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37 since the imaging resolution also varies linearly with range, a constant resolution over range is achieved, with the two variances compensating for one another. Naval mine countermeasures (MCM) crews have to generate imagery of minefields with error margins of less than 10 cm to reliably localise, detect and classify individual mines, so that active countermeasures can be engaged. The use of SAS is therefore critical here. High-frequency SSS can produce imagery accurate to 10 cm at 100 m or so, and potentially accurate to 5 cm at 50 m, which for a range of civilian and commercial applications is ideal, not to mention more affordable. However, as MCM operators increasingly look to switch from using manned vessels with high-frequency hull-mounted sonars to using UUVs, the resolution and range of SAS makes it the most well-suited sensor for this application. At 250 m a SAS with a 15 cm transmission array and a 1.2 m reception array will achieve a resolution accurate to around 7 cm while the UUV operates at speeds up to 4 knots, enabling coverage rates of up to 3.6 km 2 /h. Typically, one small hydrophone is used for transmission in SAS, because the image resolution being generated from returned signals – being constant, and independent of range – tends to be half the size of the antenna. On the other hand, the receiving antenna is longer, because the maximum range is proportional to its length. Although SAS has been in development since the 1970s, a few factors have recently boosted their applicability for unmanned systems. For one, improvements in GPUs have accelerated the running of SAS algorithms to the point that eight hours of raw data can now be processed into 3D mapping imagery in under 10 minutes. The algorithms themselves have also been developed and improved over the years to achieve higher accuracy, and more quickly. These improvements in processing enable real-time measurements at speeds and distances sufficient for SLAM navigation. The use of a fibre optic gyro (FOG) or laser ring gyro (LRG) instead of a MEMS IMU is recommended to ensure the UUV can be left to operate autonomously, as FOGs and LRGs produce far lower angular random walk and bias instability than MEMS. The stream of inertial sensor data from an onboard FOG or LRG will enable the UUV to update its heading and position more reliably within its SAS-generated map of its surroundings, to ensure it accomplishes its tasks safely and efficiently. Forward-scanning sonars In the interest of using sonar for simultaneous mapping, navigation, and sense-and-avoid purposes, significant development has gone into forward- scanning sonars that generate imagery of what is ahead of the unmanned vehicle. That not only minimises the chances of vehicles colliding with rocks or running aground, it also enables them to manoeuvre nearer to subsea structures, geological formations and even mines for closer inspections. These days a high-end forward- scanning sonar can have up to 512 channels and powerful onboard processors to accommodate the amount of data needed for actionable in-situ sense and avoid. Producing high-quality maps at such densities and speeds means these sonars also provide better hydrographic survey data than many COTS sonars on the market, which often cause UUVs and USVs to return with incomplete maps, entailing further missions and costs. Rather than having a pre-programmed (automated rather than truly autonomous) USV or AUV following a series of back-and-forth tracks across a square section of known seabed, forward-scanning sonars can enable real-time mapping of previously unknown terrain ahead of the vehicle. That could be used by the onboard autopilots to autonomously navigate along the curvature or topology of the seabed, re-orienting and adjusting its path as it encounters cliffs, obstacles or points of interest. Defence research in particular has created the bulk of the demand for such systems, for keeping their expensive Sonar systems | Focus Unmanned Systems Technology | December/January 2020 Mine detection and countermeasures operators are increasingly interested in synthetic aperture sonar for its consistent resolution and accuracy over range (Courtesy of iXblue)