OMEGA long wavelength channel: Data reduction during non-nominal stages |
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| Authors: | Denis Jouglet François Poulet Yves Langevin Jean-Pierre Bibring Brigitte Gondet Mathieu Vincendon Michel Berthe |
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| Institution: | 1. Institute of Astrophysics and Space Sciences, Observatório Astronómico de Lisboa, Ed. Leste, Tapada da Ajuda, 1349-018 Lisbon, Portugal;2. LESIA - UMR CNRS 8019 - Laboratoire d’Études Spatiales et d’Instrumentation en Astrophysique, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France;3. Université Versailles St-Quentin - DYPAC EA 2449, 47 boulevard Vauban, 78280 Guyancourt, France;4. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency - 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan;5. IRAP, Université Paul Sabatier/UMR CNRS 5277 - Institut de Recherche en Astrophysique et Planétologie, 9, avenue du Colonel Roche, BP 44346 - 31028 Toulouse Cedex, France;2. Tulane University, Urology, New Orleans, LA, USA;1. Laboratoire de Météorologie Dynamique (LMD)/IPSL, CNES, CNRS, Paris, France;2. Instituto de Astrofísica de Andalucía (IAA), CSIC, Granada, Spain;3. Laboratoire Atmospheres, Milieux, Observations Spatiales (LATMOS), IPSL, Paris, France |
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| Abstract: | The Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) instrument is a visible and near-infrared imaging spectrometer on board the European Mars Express (MEx) mission. The on-board calibration (OBC) performed at the beginning of observations on each orbit reveals that the photometric response of the C channel (1.0–2.5 μm) has been very stable since orbit insertion in January 2004. On the contrary the L channel (2.5–5.1 μm) response has varied significantly during the mission, and only orbits for which the response is close to nominal could be used with confidence. The spatial coverage of ice-free surfaces in this wavelength range is consequently limited to only ～30%, mainly during northern spring and summer. This paper presents the empirical method used to derive new instrumental transfer functions (ITF) for the non-nominal orbits. This method consists of analyzing the variation of the signal between several observations of a same region acquired at nominal and non-nominal calibration states. In the cases where the mineralogy and the atmospheric conditions between the two observations are the same, the variation in reflectance spectra is only due to the ITF variation, which provides a new ITF. We then associate these new ITFs with their corresponding OBCs to model a relationship between both. The resulting model enables us to provide a new ITF for each orbit for which the OBC is available. The new ITFs derived for the entire dataset have been validated (1) through a comparison of the C and L channel global albedo trends and (2) through a comparison of the surface temperatures derived from the L channel with those calculated from the General Circulation Model (GCM) numerical simulation of the LMD released in the Martian Climate Database. The non-nominal data processed with adapted ITFs for orbits up to 3050 increase the non-icy surface coverage of Mars to ～70% including all seasons. |
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