# Mass Processing of Sentinel-1 Images for
## 4 Results and Discussion  
### Sentinel-1 Coverage of the Mediterranean Sea  
An analysis of the Sentinel-1 coverage in the AoI is presented next. To show the full monitoring capabilities of the Sentinel-1 constellation, a time window from 29 November 2016 to 9 February 2017 (i.e., 72 days, six complete orbit cycles) was chosen for this analysis. During that window, both Sentinel-1A and Sentinel-1B were probably at full capacity and operating on an acquisition plan likely to be maintained in the future. Figure 5 displays the average revisit time (in days) during that time window for each geographic location when: (a) only Sentinel-1A acquisitions are considered; and (b) both Sentinel-1A and Sentinel-1B images are used. This figure describes more accurately the expected Sentinel-1 monitoring capabilities in the AoI than Figure 2. This is so because Figure 2 includes information from the Sentinel-1A production ramp-up phase, when the satellite was not yet operating at full capacity and modes EW and SM were being sometimes used. It has to be pointed out that the images in the 29 November 2016 to 9 February 2017 window were only used for this coverage analysis and were not analysed in the maritime surveillance study, which spans from 3 October 2014 to 30 September 2016. In addition, as in Section 2.2, images with less than 10% overage over sea were not included.  
Figure 5 shows how the average revisit time halves when both satellites are included in the analysis. Common average observation periodicities for the two-unit constellation are 1.5 days (areas in deep red colour in Figure 5b, where two ascending and two descending swaths of each satellite overlap in each 12-day orbit cycle), two days (in light red, three swaths of each satellite overlap), three days (in orange, two swaths of each satellite overlap) and four days (in light orange, imaged in one swath of Sentinel-1A and in two of Sentinel-1B). Common average observation periodicities of Sentinel-1A alone are three days (areas in orange in Figure 5a, where two ascending and two descending swaths of Sentinel-1A overlap in each 12-day orbit cycle), four days (in light orange, three swaths overlap), six days (in light green, two swaths overlap) and 12 days (in green, imaged in one  
Figure 4: Block diagram of the recurrent target analysis framework. When a new image is presented (bottom left), ship detection is applied (block A) and the relevant recurrent targets are selected (B) and removed (C), leaving only reliable detections (bottom right). All the detections are used to populate the recurrent targets DB (D), which are classified (E) in fixed structures (F) and ambiguities (G).  
swath only). The northern part of the Med is imaged more often, while the sensors remain frequently off (especially Sentinel-1A) over the southeastern part of the Med. It has to be remembered that, at the Med Sea latitudes, all the acquisitions of the Sentinel-1 satellites take place at around 6:00 a.m. or 6:00 p.m. local time.  
Figure 6 plots the daily number of acquisitions used in the maritime surveillance study and the area coverage (time frame from 3 October 2014 to 30 September 2016, only Sentinel-1A). An increase in the number of acquisitions and the stabilisation of the observation plan is seen as the Sentinel-1A operations went through the ramp-up phase. Gaps are observed (e.g., in January 2015 and June 2016) when the satellite was unavailable. Since the second half of July 2016, the operations have stabilised in a 12-day periodic pattern, with 17-24 daily acquisitions. Using an average image size of \(250\times 170\) km\({}^{2}\), the area covered ranges from 700,000 to 1,000,000 km\({}^{2}\) per day; this is, however, a small overestimate of the sea area covered since some of the images contain land parts.