# Mass Processing of Sentinel-1 Images for
## 4 Results and Discussion
### Density Maps
Looking at the map of fixed structures, the only high concentrations of detections away from the coast are in the north Adriatic Sea, in the Gulf of Gabes (off the coast of Tunisia and Libya) and off Egypt, areas with intense offshore oil & gas activities, which often employ fixed platforms. The remaining detections are near the coast. After examination of a few of these coastal places, it has been concluded that many of these detections correspond to structures near ports (e.g., buoys) or to aquaculture facilities. However, busy anchor areas east of Malta and south of Istanbul also result in recurrent detections that are categorised as fixed structures. Most of these detections are real ships anchored at exactly the same position in multiple images. They are, therefore, not fixed structures. The map of ambiguities presents detections distributed fairly uniformly in an area extending up to 200 km off the coastline. The detections closest to the coast (e.g., near Istanbul) are expected to be azimuth ambiguities not flagged by SUMO. Some of these detections have been visually verified to correspond to the third or even forth order replica of very bright coastal targets, while SUMO only checks for the first and second replicas. Detections further away from the coast are likely to be range ambiguities of coastal or near-coastal cities. The most intense clusters of such ambiguities originate from the cities of Thessaloniki, Bologna, Rome and Barcelona (these cities are indicated in Figure 1). The density of the detections is higher in the northern part of the sea, which is the most populated and built up area of the Med basin. The density of detections is much lower in the less populated southern part. This corroborates the assumption that most of these detections are range ambiguities, since only man-made structures are generally strong enough to generate visible and detectable replicas. The maximum distance of these detections from the coastline also matches the theoretical distance of the range ambiguities in Sentinel-1 IW images. Spain's Mediterranean coast is a very clear example of this: the limit of the detections replicates almost perfectly the coastline at a distance of 150-200 km to the east. At the Med's latitudes, the range direction in the SAR images is approximately \(15^{\circ}\) off east-west. This has to be remembered when looking at the detections near Morocco and Algeria: even though they are very close to the coastline, they are still likely to be range ambiguities originating from coastal structures more than 100 km away to the west or to the east of the detection. Figure 12: Density map of non-recurrent ship detections (485,342). Area off Israel with high concentration of RFI-related detections is indicated as a white polygon. Finally, Figure 12 maps the non-recurrent detections. It can be interpreted as the difference between all the detections (Figure 8) and the recurrent detections (not shown, but very similar to Figure 11a) and gives a much better representation of the true shipping patterns than Figure 8. Figure 13 displays close up sections of Figure 12 south of Sicily and around Sardinia. Many shipping details are visible, related to traffic routes (north of Tunisia), off-shore supply (Gulf of Gabes), anchor area (east of Malta), and fishing activities (around Sardinia and Malta).