## 4 Discussion
### The Influence of Scene Geometry  
Our results revealed discrepancies in shoreline position from ASC and DESC tracks depending on the scene geometry at each site. For example, in Bull Island, where the topography is quite flat, the rates of change obtained from SAR ASC SLs and SAR DESC SLs were very similar, with a correlation of \(R^{2}>0.9\). On the other hand, in areas with complex relief and high features in the backshore, such as the cliffs at Start Bay, the discrepancies are evident, with rates of change obtained from DESC and ASC indicating either that a great part of the beach is close to no change (\(\pm 0.5\) m/year) or that half of the beach is under an erosion process. These differences are linked to the varying angles between the observed scene and the satellite's path during image acquisition along ASC or DESC tracks. During the ascending orbit (south to north), the SAR sensor faces east, while during the descending orbit (north to south), it faces west. This means that the SAR signal reaches the terrain with different incident angles according to the satellite orbit, which affects SAR-SL detection in scenes with complex geometries. The illustration in Figure 6 presents an example of the result of these differences for Start Bay. In sites where the terrain is flat and devoid of cliffs (Figure 6a), the SLs from ASC (Figure 6b) and DESC (Figure 6c) tracks exhibit congruence (Figure 6d). This is the case of Bull Island, where the change rates observed in SAR-SLs from both ASC and DESC orbits yielded similar results (\(R^{2}=0.97\)), and both were in accordance with change rates calculated from the vegetation line (DSAS) (\(R^{2}=0.90\) for ASC and \(R^{2}=0.92\) for DESC) (see also Figure 10). On the other hand, in places with complex geometry, such as cliffs and dunes (Figure 6e), the differences between the results from ASC and DESC increase (as shown in Figure 6h). This is the case of Start Bay. This beach is oriented to the east, and the SAR-SLs from the DESC orbit (Figure 6g) detect the cliff toe better than the ASC orbit (Figure 6f), as the latter is affected by the layover effect and results in SAR-SLs positioned more inland. A similar problem is observed in Salinas, and a noticeable displacement between ASC and DESC is observed due to the presence of dunes, although the effect of the ground geometry on the results is less pronounced (see Figure 13). In this case, SAR-SLs from the DESC orbit are located more inland than SAR-SLs from ASC. Salinas beach is oriented to the northwest, and during the DESC orbit, detection is performed from the land to the sea, whereas during the ASC orbit, detection is performed from the sea, with a tangential view of the beach and dunes. Although both ASC and DESC data align with the TWL (see Figure 14), the seasonal pattern is more pronounced in the time series from DESC, as SAR-SLs are positioned further inland due to the layover effect caused by the dunes' geometry. During periods of high TWL, SAR-SLs from DESC appear to experience a more significant retreat. In contrast, SAR-SLs from ASC are limited to the dune toe (around 150 m from the reference line). With regards to the SAR-SLs detected seawards, there are no significant differences between DESC and ASC shorelines, and the layover effect is negligible. It is remarkable that both ASC and DESC SLs in Salinas are located on ground higher than the high-water mark. More studies are necessary to identify the actual footprints that characterize the shoreline in SAR images in Salinas. Issues related to the scene geometry were also observed in previous studies based on SAR-SLs [47; 51]. Some of those studies recommend using only the SAR scenes with a shoreward view (from the sea to the continent) to avoid the scenes in which inland features obstacle the SAR signal and affect shoreline detection. However, this would reduce significantly the number of SAR-SLs detected. More studies on the interpretation the SAR signal and its relation to ground geometry are necessary.