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47 UUVs | Insight sensor that worked exactly according to specifications when used standalone, but when operated in collaboration with other sensors it fell short of its specification, owing to EMI from another sensor,” Reynisson says. “Our engineers solved the problem though, and the system was accepted without issues.” Long-endurance navigation Long-endurance unmanned systems must be capable of precise navigation for weeks at a time without external help, so it is critical that navigation components are tested rigorously to determine how best to minimise drift. The Precise Positioning for Persistent Autonomous Underwater Vehicles (P3AUV) project, which was led by Sonardyne and ended in December 2019, was aimed at developing navigation systems for this purpose – that is, capable of maintaining underwater positional accuracy to less than 1 m without external references. “Originally we’d planned to deploy sparse-LBL seafloor transponders from a USV, so we brought L3Harris in as a partner,” says Geraint West, Sonardyne’s oceanographic global business manager. “We soon found though that this approach wasn’t of much use, owing to payload and deployment constraints. “However, it did open up new areas of study for improving our systems for remote over-the-horizon operations from a USV, as well as the core objective of closer examination of how we could improve our INS and DVL, and use that as a route for developing consistent, long-term navigation capabilities.” To that end, Sonardyne looked at a range of gyroscopes and compared them to the ring-laser gyros that currently form the basis of its Sprint-Nav INS-DVL products, and tested how they performed in missions when externally unaided. The team ended up developing a system that performs several times better in the tests than any of its current, commercially available offerings. “Part of that was down to the choice of gyroscope, but we also wanted to look closely at the current profiler, so we wound up with a much more intelligent integration between the DVL and the INS,” West explains. “Each system now uses the other’s data much more effectively, so the DVL uses the INS data to output its current profiles more accurately in real time, and vice versa, giving the Sprint-Nav an overall boost. A separate and additional upward-facing DVL was used in the tests as well as the downward-facing DVL fitted as part of the Sprint-Nav.” As well as L3Harris, the National Oceanography Centre (NOC) in Southampton, England, was a key partner during the project, providing the test platforms for the trials. One of those was an AutoSub Long Range UUV, rated to a depth of 6000 m and which can travel up to 2000 km at 1.7 m/s. Another two AutoSubs were also deployed in the final ‘swarming’ trial. “We wanted to deliver results primarily to help researchers who want to conduct long-term surveys for hydrographic or environmental purposes,” West says. “However, there are also clear benefits for the defence sector in terms of persistent anti-submarine surveillance, as well as the oil, gas and telecoms industries who need UUVs that can monitor remote infrastructures for extended periods without having to surface. “Towards the end of the trial the NOC also sought to trial a swarm of networked technologies, so we had about 15 Sonardyne devices working simultaneously across the three vehicles,” he adds. “The AUV was communicating with the seabed transponder, and an ASV provided surface tracking.” The tests took place at Loch Ness, in Scotland, which was found to be a highly advantageous trial site owing to its linear shape, depth and minimal vehicle traffic. “With minimal surface traffic to create noise, and a stable environment without large variations in sound velocity, the data set we accumulated was incredibly clean and precise,” West says. “By comparison, our previous test sites have been quite ‘noisy’, congested and shallow, so I foresee us using Loch Ness for more trials in the future.” Conclusion UUVs continue to prove themselves in tests and missions throughout the aquatic realm. As they do so, the experience and data being accrued by their engineers and operators will no doubt feed back into fresh innovations that take unmanned systems down to the deepest and furthest limits of our oceans. Unmanned Systems Technology | February/March 2020 The P3AUV trials were led by Sonardyne and used the AutoSub Long Range UUV as the testing platform (Courtesy of Sonardyne)