96 Forward-looking sonars (FLSs) are increasingly being adopted by UUV manufacturers, ROV pilots and boat crews with pole-mounted sensors for their ability to map underwater terrain ahead of the vehicle, while simultaneously detecting and avoiding obstacles. This capability is not something that other echo-sounder payloads such as side-scan sonars can achieve, and using a single sonar for both mapping and obstacle avoidance could save valuable space in UUVs. They also generate imagery at high refresh rates, function well in waters too cloudy for cameras, and they are being updated with SWaP-C improvements to enhance their appeal to marine uncrewed vehicle buyers working in bathymetry and similar applications. However, the advantages of FLSs have been historically offset by some key weaknesses in how their captured data is processed into actionable maps or ‘mosaics’. Normally, FLS mosaics are generated in one of two ways: either by taking the navigation data from the vehicle’s mission log and simply planting each captured acoustic image over the position at which it was supposedly taken, or by attempting to stitch each image together, based on common features at their borderlines, as with aerial photogrammetry. The first approach is limited by the accuracy of the UUV’s onboard positioning system. Given the total absence of GNSS underwater and the difficulty of using acoustic localisation in many places, UUVs must often rely on inertial sensors to gauge their position at each sonar pulse, and naturally, these will have some inherent inaccuracies (particularly as drift accumulates over time). Hence, the first approach can lead to inconsistent and blurry mosaics, which cannot be relied upon by end-users. Rory Jackson investigates how forward looking sonars are being used to capture data into accurate underwater maps through this company’s unique software Magic of mosaics June/July 2024 | Uncrewed Systems Technology Using IQUA Robotics’ SoundTiles software, forward-looking sonars’ acoustic data can be stitched into more accurate mosaics than ever before (Images courtesy of IQUA Robotics)
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