# Mass Processing of Sentinel-1 Images for
## 4 Results and Discussion
### Recurrent Targets Analysis  
Table 3 presents the results of the recurrent target analysis. Of the 608,326 reliable detections returned by SUMO, 122,984 are found to be recurrent and 485,342 non-recurrent. The percentage of detections that show recurrence is thus 20%. Among the recurrent detections, 75% of them are classified as fixed structures and 25% as ambiguities. In terms of percentage of the total number of reliable detections, 15% of them are classified as fixed structures and 5% as ambiguities. These numbers demonstrate the high rate of occurrence of recurrent detections in the AoI: one in every five reliable detections is recurrent. Most of these recurrent targets are of no interest for maritime surveillance purposes: the ones classified as ambiguities are false alarms, and many of the detections classified as fixed structures are of little importance in most applications (e.g., oil and gas platforms, buoys, aquaculture equipment). The Mediterranean is a relatively small sea (compared to the world's  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Number** & **Fraction** & **Fraction** \\ \hline Ship detections & 667,746 & & \\ Detections with the lowest reliability level & 59,420 & & \\ Detections with higher reliability level & 608,326 & 100\% & \\ Interpolated reported positions & 818,493 & & 100\% \\ Detections correlated to interpolated reported positions & 366,081 & 60\% & 45\% \\ Uncorrelated detections & 242,245 & 40\% & \\ Uncorrelated interpolated reported positions & 452,412 & & 55\% \\ \hline \hline \end{tabular}
\end{table}
Table 2: Summary of ship detection and correlation results (excluding off Israel area).  
occans) completely surrounded by a basin that is highly populated and has many large ports and built up coastal sections. This explains to some extent the high frequency of recurrent detections: ambiguities (range and azimuth) are mostly seen near the coasts as they are primarily produced by coastal man-made structures, and many fixed structures are associated with ports (e.g., buoys, mooring points). On the other hand, the Med's coastline is very well mapped and the tides are of low amplitude. These factors reduce the number of land masking errors (e.g. significant changes between high and low tides, reefs exposed at low tide) that can result in recurrent false alarms. In a smaller scale study in the Western Indian Ocean [22], it was estimated that 12% of the reliable detections were recurrent. In that case, most of the recurrent detections happened in wetlands and reefs that were not correctly land masked. It is thought that few such land masking errors occur in the Med.  
Among the non-recurrent detections, 34% of them remain uncorrelated, while 66% are correlated to interpolated reported positions.  
Table 3 also shows the total number of the sources of recurrence (12,300), as well as the sources categorised as fixed structures (9565) and as ambiguities (2735). These numbers give an estimate of how many locations with fixed structures outside the OpenStreetMap used in SUMO and with ambiguities from strong land-based scatterers (not already flagged by SUMO) exist in the Med and can be detected in Sentinel-1 images.