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53 Advanced Navigation Hydrus | Digest the physics model of the UUV into the INS to further optimise the latter for the former. “Having all our systems well-integrated means we get very low inertial drift. Even when we lose a bottom lock because of the distance from the seabed we’re still able to closely estimate water currents and account for those in navigation measurements. So we don’t opt for a fibre optic gyro, as the increase in inertial precision over MEMS wouldn’t be a significant improvement.” The forward-looking sonar is identifiable via four blue discs that sit equally spaced around the base of the camera housing. The pulses from it generate a 3D point cloud in a 75 º FoV, much like Lidar, principally to detect obstacles in order to avoid them or station-hold at a position relative to them as points of reference. “By and large, we arrange our sonars in a Janus configuration. That’s a concept originally developed for DVLs, where all the transducers point in different directions to perform precise measurements of range and movement speed with real-time accuracy checks,” Orr comments. It has also been engineered around a low minimum range and a high update rate to match the performance of the camera (a 60 fps system) and develop a sufficiently dense point cloud to enable SLAM, once it’s stamped with timing and inertial data. “As a bare minimum, we needed the Hydrus to be able to detect obstacles and calculate routes around them,” Orr says. “But we also didn’t want the perennial situation of a UUV returning from a survey with bad footage, so our AI engine analyses footage quality on the fly, and if say some silt gets kicked up and clouds the camera’s FoV, it will adjust the mission plan to go back – using the SLAM for accurate localisation and return route planning – and recapture that area and angle.” While sonar data is used primarily for obstacle avoidance and path planning, the camera is intended for survey purposes, although naturally the point clouds generated can be of considerable use to ocean data analysts. “The forward-looking sonar also functions as a DVL, to give ranging and speed relative to the seafloor, although to be on the safe side we also have a DVL on the underside to enable altitude hold and accurate navigation as the UUV travels between waypoints,” Orr adds. “Having DVL information feeding from both the bottom and forward sonars into the navigation really allows for superior performance in how precisely movements can be controlled, and how accurately our point clouds are recorded as a result.” The DVL has been engineered to operate using short minimum ranges – around 2.5 cm, much less than the market standard of 10-50 cm – to enable the Hydrus to navigate very closely to and along objects. Orr notes that the drawback of this approach is a reduction in the maximum range, down to around 30 m (compared with the typical 100 m), although it is suitable for the relatively short missions the UUV is likely to be used in. Survey systems The aforementioned camera is a 4K 60 fps system featuring a sapphire lens for scratch resistance. It has an 83 º FoV and is designed for working in low light, to maintain high performance in underwater environments prone to high turbidity and other lighting issues. Even so, Advanced Navigation has installed a ring of eight LEDs around the base of the camera for dynamic lighting adjustment, and the system’s embedded computer vision determines when the FoV needs illuminating. Their maximum collective output is 12,000 lumens, although typically 2000-5000 lumens is enough. “The beauty of having an FPGA, again, is that you get this really fine- Unmanned Systems Technology | April/May 2022 The camera is a 4K 60 fps system with a sapphire lens for scratch resistance, and the forward-looking sonar is identifiable by the four blue circles amid the LEDs