2-b HDO GHz. HDO is measured using the same radiometer in a time shared mode during roughly half of the 15 orbits per measurement day, each orbit corresponding to about 60 individual limb-scans. The measurement precision for HDO is about % (  5 ppbv), and single-profile information is obtained between km with an altitude resolution in the order of 3-4 km. 1 Introduction. The Sub-Millimetre Radiometer (SMR) on board the Odin satellite, launched in February 2001 in a polar sun-synchronous orbit, employs 4 tunable single-sideband Schottky-diode heterodyne receivers in the GHz spectral range. A 1.1 m telescope is used for passive observations of thermal emissions originating from the Earth’s limb. Spectra are recorded using two high resolution auto-correlator spectrometers. Measurements are performed in a time sharing mode with astronomical observations. In aeronomy mode, various target bands are dedicated to observations of trace constituents relevant to stratospheric / mesospheric chemistry and dynamics such as O 3, ClO, N 2 O, HNO 3, H 2 O, CO, and isotopes of H 2 O and O 3. Profile information is retrieved from the spectral measurements of a limb scan by inverting the radiative transfer equation for a non-scattering atmosphere. A retrieval algorithm based on the “Optimal Estimation Method” has been adopted for the ground segments of Odin-SMR in Sweden and in France. The water isotope mode is specifically designed for observations of stratospheric H 2 O, H 2 O-18, and HDO using two 800MHz wide bands centred at and GHz. Achieved observation capabilities of the SMR radiometer for these target species are presented and examples for the global water vapour data set are given. Potential of the Odin Sub-Millimetre Radiometer for the Study of Stratospheric Water Vapour and its Isotopes J. Urban (1), N. Lautié (2), D. Murtagh (2), Y. Kasai (3), J. de La Noë (1), E. Dupuy (1), L. El Amraoui (1), P. Eriksson (2), U. Frisk (4), C. Jimenez (1), E. Le Flochmoën (1), M. Olberg (5), and P. Ricaud (1) (1) Observatoire Aquitain des Sciences de l’Univers / L3AB, Floirac, France; (2)Chalmers University of Technology, Göteborg, Sweden; (3)Communications Research Laboratory, Tokyo, Japan;(4) Swedish Space Corporation, Solna, Sweden; Contact: (5)Onsala Space Observatory, Onsala, Sweden. Potential of the Odin Sub-Millimetre Radiometer for the Study of Stratospheric Water Vapour and its Isotopes J. Urban (1), N. Lautié (2), D. Murtagh (2), Y. Kasai (3), J. de La Noë (1), E. Dupuy (1), L. El Amraoui (1), P. Eriksson (2), U. Frisk (4), C. Jimenez (1), E. Le Flochmoën (1), M. Olberg (5), and P. Ricaud (1) (1) Observatoire Aquitain des Sciences de l’Univers / L3AB, Floirac, France; (2)Chalmers University of Technology, Göteborg, Sweden; (3)Communications Research Laboratory, Tokyo, Japan;(4) Swedish Space Corporation, Solna, Sweden; Contact: (5)Onsala Space Observatory, Onsala, Sweden. Acknowledgements: Odin is a Swedish-led satellite project funded jointly by the Swedish National Space Board (SNSB), the Canadian Space Agency (CSA), the National Technology Agency of Finland (Tekes) and the French Centre National d’Études Spatiales (CNES). Top: the Odin satellite platform with its 1.1 m telescope, solar-panels and -shields just before the launch. 2-a H 2 O GHz. Single-profile information of stratospheric water vapour is retrieved between 20 and 70 km with a vertical resolution in the order of 3 km and a precision of % (0.5-1 ppmv). Measurement noise might be reduced by data averaging. The Figures show for example zonally averaged water vapour fields as observed by Odin/SMR on April and Octobre 26-27, Odin/SMR water isotope mode measurements are performed in about weekly intervals. 2-c δD (HDO). The zonal mean depletion of stratospheric deuterium with respect to its isotopic ratio in Standard Mean Ocean Water (SMOW) of ×10 -4 can be derived from the Odin/SMR measurements of H 2 O and HDO. This variation is usually expressed using δ-notation: δD = { (2[HDO]/[H 2 O] - R 0 ) / R 0 }×100 [%], where R 0 is a standard isotope ratio (e.g. SMOW). The Figures show preliminary results obtained for April and October 26-27, A maximum depletion in the order of 60 % is observed in the lower stratosphere of the tropics and δD increases with altitude above. Results are in qualitative agreement with 1-d and 2-d model calculations [e.g. Ridal, JGR, 2001; Bechtel & Zahn, ACPD, 2003] simulating δD as function of the relative contributions of the different sources of stratospheric water vapour such as methane oxidation and transport through the tropical tropopause. 3 Discussion. For a more quantitative interpretation of the Odin/SMR measurements, the accuracy of the H 2 O, HDO, and H 2 O-18 data has to be determined, which is a function of various instrumental (calibration accuracy, antenna and sideband suppression knowledge) and spectroscopic uncertainties. Most critical spectroscopic errors are for example the pressure broadening parameters of the target lines, which will have to be determined by laboratory experiments April October 2002 H2OH2O H2OH2O HDO δD (HDO) H 2 O-18 δ 18 O (H 2 O-18) 2-d H 2 O GHz. A H 2 O-18 line is measured simultaneously with H 2 O. The single-profile precision is in the order of % (  3 ppbv) and the altitude resolution is 3-4 km. Information is obtained between approximately 20 and 60 km. 2-e δ 18 O (H 2 O-18). Chemical and physical isotope fractionation also affects the isotopic ratio of 18 O/ 16 O in stratospheric water vapour. The Figures show observed zonal mean variations of δ 18 O = { ([H 2 O-18]/[H 2 O-16] - R 0 ) / R 0 } × 100 [%].