2 km, i.e. there is a conspicuous orbit-periodic feature. The analysis of the correlation between the instrument's viewing angle azimuth and differential mispointing supports the hypotheses that a major part of this latter phenomenon can be attributed to an uncorrected roll angle of the satellite/instrument system of approximately 54 mdeg. Complementary to this, ESA operational LOS calibration results were used to characterize MIPAS pointing. For this purpose MIPAS is used as a radiometer while the passage of infrared bright stars through the instrument's field of view is recorded. Deviation from expected time of passage gives information about mispointing. A pronounced seasonal variation of the LOS is seen before a correction of on-board software took place in December of 2003. Further a pitch bias of 24 mdeg with respect to the platform attitude information is found.