## 6 Conclusions  
In this paper, an innovative procedure for estimating the water flow elevation of rivers is proposed. Today, a shared and worldwide database containing historical and reliable data concerning the water surface elevation of rivers has not been completed. Many areas are still unmonitored and due to the large importance of fresh water, an evaluation of the river system is fundamental. The main scope of this research is contributing to solving this information-gap problem by designing a SAR signal  
Figure 16: Scatterplot of the water levels estimated by satellite and observed by in-situ stations for Po at Pontelagoscuro (**left**), Tiber at Ponte Nuovo (**center**) and Tiber at Ripetta (**right**).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**STATION** & **R** & **NS** & **RMSE [m]** & **RRMSE** \\ \hline Pontelagoscuro & 0.88 & 0.77 & 0.91 & 0.10 \\ Ponte Nuovo & 0.65 & \(-\)0.03 & 0.68 & 0.39 \\ Ripetta & 0.93 & 0.85 & 0.55 & 0.21 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Performance indicators of the simulated water levels versus the ground-based observations.  
processing technique having the capability to perform water flow level estimation. The problem of measuring such data is usually solved by designing an ATI SAR geometry, which is constituted by two radars spatially distanced by a baseline extended in the azimuth direction. In the case of space-borne missions the performing of ATI can be an unusual and difficult task. For several single-antenna spaceborne SAR satellite systems, the refocusing of ATI observations from one raw data is a problem because of the not-oversampled nature of the received electromagnetic bursts. This phenomenon makes raw data very similar to a white random process and appearing interlaced Doppler bands completely disjoint. After the range-Doppler focusing process, this problem causes decorrelation when observing the ATI phase of distributed targets. Spaceborne LOS level measurements could be taken into consideration only for small and very coherent targets and in any case were located within the same radar resolution cell. This paper proposed the application of a robust technique for tracking the double-bounce reflections of some principal bridges crossing the rivers and to measure the gap space existing between the river surface and the bridges. The developed algorithm tracked over time the double-bounce scattering event position reflected on the river surface with respect to the single-bounce and direct backscattered echoes form the principal structures of the bridge. River water-flow data were indirectly retrieved by converting the time-domain water surface variation in velocities. The experiments were evaluated by processing simulated and a long temporal series of COSMO-SkyMed data.  
Conceptualization, F.B., A.T., P.A., C.C. and D.O.; Data curation, F.B. and A.T.; Formal analysis, F.B., A.T., P.A., C.C. and D.O.; Funding acquisition, F.B.; Investigation, F.B. A.T., P.A., C.C. and D.O.; Methodology, F.B., A.T., P.A., C.C. and D.O.; Resources, F.B., A.T., P.A. and C.C.; Software, F.B.; Supervision, F.B. and D.O.; Validation, F.B. and D.O.; Visualization, F.B.; Writing original draft, F.B. and D.O.; Writing review & editing, F.B., A.T., P.A., C.C. and D.O.  
This research received no external funding.  
The authors declare no conflict of interest.