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

38 Synthetic aperture sonar Synthetic aperture sonar (SAS) depends to a large extent on high-performance digital signal processing. SAS combines a number of acoustic pings, often at different frequencies, to form an image with much higher resolution than conventional sonars, typically ten times higher. This higher resolution can also be traded for longer range at a lower resolution, and is largely used for military applications such as mine hunting. For example, a wideband SAS sonar with frequency range of 70-100 kHz can produce images with a resolution down to 4 cm. It can be tightly integrated with the AUV’s inertial navigation system and motion sensing platform, using modern signal processing such as displaced phase centre analysis to turn the raw data into images. A sonar’s azimuth resolution (that is, the width of ground ‘illuminated’ by the pings) is derived from the ratio between the acoustic wavelength and the length of the array. For typical sidescan sonars, this is of the order 1:60 to 1:400 (meaning a resolution of 1 m at 60 m and 1 m at 400 m). A longer array will increase the ratio, but using one is not always possible or practical on underwater vehicles. Operating at a higher frequency will increase the ratio, but will at the same time limit the achievable range owing to higher absorption of the sound waves by the water. SAS uses data from several consecutive pings to synthesise a longer sonar array, but faces a limit as the maximum range is proportional to the length of the receiving array, and inversely proportional to the AUV’s speed. An AUV can have two full-length receiving arrays on each side, forming two SAS images of the same scene with slightly different geometry. That adds considerable robustness to the system, and allows very high resolution from the data by making use of the phase information from the two different receivers. The signal processing algorithms are an integral part of an SAS system. One post-processing software package provides four elements, from navigation to rapid and high-quality imaging. The software can be used either inside the AUV or externally once the data is retrieved from the craft. Multi-aperture sonar Using multiple beams and receivers gives even more flexibility for the sonar system, but requires even more high- performance processing. The latest multi-beam sonar weighs less than 400 g for small AUVs and uses dual frequencies to provide flexibility between range and resolution. For example, a 256-beam, 375 kHz system provides a range from 20 cm to 200 m with an 8 mm resolution. The beams have a separation of 0.5 º , and provide a field of view of 130 º horizontally and 20 º vertically. Compare that with a 512-beam system which will have a range of 120 m at 750 kHz with a 4 mm resolution, improving to a resolution of just 2.5 mm at a range of 40 m using a 1.2 MHz signal. At its highest frequency, of 2.1 MHz, the 512 beams give the same 2.5 mm resolution but a range of 30 m. The advantage is a lower angular resolution (0.4 º rather than 0.6 º ) which allows the tighter resolution across a smaller area for more accurate seabed mapping. By contrast, the best angular resolution from a sidescan sonar array is 0.15 º . Acoustic comms Communicating across oceans relies on technology similar to sonar but implemented in very different ways. NATO’s Science and Technology Organization’s Centre for Maritime Research and Experimentation has adopted a standard for underwater acoustic comms called Janus, which is recognised as a NATO standard. It is the first international digital underwater comms protocol, and opens the way to developing new underwater comms applications. Janus is a digital underwater signalling system that can be used to contact underwater devices using a common August/September 2017 | Unmanned Systems Technology This multi-beam sidescan sonar towfish uses dynamic beam steering image stabilisation to remove artefacts in the picture caused by tow vehicle motion (Courtesy of Klein Marine Systems)