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6 Mission-critical info for UST professionals Platform one Three engineers in Denmark have developed a lightweight, reusable underwater probe that can be carried by a UAV and be used at depths of up to 300 m (writes Nick Flaherty). The probe weighs 2.6 kg and can measure salinity, temperature and pressure in the sea near glaciers, taking at least three measurements per metre before it returns to the surface. When it reaches a set depth, a small heating element burns through thin plastic strips that hold two iron weights. Relieved of the ballast, the probe rises again towards the surface. The probe is designed to be carried by a customised quadrotor UAV to the survey area. The UAV is also used to collect the probe. The 23 kg UAV has a 600 Wh battery pack that gives the fully loaded aircraft 20 minutes of flight time from the four U8II KV190 motors from T-Motor, each with 7.3 kg of thrust. The probe and UAV were developed by Ebbe Poulsen, Mathias Eggertsen and Erik Hinge Jepsen, and have been tested during a voyage aboard inspection vessel Knud Rasmussen in north-east Greenland. The modifications for the UAV were concerned mainly with the probe’s release and retrieval, said Poulsen. During release, a servo on the UAV actuates a trigger that releases a strap fastening the probe to the UAV. When the probe resurfaces after a dive, two arms suspend a line from it into the water. A servo on the UAV then drops a hook that is dragged through the water to catch the line and pull up the probe. A magnetic system for retrieval was not deemed feasible, because of the required precision of positioning between the buoy and the UAV, Poulsen said. The UAV also needed an inertial navigation system, as satellite and magnetic sensors are less accurate in northern latitudes. “It’s far too dangerous to sail in front of a glacier, and helicopter time is wildly expensive,” explained Professor Soren Rysgaard of the Arctic Research Centre at Aarhus University, in Denmark. “Therefore, the idea arose of developing a lightweight probe that a UAV can fly in and drop off right in front of the glacier. “The costs of research projects increase significantly when the equipment becomes too costly, and in the Arctic there is a high risk of losing equipment. The objective was therefore to develop a probe costing about one-tenth of the price of commercial versions.” The probe is 3D-printed and has a pressurised container for the electronics and to provide the buoyancy for the return to the surface. The team compared the data from its probe with commercial probes that are dropped from a boat on a copper cable. The data was almost identical. “This opens up major perspectives,” said Claus Melvad, senior associate professor in the Department of Mechanics and Production at Aarhus, who oversaw the system’s development. “The sensors in this home-made probe cost only a few thousand Danish kroner [a few hundred US dollars], and the data resolution they deliver is fully adequate.” In time, the probes could be equipped with a transmitter that automatically sends data when the buoy reaches the surface. “If, one day, we succeed in producing the probes in biodegradable material, any ship sailing in Greenland’s waters could drop a probe,” said Prof Rysgaard. “This will provide a lot of very important data that will help us to understand how climate change is affecting the sea currents around Greenland – we have incredibly limited data from this area.” Sensors Probe goes with floes The lightweight probe can be dropped from a UAV and recovered to measure ocean properties close to a glacier April/May 2022 | Unmanned Systems Technology