ENVISAT radar altimeter measurements over continental surfaces and ice caps using the ICE-2 retracking algorithm

This study presents the first results obtained with the ENVISAT satellite radar altimeter RA-2 over lands and ice caps. As radar altimetry was firstly devoted to ocean study, this paper starts with a detailed description of the radar altimetry principle over continental surfaces: a general formulation of the radar altimeter echoes according to different surface types is given and the retracking procedure ICE-2, specially implemented on the ENVISAT satellite ground segment to provide output parameters over continents, is clearly presented. The ENVISAT RA-2 datasets processed with the ICE-2 procedure are then mapped given an overall description of the output parameters over various surfaces. The behaviour of these ENVISAT altimetric parameters are analysed and related to main land surface characteristics such as surface/volume echo, specularity, roughness.... Apart from few unexpected signals which will need to be further investigated, the new ENVISAT dataset confirms the potential of inland altimetry and provides a new opportunity to monitor continental surfaces. The Ku band measurements confirm previous results recently obtained with the altimeters on board ERS or Topex-Poseidon and offer a temporal continuity. The S band altimeter measurements are in accordance with theoretical expected ones and confirm previous Topex-Poseidon observations in C band. The S band and the use of the dual-frequency thus bring new capabilities for altimetric applications with important deliveries for temporal evolution survey. For instance, S band will be used over continents to better detect wetlands located under vegetated areas or to better quantify wave penetration in terrestrial snowpack to accurately retrieve snow depth evolution. Over ice sheets, the dual-frequency S and Ku band waveforms parameters will provide a more accurate height estimation as it will be possible to quantify the error on height measurements induced by snowpack structure change over time. This will then provide a more confident survey of the ice sheets surface topography evolution. The combination and complementarities with the other altimetry missions such as ERS, Topex-Poseidon and Jason look already promising and offer a 10-year span dataset for continental altimetry applications. (c) 2005 Elsevier Inc. All rights reserved.