Future use of microwave observations in support of Cryosat C. Ruiz (1) , E. Jeansou (1) , J. D. Flach (2) , K. Partington (2) , M. Drinkwater (3) , F. Rémy (4) (1) Noveltis, Parc Technologique du Canal, 2 Ave de lEurope, 31520 Ramonville Saint-Agne, France, christian.ruiz@noveltis.f r, eric.jeansou@noveltis.fr (2) Vexcel UK, Great Farm Offices, West Woodhay, Newbury, UK, kim.partington@vexcel.co.uk , dominic.flach@vexcel.co.uk. (3) European Space Agency-ESTEC, Postbus 299, 2200 AG Noordwijk, Netherlands, Mark.Drinkwater@esa.int (4) Legos, 14 Ave Edouard Belin, 31400 Toulouse, France, frederique.remy@notos.cst.cnes.fr ABSTRACT/RESUME Electromagnetic models are used as the basis for a least squares inversion technique to estimate the dry snow zone surface properties of the terrestrial ice sheets from active and passive microwave satellite data. Retrieved parameters include grain size, density, layer thickness and accumulation rate. The prime motivation is to provide information of direct value to the Cryosat altimeter mission. The derived parameters can be used to convert from elevation change to snow mass change. They can also be used to predict geophysical retracking errors in altimeter data and to estimate the resulting uncertainty in the altimeter elevation measurement. With this technique, snow accumulation rate can also be estimated using passive microwave data. These data can then be compared to historical ERS altimeter data in order to assess the impact of interannual variability in accumulation rate on the significance of rates of elevation change. The technique is in the preliminary stages of assessment but is demonstrated using ERS-2 altimeter data in conjunction with spatio-temporally co-located SSM/I and QSCAT data. It is planned to apply the technique ultimately to Cryosat. This work was performed in the frame of ESA-ESTEC project “Synergetic Use of Remote Sensing Data in IceSheet Snow Accumulation and Topographic Change Estimates” (16556/02/NL/GS Contract) [1] 1 INTRODUCTION Despite the invaluable nature of the adavanced Cryosat radar altimeter measurements, there are some ambiguities which are difficult to resolve without the use of uncertain assumptions. In particular, a change in surface elevation is proportional to a change in mass only if the density profile remains constant. The measurement position may alter within the vertical column of snow and firn if the altimeter data are not retracked correctly. In general, the interpretation and significance of the elevation change measurements is dependent on how other surface parameters vary over time and these cannot be estimated using altimeter data alone. In this paper, we outline a method for using non-altimeter microwave remote sensing data sources to invert surface and near-surface properties of the ice sheets, which can in turn be used to convert the altimeter height measurements into a mass change and can be used to estimate retracking error. The technique, which could be used simultaneously with Cryosat from 2005, is designed for dry snow zones of the ice sheets and has been tested on the Greenland dry snow zone. 2 METHOD The inversion procedure is based on simple Rayleigh scattering based microwave models combined with a model of the dry snow zone stratigraphy. The technique was limited to the dry snow zone by the assumptions of the models described below. A result of this is that the technique is applicable to the central region of Greenland, where it is demonstrated and tested, and to most of Antarctica.