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The SCanning Imaging Absorption spectroMeter for Atmospheric\udCHartographY\ud(SCIAMACHY) on Envisat (2002–2012) performed nadir, limb, solar/lunar\udoccultation and various monitoring measurements. The pointing information of\udthe instrument is determined by the attitude information of the Envisat\udplatform with its star trackers together with the encoder readouts of both\udthe azimuth and the elevation scanner of SCIAMACHY.\ud  \udIn this work, we present additional sources of attitude information from the\udSCIAMACHY measurements itself. The basic principle is the same as used by\udthe star tracker: we measure the viewing direction towards celestial\udobjects, i.e. sun and moon, to detect possible mispointings.\ud  \udIn sun over limb port observations, we utilise the vertical scans over the\udsolar disk. In horizontal direction, SCIAMACHY's sun follower device (SFD)\udis used to adjust the viewing direction. Moon over limb port measurements\uduse for both the vertical and the horizontal direction the adjustment by the\udSFD. The viewing direction is steered towards the intensity centroid of the\udilluminated part of the lunar disk. We use reference images from the USGS\udRobotic Lunar Observatory (ROLO) to take into account the inhomogeneous\udsurface and the variations by lunar libration and phase to parameterise the\udlocation of the intensity centroid from the observation geometry.\udSolar observations through SCIAMACHY's so-called sub-solar port (with a\udviewing direction closely to zenith) also use the\udSFD in the vertical direction. In the horizontal direction the geometry of the port defines the viewing\uddirection.\ud  \udUsing these three type of measurements, we fit improved mispointing\udparameters by minimising the pointing offsets in elevation and azimuth. The\udgeolocation of all retrieved products will benefit from this; the\udtangent heights are especially improved. The altitudes assigned to SCIAMACHY's solar\udoccultation measurements are changed in the range of −130 to −330 m, the lunar occultation\udmeasurements are changed in the range of 0 to +130 m\udand the limb measurements are changed in the range of −50 to\ud+60 m (depending on season, altitude and azimuth angle). The horizontal\udlocation of the tangent point is changed by about\ud5 km for all measurements. These updates are implemented in version 9 of the SCIAMACHY Level 1b\udproducts and Level 2 version 7 (based on L1b version 9)

Improved pointing information for SCIAMACHY from in-flight measurements of the viewing directions towards sun and moon