# Preliminary Results of Ship Detection Technique by Wake Pattern Recognition in SAR Images
###### Abstract  
Recently, international agencies for border security ask for an improvement of the actual Maritime Situational Awareness. This manuscript presents preliminary results of a detection technique of go-fast boats, whose utilization in illegal affairs is strongly increasing. Their detection is very challenging since: (i) their echo is not visible in SAR images, and (ii) the illegal activities are carried out in the nighttime making useless the optical sensors. However, their wakes are very persistent and extent in SAR images for some kilometers. Hence, the manuscript shows an innovative deterministic methodology for the ship detection based on the wake signature. It firstly identifies pixels crossed by the wakes, whose presence is, then, validated in two steps. The first level of validation estimated how prominent the wake components are with respect to their background. The second level of validation exploits the presence of the wakes among neighbor pixels. The approach has been applied on ships imaged by TerraSAR-X mission showing the same peculiarities of go-fast boats. Results highlight the potentialities of the proposed approach, which can be also conceived as a subsequent step in a hybrid system, whose preliminary wake detection screening is carried out by different techniques. m aritime situational awareness; ship detection; SAR images; wake detection 20  
face high-speed operations, such vessels are planing hull forms. They usually have a length between 10 to 15 m, and they can reach speeds greater than 80 knots in calm water, and up to 50 knots in rough sea depending on their operational limits, being equipped with powerful engines delivering up to 1000 hp [10; 11]. Their hulls are usually made of fiberglass with a sharp, vertically rounded bow, hard chine and a transom stern in order to reduce resistance (hard chines and transom stern facilitate flow separation around the hull), and high values of the deadrise angle (\"deep-V hull\") to improve seakeeping and maneuverability performances. In addition, due to materials, size, and velocity, the ship echo could not be distinguishable. In fact, the faster the ship, the stronger the smearing and the defocusing in the SAR image are and residual ship signatures can become so weak that they are masked by sea clutter or image noise [13; 14]. Moreover, in order to complicate their detection, the European border and coast guard agency, Frontex [10], has confirmed that the illegal activities are typically carried out during the nighttime, making impossible the utilization of the optical sensors. However, the characteristics, making the go-fast boats so attractive, also have an interesting drawback. Thanks to their high velocity and narrow beam, the go-fast boats are likely to produce long visible wakes. Such wakes are very persistent, lasting for distances of several kilometers [8; 15; 16; 17], and they can be observed in Synthetic Aperture Radar (SAR) imagery. Hence, the core concept of the manuscript is that the go-fast boats can be detected in SAR images by using their wake as a signature of their presence. Ship wake in SAR images appears as a fan of bright linear components around a central dark line, representing the turbulent wake [17]. It is generated by the ship's propeller that dampens the surface waves, reflecting less energy back to the radar, and propagates along the ship longitudinal axis till 20 to 30 ship lengths at [18]. Two bright linear components are typically imaged next to the turbulent wake on both opposite sides. It is widely assumed that such bright components appear within 4\({}^{\circ}\) from the turbulent wake and they are classified as narrow-V wakes. The most external pair of wakes is representative of the divergent component of the Kelvin pattern, which is composed also of transverse waves. The latter ones travel mostly along the motion direction, which is aligned to the ship's main axis, whereas the divergent wakes appear as diagonal crests moving outwardly from the ship's location.