102 enable localisation of UUVs and divers accurate to around 1 m. As with traditional LBL, the Inca AUVs can be localised as long as at least three Mayas are on the surface to form a perimeter around the antenna, although four Mayas is a better number for improving the Incas’ positioning accuracy. From there, they work via multilateration using the times-of-flight between the signals between the LBL beacons and the AUVs’ transponders, with the difference between the four times-of-flight (assuming four beacons) and each AUV being key to triangulating that AUV’s position. “The Mayas are first deployed in the water to form a long baseline on the surface in which each Maya’s position is accurately known, thanks to their RTKGNSS satellite receiver,” Brizard says. After being launched into the water, the Incas broadcast acoustic positioning signals that are captured by the Mayas. Each Maya transfers the time of arrival (ToA) of the positioning signals to the C2 centre, which computes the position of the Incas by reference to the Maya’s LBL. Then, to allow the Incas to form the predefined, passive sonar antenna grid, the C2 centre sends to each Inca (via the Mayas) a position adjustment command, which is executed by the Incas to reach and hold their position in the antenna grid. “Traditional LBL beacons go on the seafloor, simply because that’s how you would fix their locations in place, but it takes great time and work to install those on the seafloor and then triangulate their GNSS position so you can localise your UUVs – and then, if the UUVs need to move somewhere outside of the spread of beacons, you have to move your beacons and go through the whole arduous process again,” Brizard says. “The only reason people hadn’t thought to put LBL beacons on the surface before was because they’d move around, and so you couldn’t quite get their position accurately enough. GNSS would only localise the beacons to several metres, but now, by equipping LBL systems and new RTK-GNSS and DGNSS systems onto a single node, we can have the beacons float on the surface and still locate them accurately.” Hence, as long as the Mayas are on the surface, they maintain a RTK-GNSS fix to 2 cm horizontal position (4 cm vertical), which, combined with the LBL positions of the Incas below, means the C2 centre can receive position updates for every AUV constantly, with the Incas being localised in real time to around 1.5 m accuracy. “This isn’t some secret technology we’ve invented all by ourselves. In fact, approaches like this are rapidly becoming very popular for underwater localisation, especially as RTK corrections have become more and more affordable over the past few years,” Brizard notes. The Mayas return to their Incas through guidance from the proprietary SEAKER USBL, composed of a directional April/May 2024 | Uncrewed Systems Technology Each Inca is positioned to 1.5 m accuracy – a key factor in ensuring the swarm can carry out beamforming as an antenna At the close of the mission, a vast net with a beacon signals the AUVs to home in and swim inside it, enabling them all to be recovered at once
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