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36 O ver the past two years, a score of transnational and environmental concerns have driven calls for UUVs to operate for longer stretches and in deeper waters than ever before. Companies in the sector are starting to provide capabilities for applications such as rapidly locating airline wreckage, surveying seabed resources (potentially in disputed waters) and inspecting critical assets such as subsea cables and pipelines, at depths down to 6000 m. With companies such as Boeing and Cellula developing UUVs to carry out missions lasting months at a time, the safety and efficiency of UUVs depends on enhanced autonomy. Acoustic systems must therefore generate imaging and navigation data more rapidly, accurately and at greater ranges than ever. As a result of these trends, sonars and other acoustic systems have seen a range of technological leaps and shifts in the past few years. Synthetic aperture sonars The nature of sonar imaging is such that the results from it are typically improved by the length of the array. Eventually though there are of course limitations to hardware integration on UUVs (and all other unmanned vehicles), with only a few hulls being long enough to carry a 20 m-long system. However, an array of 30-40 m in length can be generated synthetically (giving the array a ‘synthetic aperture’), using a physical antenna measuring only 1 or 2 m as well as a combination of acoustic pings at varying frequencies, all focusing on the same given area of seafloor. Stitching together the data from those pings gives not only an image with a far higher resolution than that offered by typical sidescan sonar (SSS), the resolution is also captured independently of range. SSS works using angular resolution: its beam has an angular aperture, often about 0.5 º . That means ‘viewing’ an object using it from 100 m away will yield an approximately double-sized image of it compared with scanning it at 200 m. In essence, SSS resolution is not consistent, as it changes with range – unlike SAS. The size of the synthetic aperture is directly linked to the footprint of the transmitting beam on the seafloor, and varies linearly with the beam’s range. But UUV autonomy and the need for extended missions are spurring fresh advances in sonar technology. Rory Jackson reports The longer view December/January 2020 | Unmanned Systems Technology Advances in acoustic imaging and positioning are enabling greater autonomy and more detailed data for UUV operators than ever (Courtesy of Advanced Navigation)