Abstract
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 −330 m, the lunar occultation measurements are changed in the range of 0 to +130 m and the limb measurements are changed in the range of −50 to +60 m (depending on season, altitude and azimuth angle). The horizontal location of the tangent point is changed by about 5 km 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).
Highlights
Satellite measurements allow global observations of atmospheric constituents
The pointing information of the instrument is determined by the attitude information of the Environmental Satellite (Envisat) platform with its star trackers together with the encoder readouts of both the azimuth and the elevation scanner of SCIAMACHY
In Bramstedt et al (2012), precise pointing knowledge has been derived for SCIAMACHY’s solar occultation measurements. It used the sun over limb port measurements, which are the solar occultation measurements continued above the atmosphere
Summary
Passive remote sensing instruments use three basic types of geometry: nadir, limb and occultation. Critical point in the latter two is always the pointing knowledge, i.e. the precise knowledge of the viewing direction and with that the altitude grid of the observations. In Bramstedt et al (2012), precise pointing knowledge has been derived for SCIAMACHY’s solar occultation measurements. It used the sun over limb port measurements, which are the solar occultation measurements continued above the atmosphere. We will use the moon over limb port measurements and the sun over sub-solar port observations to derive further pointing information. Appendix B lists the important abbreviations used throughout this paper for the reader’s convenience
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