## 4 Results  
### Selecting the Waveform Portion Using a DEM  
When a waveform is influenced by land-based contamination, the waveform will contain small peaks due to off-nadir land or land coverage that follow the large peak, which corresponds to the water reflection within the altimeter footprint. In this case, as shown in Figure 5 (two outstanding peaks in Figure 5 waveform), the portion of the selected waveform provided a more precise tracking point for the two-step physical-based retracker and OCOG retracker, especially for the OCOG retracker, whose retracking point was the front edge of the COG window. The footprint of the signal was very close to the shore, thereby introducing a large peak at the trailing edge. The DEM information was used to select the portion of the waveform closer to nadir. The closest peak and valley after nadir were selected for retracking. Figure 5a,c shows the location of nadir in a black vertical line. The portion being selected is under the SWH fitting or MSS fitting line in Figure 5a with the two-step physical-based retracker, and is marked out in red crosses as the L1B-Waveform portion in Figure 5c with OCOG retracker. For the two-step physical-based retracker, the fitting using the selected waveform portion (Figure 5a) resulted in a 99% accuracy, while the fitting using the whole waveform (Figure 5b) resulted in an accuracy of 83%, making the location of the tracking point on the leading edge slightly different. In this study, the objective of the two-step physical-based retracker was to ensure proper retrieval of the surface height over inland waters, when exploiting the SAR ocean retracker in [65] with minimal changes on its operation; the significance and interpretation of other geophysical parameters that  
Figure 4: Overview of the workflow.  
can be inferred (SWH and MSS) needs to be further investigated and it is out of the scope of the current study.  
For the OCOG retracker, the COG when using the selected waveform portion was located inside the first peak. However, when using the whole waveform, it was located in between peaks, resulting in different tracking points that were the front edge of the COG window. For the threshold retracker, the tracking point did not change in this case. The threshold retracker depended only on the largest peak, which, as shown in Figure 5, was the same with or without waveform portion selection. However, in the case of the largest peak not being from the nadir water body but from the surroundings or other objectives, the threshold retracker with and without waveform portion selection led to different results.  
The differences between the performance using the waveform portion selection method, and the method using the whole waveform, are shown in the time series results over the Ebro Reservoir in Figure 6. The Level-2 data were obtained from the Sentinel-3 Level-2 product using the ocean retracker, which does not include any waveform portion selection. The waveform portion selection method eliminated most outliers compared with the method of using the whole waveform.  
Figure 5: Comparison of the waveform portion selection method with the method using the whole waveform for the two-step physical-based retracker and OCOG retracker: (**a**) two-step physical-based retracker with the selected waveform portion; (**b**) two-step physical-based retracker with the whole waveform; (**c**) OCOG retracker with the selected waveform portion; and (**d**) OCOG retracker with the whole waveform. Red stars mark the selected portion of the waveform. r (1 st) is the correlation coefficient for the SWH fit with the original waveform, and r (2 nd) is the correlation coefficient for the MSS fit.