# Envisat RA-2 Individual Echoes: A Unique Dataset for a Better Understanding of Inland Water
## 5 Discussion and Concluding Remarks  
Envisat collected 1-s bursts of 1984 \(\times\) 128 range bins of IE complex echo returns every 3 min. The available IE data cover all sorts of water targets, including open ocean, coastal zone and inland water. A systematic study of global echoes, although not yet exhaustive, shows a generalized specular behavior in most small and medium inland water targets across the world. This paper illustrates the expected electromagnetic behavior of inland water targets by exploiting the classical radar cross-section (RCS) theory for specular surfaces. Results coming from the theory are compared with those from real IE data, collected by the Envisat satellite in three selected different case studies regarding inland water targets chosen in different contexts. Over the floodplain of Prek Toal, the Envisat altimeter behaved reasonably well. The most interesting result was the capability to identify a gradient during full flood. In fact, the untracked radargrams showed the expected steady sloping valley floor in the northern direction, with a noticeable difference in amplitude between the dry season and the flood season. The Prek Toal case demonstrates the use of specular echoes to measure levels of flood areas as well as appreciate small geoidal corrections as a bonus. The two case studies (Sections 4.2 and 4.3) involving rivers demonstrate the potential of specular echoes to measure river levels, but also point out that this is possible only for rivers at or very close to nadir. The waveforms collected in the Lake Edwards vary from weak to moderate Brownian, while the river Semliki is always specular. Rio Nahuapa is a much narrower river than Semliki, and the satellite track makes multiple crossings of the river, providing new insights into the detectability of small rivers. The results are promising because they demonstrate the feasibility of using satellite radar altimetry in rivers much smaller than what was considered possible until now. The echo power is accurately described by the RCS model. The model very closely matches the data. The width of the main lobe, the position of the side-lobes, and the 20 dB difference between main and side lobes, are all consistent with the data. These findings indicate that most rivers can be modeled with the RCS of a specular surface. The most interesting results of this work can be resumed as follows: (1) specular echoes are coherent for many independent echoes; (2) specular echoes have a well defined Doppler evolution and can be SAR processed for water level estimation, avoiding any classical retracking; and (3) there is a relationship between target size and lobe size, with very interesting implications to the determination of an optimal burst length, which is an important point for further studies. The advantage of IEs is that the averaging can be precisely centered on narrow water bodies and can be coherently (zero Doppler) averaged. Coherent averaging further increases the accuracy of the level estimation. The Individual Echoes produced by the Envisat RA-2 platform represent the only currently available dataset for the experimentation of continuous coherent processing on real data, making this dataset extremely important and under-exploited until now. Satellite and radar configurations (PRF, orbit parameters, etc.) can exploit positively the peculiarities of specular echoes, and serve as a basis for the development of future missions. The results of this work may also support a more effective use of the currently available data, in the prospect of building a global inland water level monitoring by satellite observations, inclusive of small size water bodies. **Acknowledgments:** The analysis of Envisat IE data has been inspired by the ESA funded COASTALT project that was aimed at exploiting Envisat altimeter data in the coastal zone. The help of several individuals is graciously acknowledged. [PERSON] of Eumetsat and [PERSON] of the European Space Agency made possible the access to the IE archive.