60 In operation | Dropla mine countermeasures Multi-sensor strategy Dropla’s UAVs fly with three categories of sensors, broadly categorised as “optics”, “magnetics” and “electromagnetics” (EM). Within “optics” are RGB cameras, multispectral cameras, thermal cameras and Lidar. The former two naturally serve well in daytime visual surveys for signs of minelaying, but they are limited when landmines are hidden by the presence of dense vegetation, being located underground, and similar factors. “Thermal cameras can detect targets of substantial sizes hidden deep underground, such as anti-tank mines encased in metal, by measuring the signature of heat released over time; for instance, the temperature drop of an object between noon and just after sunset. If its temperature has dropped by a differential as small as 2 C, compared to the soil around it, that can be enough to confirm it as a potential landmine,” Shvaydak says. The Lidar, meanwhile, serves more in feature extraction, including both the identification of soil disturbances typical of landmine placement (sapper work often leaving very precise signs that can be identified via Dropla’s Dropla.Vision analytics software during data processing) and the creation of a 3D map of the terrain the UGVs will use to navigate later on. Within “magnetics” solutions are magnetic gradiometry sensors, which measure deviations in the Earth’s magnetic field caused by the presence of conductive objects (such as metals) in the ground. “When working with magnetic gradiometry, however, you’re working with tensor fields, which follow the inverse square law of magnetics, which means a target of 10 kg lying 10 m deep will look just the same as a 1 kg target at 1 m depth,” Shvaydak explains. “So you can’t run generalisation algorithms on their output data to determine whether it’s one or the other, but we do use localisation algorithms to calculate the probable exact centre of detected anomalies, which is still very helpful.” Magnetic sensors are also limited by metal clutter contaminations, meaning, for instance, that a 50 kg piece of metal will be detected as an anomaly, spanning about 3-6 m in diameter, which could be hiding a second, smaller landmine within its radius. Hence, like the “optic” sensors, it cannot be used as a standalone mine detector. The EM detection systems used are ground-penetrating radars (GPR) and pulse induction (PI) metal detectors. The former include highly regarded sensor technologies, such as synthetic aperture radar (SAR), but its accuracy reduces with the moisture content in the ground; very wet soil will back-scatter 95% of SAR’s EM radiation into the surrounding environment. “We can’t rely on GPR if the ground’s moisture content exceeds 70%; it just renders the radar useless,” Shvaydak says. “There’s a very limited window of opportunity to use GPR and that’s when we have a warm, dry month, but in that window, GPR honestly becomes an amazing technology, capable of revealing the hardest targets of all: anti-personnel, plastic landmines. Those make up 80% of missed targets, simply because they don’t throw up a signature for any other sensors to use. “Overall, the back-scattering of EM waves in highly non-homogenous environments remains an unsolved mathematical issue, although there is a multinational research group and opensource software developer called gprMax that is trying to resolve the problem by simulating that back-scatter specifically for landmine detection, as well as a few other pointed use-cases. Its progress will likely be hugely valuable in the fight against anti-personnel mines.” PI metal detectors are well-established and widely used devices, which function by producing electromagnetic pulses via copper coils, but they suffer from a high rate of error – a ratio of around 100:1 false positives to actual metal targets. Therefore, other sensors are key to reducing ground clearance work. With all these sensors combined (via real-time processing, post-processing and data fusion), the UAV-based sensing narrows surveyed SHAs down to the 5% of actually dangerous CHAs, where potential landmines lie. February/March 2025 | Uncrewed Systems Technology By using multiple UAVs at once, Dropla is scanning 30 ha per day, compared with conventional, single-UAV approaches that take many days
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