Unmanned Systems Technology 015 | Martin UAV V-Bat | William Sachiti | Sonar Systems | USVs | Desert Aircraft DA150 EFI | SeaCat AUV/ROV | Gimbals

35 per second to build up a picture. This was originally developed in the 1940s for submarines and ships. It has its limitations though, particularly when the hull of a boat blocks the sonar signal and creates interference from multi-path signal reflections, so sidescan sonar was developed. Here, the transmitter is housed in a craft towed by a boat (called a towfish) and scans the seabed. The return signals are recorded, processed in real- time and stacked, creating an image of the seabed and any other objects within range of the pings. Having the transmitters and receivers on a tow also allows the sonar to be closer to the seabed, to provide higher resolution in shallow waters where multi-path reflections can again create interference and reduce image quality. This is seeing a resurgence as unmanned surface vessels (USVs) can use it for both shallow- and deep- water applications. That is because of the lower cost compared with synthetic aperture and multi-beam systems, and improvements in digital signal processing hardware and software that produce imagery from it at a higher resolution image than before. It also allows a larger sonar array to be used than can be fitted directly onto an AUV. Using a larger array on a towfish gives higher quality images than an AUV for the same single-beam technology. A USV with a towfish successfully completed sea trials during the first quarter of 2017 at USV speeds of up to 10 knots and various depths for the towfish, and with towed sidescan sonar (TSSS) arrays of various sizes, to provide high- resolution seabed images. Using a TSSS also allows the sonar to be fully remotely monitored from a ground station in real time through the USV’s radio link. That avoids the problems faced by an AUV that has to use an underwater comms system (which is low bandwidth) or storing the sonar data to be relayed when the craft surfaces. However, the towing cable limits the depth to which the towfish can go, and while that is not a problem for surveying the seabed in shallow waters, a higher performance sonar array may be needed to reach the seabed for mapping applications. For application at depths of more than about 500 m, an AUV becomes more appropriate, given the performance limitations of a smaller single-beam array or the cost implications of using a higher performance technology such as synthetic aperture or multi-beam sonar. This is where the trade-off between the type of sonar technology and the type of unmanned craft becomes important. A towfish with a single-beam sidescan sonar will typically include an auto- altitude mode to keep the sonar at a constant height above the seabed, as well as automatic obstacle avoidance to avoid having to use a complex control loop back to the USV controller. Using internal sensors coupled with an ultra-short baseline (USBL) positioning system, the position of the TSSS can be controlled in real time, and the sonar data stamped with the time and the location (geo-localised). The USBL uses an acoustic transceiver under a ship, and a responder on the towfish. The transceiver head normally contains three or more transducers separated by a baseline of 10 cm Sonar systems | Focus Unmanned Systems Technology | August/September 2017 The Oculus M multi-beam sonar systems weigh as little as 400 g (Courtesy of Blueprint) Sonar survey results of the Holmengraa, a tanker that sank off the coast of Norway in 1944 (Courtesy of Kongsberg Maritime)