Uncrewed Systems Technology 049 - April/May 2023

100 Focus | Sonar systems systems on crewed survey vessels might have been limited to sliders preset to choose figures such as 210, 220, 230 kHz and so on. That feature can be less important for UUVs, as they are not constrained to the ocean surface as USVs are. However, USVs remain a less expensive option for many users, and also come with operator benefits such as real-time monitoring and analysis of survey data as well as in- the-loop control if needed. More interesting to UUVs are other AI- powered abilities intended to make better use of their close-up survey capabilities, such as automatic pipe detection and tracking. Advances in machine learning (ML) have accelerated the development of algorithms for UUV computers to recognise structures such as oil pipelines or similar structures that need regular high-resolution inspections in their 2D and 3D sonar imagery. In addition, recognising a pipeline enables onboard processors to track along its length. While it might be assumed that surveying a pipeline is a simple matter of plugging waypoints with their approximate depths and coordinates into a UUV’s GCS planner, it actually leaves such surveys as open- loop operations with no real-time checking of whether the UUV is surveying the way its owner wants it to. In reality, the acoustic waves from a sonar suffer reflections or distortions upon contact with many pipe surfaces, owing to the metals, concrete, plastics and resins used in their construction, which vary with the pipe’s inclines, twists or other directional and shape changes along its length. That introduces unclear details about pipeline inspection data, with sections of the pipe seeming to show blurring or otherwise erroneous imagery such as areas that are actually above, below or further along the pipe than the section being viewed. One last area worth considering is the active stabilisation of sonars. Much as UAV camera gimbals need to be able to correct for vibration and movements of their platforms, sonars – particularly on large USVs – will be swayed to and fro by waves and currents, which will have an impact on how beam densities are distributed over survey targets if they are not corrected for. Wind farm inspections in particular tend to have very strict survey specifications, often with a set number of ‘hits per cell’ or points per grid square of the area to be surveyed. If the action of waves is causing a vessel to heave, roll, pitch and yaw, the hits will naturally be excessive in some cells and insufficient in others. Roll stabilisation is nothing new: when the INS reports that its USV has rolled 5o to port, the sonar can be commanded to alter its angle by 5o to starboard. Only more recently however have pitch and yaw stabilisation also been tackled by sonar developers, with greater understanding of how the transmit and receive arrays must be controlled in order to mitigate the effects of weather bucking the vessel in these axes. Forward-looking sonar The use of sonars in a forward-looking capacity, rather than the downward- looking integrations typical of bathymetric surveys, is traditionally tied more closely to autonomy and AI than actual mapping work, as their intended use case has been to scan ahead so that a USV or UUV can clearly see rocks, reefs, or other seabed features that represent a collision hazard. Naturally, this is more important for UUVs, as they operate closer to the seafloor than USVs, and are thus at greater risk of being lost through collisions with such obstacles. Nowadays however, most forward-looking sonars are less powerful and have a smaller form factor than those used in bathymetry. Highly SWaP-optimised imaging sonars are used increasingly in 100-200 kg UUVs and 3-25 kg micro-AUVs alike, to enable real-time forward obstacle detection and avoidance as well as a limited degree of mapping in the water column in front of the UUV, particularly as it dives down to the seabed. That can provide actionable data for some users, on top of enabling the UUV to reactively plan out a survey or avoidance path of the seafloor ahead. Imaging sonars used in this way are April/May 2023 | Uncrewed Systems Technology Highly SWaP-optimised imaging sonars are increasingly used as forward-looking obstacle avoidance sensors in UUVs (Courtesy of Impact Subsea)