# Inland Water Level Monitoring from Satellite Observations:
## 3 Review Results
### Waveform Retracking
#### 3.2.1 Novel and Improved Retracking Algorithms
It can be seen that the water levels obtained by MWaPP+ are more robust for both Sentinel-3 and CryoSat-2 than the other algorithms having the lowest std (0.25 m-0.30 m). Also, MWaPP+ is capable of processing poor quality complex waveforms obtaining robust water surface heights in contrast to MWaPP. In the following three publications, all novel or improved retracking algorithms proposed were evaluated using Sentinel-3 data. Initially, a new waveform portion selection method with consideration of the DEM was proposed together with the Threshold, OCOG, and two-step SAR physical-based retrackers [128; 129]. Although there was no significant difference in the results of the three retrackers, the use of the novel selection method greatly improved the accuracy of measurements. The RMSEs for different water bodies range between 0.17 m and 1.39 m, while all retrackers gave better results than the Level-2 ocean retracked data without the waveform portion selection method. Moreover, a new bimodal correction algorithm was proposed for the retrieval of inland water surface elevation, mainly over ice-covered lakes [130]. The four standard SRAL SAR retrackersused to retrieve water level measurements are Ice-sheet, SAMOSA-3, OCOG, and Sea-Ice [59; 131]. However, these retrackers generated erroneous measurements when inland water bodies were covered by ice, and therefore to address the negative measurements a new algorithm was proposed. The evaluation of this study shows that the Sea-Ice retracker fails to provide continuous measurements of water levels, while the accuracy of the other three retrackers is similar. The mean-RMSE over lakes is 0.12 m for OCOG, 0.11 m for SAMOSA-3, and 0.11 m for Ice-sheet. Also, correlation coefficients r, with ice-affected estimates to be excluded, are higher than 0.87 for Ice-Sheet, greater than 0.79 for OCOG, and higher than 0.87 for SAMOSA-3. Moreover, with the use of the new bimodal correction algorithm including the ice-affected estimates is shown that the accuracy of measurements over ice-covered lakes is extremely higher than without the use of the new method. Using the new algorithm over Great Slave Lake is shown that r = 0.93 and RMSE = 0.05 m instead of r = 0.13 and RMSE = 0.6 m using SAMOSA-3 retracker. As it can be seen the new method provides accurate continuous water level measurements over also ice-covered water bodies. Finally, in 2022 a physical retracking algorithm based on a priori numerical simulations was proposed for water level retrievals over lakes [132]. The novel retracker takes as input both the lake contour and the instrument characteristics. The methodology was inherited from previous developments [133]. They used in-situ data and OCOG retracker to evaluate the performance of the new retracker. The results indicated that the unbiased RMSE is better than 0.07 m over medium-sized lakes, while even over small lakes, the unbiased RMSE is better than 0.14 m showing that the proposed method retrieves water level measurements with high accuracy. On the other hand, for inland water bodies with very narrow widths of less than 200 m, the OCOG algorithm did not provide accurate results. The publications including the performance of novel or improved retracking algorithms are presented in Table 4.