Precise pointing knowledge for SCIAMACHY solar occultation measurements

[摘要] We present a method to precisely determine the viewing direction for solaroccultation instruments from scans over the solar disk. Basic idea is the fitof the maximum intensity during the scan, which corresponds to the center ofthe solar disk in the scanning direction. We apply this method to the solaroccultation measurements of the satellite instrument SCIAMACHY, which scansthe Sun in elevation direction. The achieved mean precision is 0.46 mdeg,which corresponds to an tangent height error of about 26 m for individualoccultation sequences. The deviation of the derived elevation angle from thegeolocation information given along with the product has a seasonal cyclewith an amplitude of 2.26 mdeg, which is in tangent height an amplitude ofabout 127 m. The mean elevation angle offset is −4.41 mdeg (249 m).SCIAMACHY's sun follower device controls the azimuth viewing direction duringthe occultation measurements. The derived mean azimuth direction has anstandard error of 0.65 mdeg, which is about 36 m in horizontal directionat the tangent point. We observe also a seasonal cycle of the azimuthmispointing with an amplitude of 2.3 mdeg, which is slightly increasingwith time. The almost constant mean offset is 88 mdeg, which is about5.0 km horizontal offset at the tangent point.

[学科分类] 几何与拓扑