# Preliminary Results of Ship Detection Technique by Wake Pattern Recognition in SAR Images
## 4 Discussion  
Further investigations are needed on the missed wake (Figure 8a). In Figure 9, which shows that direction of the turbulent wake in each grid point, two grid points are located on the missed wake and, in such pixels, the reconstructed wake direction is aligned with the turbulent wake. This allows us to affirm that the candidate wake identification is properly performed. However, the wake does not satisfy Equation (4) due to the very low values of merit indexes relative to narrow-V wake. Indeed, in wake \(a\), which is the most prominent of the image, the values of the turbulent and narrow-V wake are 0.57 and 0.15; instead, their values in wake \(c\) decrease until 0.39 and \(-\)0.04. This is due to the appearance of the wake \(c\), which shows bright narrow-V wakes only close to the ship location, whereas they appear very weak elsewhere and, in particular, where the grid points lie on the turbulent component. Figure 8: (**a**) Score logic approach applied over the full yellow circle in Figure 6. (**b**) Results of the second level of validation. As mentioned in the Introduction, wakes observability increases if [21; 38]: (i) the ship is large, (ii) the ship is fast, and (iii) the wind speed is low. Such parameters are also related each other. Indeed, it has been stated [21] that the wake is clearly distinguishable for high ship velocity along with low wind speed. In addition, such improvement increases with the ship length. Additionally, the looking direction of the radar beam can darken some components [38; 39]. From an operational user's perspective, the high speed reachable by the go-fast boats (up to 50 knots) can be considered as an important enhancing factor of the wake observability in SAR images. In fact, the ship velocity has a monotonic influence on the detectability: when the speed increases, the detectability increases. Furthermore, the ship speed is the parameter with the most influence. In [38], it has been shown that, when the ship velocity is about 23 knots (12 m/s), the probability of wake detection is: (a) larger than 80% no matter the course-over-ground and with ship length of 20 m, and (b) larger than about 70%, even with wind condition corresponding to 5 btf--fresh breeze (i.e., 17-21 knots on the Beaufort scale). Focusing on sea conditions only, assuming that flat or low waves in the sea are ideal both for completing illegal activities in the smallest time and for the wake detection, the auxiliary exploitation of meteorological data, to decide if/when the wake detection can be carried out, can strongly help both processing resources and successful rate. To investigate the operative utilization of the proposed detection technique, data availability and latency have to be discussed. Different issues shall be considered: (a) time of satellite passage, (b) revisit time of constellation, and (c) data latency. The nighttime observation capabilities of the SAR are indisputable, but the most exploited SAR satellites operate in dusk/dawn orbit and, practically, share the same line-of-nodes. The former one poses a serious obstacle to the monitoring of illegal activities during nighttime, and the latter one strongly limits the achievable revisit time, also considering the synergic exploitation of several constellations. A revisit time analysis of the COSMO/SkyMed constellation has been carried out in [40], showing that the Mediterranean area is recovered in about 12 h, which slightly reduces if the entire globe is considered thanks to the lower revisit time of the high latitude. However, an important trend in satellite remote sensing technology is towards system miniaturization, e.g., spaceborne SAR missions have been successfully deployed relying on very compact platforms, i.e., the Israeli TecSAR (300 kg launch mass) [41], the Indian RISAT-2 (300 kg launch mass) [42], and the Finnish ICEYE-X1 (70 kg launch mass) [43]. This is strongly fostering the realization of SAR satellite constellations based on  
Figure 9: Zoom around missed wake \(c\).