1041 Satellite-pointing retrieval from atmospheric limb-scattering of solar UV-B radiation J.W. Kaiser, C. von Savigny, K.-U. Eichmann, S. Noël, H. Bovensmann, and J.P. Burrows Abstract: We present a new algorithm for tangent height retrievals from limb-scattering observations. These observations are performed by satellite-based spectrometers operating in the UV, visible, and short-wave IR spectral ranges. They record the solar radiation scattered in Earth’s atmosphere in limb-viewing geometry and aim at vertically resolved retrievals of the atmospheric composition with global coverage. Inaccuracies in the knowledge of the instrument’s pointing frequently dominate the error budgets of the atmospheric composition products. Therefore, additional information on satellite pointing is crucial for the quality of all products derived from limb-scattering observations. The information is commonly expressed in terms of the tangent heights. The presented algorithm determines the tangent heights directly from the observed limb radiances by analyzing the shapes of the so-called knee in several UV-B radiance profiles.All vertical pointing information contained in a UV-B fit window is exploited by simultaneously retrieving the ozone profile. The algorithm has been implemented in the toolbox SCIARAYS and named Tangent height Retrieval by UV-B Exploitation (TRUE) knee method. We have applied it to five orbits of SCIAMACHY’s limb observations. It achieves a precision of about 200 m when applied to an individual limb scan. The broadband structure of the observations can be reproduced within 1% RMS. A compari- son of the retrieved tangent heights with the engineering ones delivered by ESA reveals that the engineering tangent heights exhibit a systematic error, which varies with an amplitude of about 3 km. Its origin is traced to the on-board orbit model of Envisat. PACS Nos.: 94.80.+g, 42.68.Ay, 92.60.Ta, 42.68.Wt, 07.87.+v, 92.70.Cp, and 07.60.Rd Résumé : Nous présentons un nouvel algorithme pour déduire les hauteurs tangentes des observations de diffusion dans le limbe. Ces observations sont faites par des spectromètres à bord de satellites et opérant dans l’UV, le visible et les IR de courte longueur d’onde. Ils enregistrent la radiation solaire diffusée dans le limbe de l’atmosphère terrestre, dans le but d’en déterminer la distribution verticale des composants dans une vue globale. Les imprécisions dans la direction où pointent les instruments sont la plus grande source d’erreurs Received 1 June 2004. Accepted 19 October 2004. Published on the NRC Research Press Web site at http://cjp.nrc.ca/ on 29 November 2004. J.W. Kaiser. 1 Remote Sensing Laboratories, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland. C. von Savigny, K.-U. Eichmann, S. Noël, H. Bovensmann, and J.P. Burrows. Institute of Environmental Physics and Remote Sensing, University of Bremen, Otto-Hahn-Allee 1, 28334 Bremen, Germany. 1 Corresponding author (e-mail: johannes@uni-bremen.de). Can. J. Phys. 82: 1041–1052 (2004) doi: 10.1139/P04-071 © 2004 NRC Canada