Water bodies represent important economic and cultural resources but also much economic activity and development take place close to the shorelines of lakes and can be adversely affected by flooding [Rapley & al., 1987]. Moreover lake volumes respond to changes in precipitation integrated over their catchment basins and so can act as important, though indirect indicators of climate change on both regional and global scales. Major river systems are important targets of research covering a wide range of applications such as transport, flooding hazard, water and food resource management, studies of the hydrological cycle, and addressing the impact of land use and climate change [Leopoldo & al., 1985].

For certain major rivers and wetlands, hydrological information can often be difficult to obtain due to the inaccessibility of the region, the sparse distribution of gauge stations, or the slow dissemination of data. Satellite radar altimeters have the potential to monitor height variation over inland waters [Birkett, 1998]. Hydrological products from satellites avoid political and logistical problems and can give an accurate height measurement not only for lakes but also for large rivers such as the Amazon, which has been a primary target of study over the last 10 years.

Since 1982, research into the application of altimetry for monitoring river and lake levels has been carried out. This highlighted the advantages of using data derived from satellites due to the global coverage and regular temporal sampling of the processed data, but also identified the difficulties in interpreting radar altimeter measurements made over inland water.

The first reason for studying altimetry over lakes was in fact to validate altimeter measurements. Evaluating an altimeter system over lakes has a number of advantages. Lakes have minimal tides and little dynamic variability in comparison to the oceans; thus the spatial change in lake level closely follows the geoid. Many lakes are monitored so that changes in lake levels can be accounted for in the evaluation [Morris & Gill, 1994]. The first zone of application of this new method to validate instruments was over the American Great Lakes with the Seasat [Brooks, 1982] and TOPEX/Poseidon [Morris & Gill, 1994] altimeters. From the results of these studies the great potential of altimetry to monitor inland water levels rapidly became apparent. This has been applied several times not only over the American Great Lakes [Morris & al., 1994] but also over the Caspian Sea [Cazenave & al., 1997], over East African lakes [Birkett & al., 1999] and the largest rivers including the Amazon [Koblinsky & al., 1993, Mercier & al.,2001, De Oliveira Campos & al., 2002]. Recently, the potential for generating river and lake heights on a global scale by reprocessing the individual altimeter echoes was demonstrated [Berry, 2002].

In general, the great improvement of altimeter measurement accuracy over the past decade has been due to the enhancement of the altimeter instrumentation together with substantial improvements in the precision of satellite orbit calculation. Furthermore, satellite altimetry coverage over land surfaces has been greatly improved due to the inclusion by ESA of additional tracking modes on the ERS and Envisat altimeters, which enable the instruments to track rapidly changing surfaces. This has led to substantial advances in altimeter research over ice, land and inland water.

Nowadays there are several teams in the world involved in satellite altimetry over inland water, including NASA (National Aeronautics and Space Administration) with C. Birkett, and CNES (Centre National d'Etude Spatiales) with A. Cazenave and DMU (De Montfort University) with P. Berry. At DMU, the understanding of land surface response to the altimeter pulse has been researched, and this has allowed characterisation of the relationship between the underlying terrain and the individual echo shapes.

Recognizing the past achievements and current improvements in this field, ESA launched a development, awarded to DMU, of an easy-to-use and accurate product to monitor River and Lake levels.