## 4 Results
### Extended Mask Assessment  
This subsection analyzes the results obtained from the extended water mask building algorithm and the benefits of including across-track measurements from extended water masks compared to just selecting nadir measurements. Figure 8 shows the resulting water masks built for all the reservoirs under study. Comparing the water occurrence of all figures, it can be observed that the presence of water is much less frequent in Itoiz, especially at the transect with the satellite's nominal track. The transect in Itoiz is divided into two small crossings 1.7 km apart, each narrower than 100 m. The southern crossing can disappear during seasons of low WL\({}_{t}\)s in the reservoir. Regarding the quantification of the increase in the number of points provided by across-track measurements, Table 3 shows the average number of measurements obtained over all reservoirs for FFSAR and UFSAR. It also displays the difference in the number of measurements between the proposed extended mask and a strict mask, which only considers nadir measurements. The average relative increase in measurements as a consequence of the extended mask is 36.6% for UFSAR and 47.7% for FFSAR. During the study period, both satellite tracks, 111 and 248, drifted up to 1.5 km to the east. This is highly beneficial for the Mequinensa-Caspe target because, due to the sinuous contour of the reservoir, eastward drifts from the nominal track can lead to a seven-fold increase in the transect length. In contrast, the transect of Itoiz was reduced due to the eastward drift (Figure 8). Regarding the accuracy of across-track and nadir measurements, the graph in the middle of Figure 9 shows the error of both types of measurements for all observed FFSAR-derived WL\({}_{t}\)s before filtering, confirming the convenience of considering such immediately cross-track measurements as the errors reported are in line with the nadir measurements. The bottom plot of Figure 9 presents the same analysis for UFSAR-derived WL\({}_{t}\)s using the OCOG retracker. From these two figures, we can make the following observations:  
* For FFSAR-derived WL\({}_{t}\)s, an average of 46.4% of nadir measurements and 51.5% of across-track measurements had an error below 10 cm. For UFSAR-derived WL\({}_{t}\)s, an average of 61.0% of nadir and 57.8% of across-track measurements had an error below 30 cm. These results indicate that, despite the introduced bias of across-track measurements, their accuracy is very similar to that of nadir measurements. * A threshold of 25% of UFSAR measurements with an error below 20 cm was not reached in any reservoir, for either nadir or across-track measurements. This explains why achieving an MAD below 20 cm in these targets is challenging. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Averaged Num. of Samples Per Pass**} & \multicolumn{3}{c}{**Relative Increase (\%)**} \\
**Target Name** & \begin{tabular}{c} **Strict Water Mask** \\ **UFSAR** \\ \end{tabular} & \
\begin{tabular}{c} **Extended Water Mask** \\ **FFSAR** \\ \end{tabular} &
\begin{tabular}{c} **Relative Increase (\%)** \\ **UFSAR** \\ \end{tabular} \\ \hline Yesa & 8.9 & 518.2 & 10.1 & 578.9 & 13.5 & 11.7 \\ Itoiz & 0.5 & 23.2 & 0.9 & 52.5 & 80.0 & 126.3 \\ Meq. North & 2.9 & 171 & 4.1 & 241 & 41.4 & 40.9 \\ Meq. South & 6.9 & 407.4 & 8.7 & 511.6 & 26.1 & 25.6 \\ Meq. Caspe & 7.8 & 453.2 & 10.3 & 606.7 & 32.1 & 33.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Averaged number of measurements obtained at the along-track resolution of FFSAR and UFSAR using two different water masks. The former water mask only includes water pixels whose water occurrence was above 25% and falls below the satellite’s nadir. The latter mask includes across-track measurements, as long as there are water pixels closer than 250 m across-track, as explained in Section 3.4. * When considering nadir and across-track measurements together, approximately 73.8% of FFSAR measurements had an error below 20 cm, in contrast to only 9.6% for UFSAR.