Uncrewed Systems Technology 049 - April/May 2023

104 Focus | Sonar systems capabilities into marine autopilots (or sonar computers) so that uncrewed sonar platforms can zero in on items that are of interest to their operators. However, open-access sonar data libraries for training algorithms to enable such intelligence are scarce, so it falls to vehicle and sonar companies to amass and organise such data themselves in order to gain that next-level edge in forward- looking survey and inspection. Sonar hardware On top of these software enhancements, echo sounder architectures are improving to the point that beamwidths as fine as 0.5o are commercially available in systems that are SWaP-optimised enough for integration into micro-AUVs and small USVs. There is a demand for such ultra-high spatial resolutions frommilitary as well commercial operators, although which customers prefer 0.5o beamwidths over, say, 1o or 2o systems is unclear. In essence, however, the primary consequence of this higher resolution is an improved capability of target detection, by being able to distinguish between two or more objects at the 0.5o level. For asset owners who need to inspect their pipelines, wind farm foundations and other critical infrastructure, such improvements in spatial resolution are vital to identifying microfractures, joint wear and similar signs of wear in potentially murky environments, particularly where biofouling and shadows could render even highly illuminated camera imagery and underwater Lidar effectively useless for spotting where maintenance and repairs are needed most. Transducer design is a critical part of the continuing refinement of sonar hardware, as they are the systems by which electrical signals are transformed and emitted as sound waves, making them in many ways one half of the operative part of a sonar. Advances in transducer engineering are particularly important for MBEs, which need transducers capable of ever-higher power levels and which can be precisely directed. The latter entails building a transducer to transmit pulses in a stable manner, so if the transmission is not robust enough to withstand being altered over ranges of temperature and pressure, it will inevitably fall short of the requirements of UUV and USV customers who increasingly want to be able to operate in environmental extremes such as 6000m below sea level or in polar climates. Transducers are only half of the equation for sonar mechanical design though; hydrophones are the other. As a hydrophone is essentially an underwater microphone, optimising it for sensitivity is the key to having it detect the specific sound waves and frequencies from the transducer, without interference from electrical or other sources disturbing the information in the received waves. Some modern hydrophones can detect sounds that are quieter than those generated by a sea state of zero, where only thermal ‘noise’ (from Brownian motion of seawater particles) is present, as well as being able to detect across very broad bandwidths. Some for instance can be used to detect sound waves from just a few Hertz up to 100 kHz, while others can detect at more than 1 MHz. Although these two main components form the transmit and receive arrays of sonars, not all sonars are built as two- array systems. Some modern systems are able to use hydrophones to transmit as well as receive sound, while some more powerful architectures are increasingly key to enabling high-power transmissions while still sensing high densities of low- power wave returns on the receive side. In addition to the core subsystems essential to any functional sonar, April/May 2023 | Uncrewed Systems Technology Testing and improving sonars for longer MTBFs will be critical as UUVs are increasingly adopted as resident systems for subsea infrastructure maintenance (Courtesy of Norbit) Some hydrophones can detect sounds that are quieter than in a sea state of zero, where only noise from Brownian motion of seawater particles is present