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

34 A lthough sonar systems have been in use for many years, by surface as well as sub-surface craft, the growing use of autonomous underwater vehicles (AUVs) is spurring fresh advances in the technology. There are several types of sonar, with different performances, sizes, weights and power capabilities covering a range of applications. Sonar uses sound waves in the 18 kHz to 2 MHz frequency range to detect objects underwater and map the sea floor. Each audio pulse, or ping, from the transmitter is reflected by an object or the seabed, and picked up by a receiver, which calculates the distance to the object. Successive pings build up a picture of the target area using signal processing algorithms. This provides a greater range than visible-light cameras, up to 200 m for sonar compared with 20 m for cameras, which of course need additional lighting at depth anyway. With AUVs travelling the world’s oceans independently there is also an increased need for underwater communications technology to avoid the need for the craft to surface in order to communicate or update its position. There is a fundamental difference between low- frequency techniques that can travel long distances underwater but carry very little data, and higher frequency systems carrying usable data. This is leading to a greater focus on developing the sensors to enable new services. It is a general long-term migration by many technology providers away from merely selling products. Developing the sensors is becoming just as important as the AUV platform that carries them, as operators look to provide high-resolution imagery by the hour via what is known as Robotics as a Service (RaaS). Here, instead of selling hardware, pure-play RaaS companies will build, own, operate and maintain AUV fleets so that customers do not have to buy and maintain their own craft. In this way the AUV developers become service providers, with a fleet of craft that can be sent to a location to provide the required data, rather than relying on a remotely operated vehicle (ROV) with a support ship. That combination can cost more than $150,000 a day to operate, and the ROV umbilical and tether management in harsh ocean environments can create hazardous conditions for both shipboard and subsea equipment. However, as well as autonomous operation for the AUVs, this new model will rely on more sophisticated sonar for collision detection and to provide high- resolution imaging and new underwater comms technologies. Single-beam scanning Single-beam scanning sonar is the most basic system, with a single transmitter scanning a wide field of view in front or to the side of an AUV, typically 10-30 º . When it picks up a return signal, the field of view is reduced to produce more pings Nick Flaherty reports on new developments in AUV sonar and underwater comms technologies for surveying and mapping Sound and vision August/September 2017 | Unmanned Systems Technology A towed Synthetic Aperture Sonar system with built-in stabilisation (Courtesy of Klein Marine Systems)