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Volume 10, issue 7 | Copyright
Atmos. Meas. Tech., 10, 2413-2423, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 05 Jul 2017

Research article | 05 Jul 2017

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

Klaus Bramstedt1, Thomas C. Stone2, Manfred Gottwald3, Stefan Noël1, Heinrich Bovensmann1, and John P. Burrows1 Klaus Bramstedt et al.
  • 1University of Bremen, Institute of Environmental Physics (IUP), Otto-Hahn-Allee 1, 28359 Bremen, Germany
  • 2US Geological Survey, 2255 North Gemini Drive, Flagstaff, AZ 86001, USA
  • 3German Aerospace Center, Remote Sensing Technology Institute, Münchner Str. 20, 82234 Wessling, Germany

Abstract. The SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) on Envisat (2002–2012) performed nadir, limb, solar/lunar occultation and various monitoring measurements. The pointing information of the instrument is determined by the attitude information of the Envisat platform with its star trackers together with the encoder readouts of both the azimuth and the elevation scanner of SCIAMACHY.

In this work, we present additional sources of attitude information from the SCIAMACHY measurements itself. The basic principle is the same as used by the star tracker: we measure the viewing direction towards celestial objects, i.e. sun and moon, to detect possible mispointings.

In sun over limb port observations, we utilise the vertical scans over the solar disk. In horizontal direction, SCIAMACHY's sun follower device (SFD) is used to adjust the viewing direction. Moon over limb port measurements use for both the vertical and the horizontal direction the adjustment by the SFD. The viewing direction is steered towards the intensity centroid of the illuminated part of the lunar disk. We use reference images from the USGS Robotic Lunar Observatory (ROLO) to take into account the inhomogeneous surface and the variations by lunar libration and phase to parameterise the location of the intensity centroid from the observation geometry. Solar observations through SCIAMACHY's so-called sub-solar port (with a viewing direction closely to zenith) also use the SFD in the vertical direction. In the horizontal direction the geometry of the port defines the viewing direction.

Using these three type of measurements, we fit improved mispointing parameters by minimising the pointing offsets in elevation and azimuth. The geolocation of all retrieved products will benefit from this; the tangent heights are especially improved. The altitudes assigned to SCIAMACHY's solar occultation measurements are changed in the range of −130 to −330m, the lunar occultation measurements are changed in the range of 0 to +130m and the limb measurements are changed in the range of −50 to +60m (depending on season, altitude and azimuth angle). The horizontal location of the tangent point is changed by about 5km for all measurements. These updates are implemented in version 9 of the SCIAMACHY Level 1b products and Level 2 version 7 (based on L1b version 9).

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Short summary
The satellite instrument SCIAMACHY on board the ESA's platform Envisat (2002–2012) performed observations of the Earth's atmosphere. Using sun and moon observations of the instrument itself, we derived a set of correction parameters for the determination of the viewing directions of the instrument. From this work, all vertical profiles of atmospheric parameters from SCIAMACHY's limb and occultation measurements will be improved by a more accurate altitude information.
The satellite instrument SCIAMACHY on board the ESA's platform Envisat (2002–2012) performed...