The Radar altimeter is essentially an instrument for determining the two-way delay of the radar echo from the Earth's surface to a very high precision (less than a nanosecond) level. It also measures the power and the shape of the reflected radar pulses.
Altimetric data collected over continents are usually processed and optimized for ocean surfaces which restrained their use for continental applications. With the introduction of radar altimetry applications over the ice sheets, a new retracking procedure was developed, the so called ICE-2 retracker. It has been designed to process altimeter waveforms obtained over most of the non-ocean surfaces. This technique was first applied with success to the ERS altimeter data (Legresy & Remy, 1997; Remy et al., 1999; Vinay et al., 2002). This processing has been now included directly in the ENVISAT radar altimeter ground segment, which enables retracked data to be delivered to the users for continental surface applications.
The OSCAR project intend to maintain and develop the use of satellite radar altimetry over continental surfaces. Here we show the processing chain that has been developed at LEGOS and which we use to qualify the ENVISAT and ERS data. The validation is based on a crossover analysis. We will show the steps of the validation process, the results on various parameters, like height, but also backscatter, leading edge and trailing edge of the waveforms. The validation process allows us to deliver reports, but also a validation table which is available on our website as well as other meta product. The validation table gives for each track of each cycle a validity flag and a qualifying flag giving the reason of rejection if necessary. The waveform parameters as well as the geophysical and instrumental corrections are checked during the validation process. Over ice caps further echo and geographic corrections are computed which allows surveying the altimetric accuracy with time, area, surface slope.
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.