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Along track repeat altimetry

Satellite altimetry is the unique possibility for continuous and extensive survey of the large polar ice sheets volume change. With ERS1 it became possible to measure the surface topography of 80% of the Antarctic and quite all of the Greenland ice sheets with an unprecedented accuracy. The accuracy of the classical radar altimeter measurements over continental surfaces is however limited by a number of factors of which the first is the topographic induced error (commonly called slope induced error). In addition volume echo induce penetration effect on the altimeter waveforms. The temporal survey of the surface height is classically made using crossover points differences in order to limit the topographic induced errors. However the measurements show difference as to volume echo induced errors betweenascending and descending tracks. A method has been developed at LEGOS to survey along track by taking into account the fluctuations across track of both the height measurement and the waveform shape parameters. This method has the advantage to avoid the ascending/descending difference in echoing and also to lead to around 100 times more measurements available to survey the evolution. It also helps to look at the time evolution of the ice sheet surface at small scales of the order of few km in regional or local studies. In this presentation, we show the principle and aspects of the methods and the impact in terms of accuracy and local signal on 2 regions, one with very small expected fluctuations (Vostok lake area) and another one with large fluctuations expected (Pine island glacier basin). We discuss the interest of the method in various aspects and it's applicability on other land surfaces.

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