Sea Technology

MAR 2018

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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www.sea-technology.com March 2018 | ST 13 W ith the continuing development of underwater drones (AUVs) comes the questions of how to facil- itate their launch, recharging and data retrieval. In 2016, two industry players, Naval Group, a European naval defense company, and RTsys, a specialist in underwater acoustics, pooled their skills to offer a new and innova- tive way to launch and recover drones using an under- water docking station. This system allows us to imagine new uses for AUVs, with the possibility of leaving them permanently submerged for continuous monitoring and the possibility of more rapid deployment. From the beginning, the project had multiple chal- lenges: how to guide the target to its base, recharge it underwater, prevent biofouling while submerged for months at a time, retrieve data, and program missions. These challenges were met in just 10 months in 2016, resulting in a seabed station and docking kit that can be adapted to almost all drones currently on the market. A Homing Device For the drone to find its station at the end of its mis- sion and enter it with precision, it was necessary to de- velop a homing device. This device is based on a posi- tioning system exclusively using underwater acoustics, thus overcoming turbidity issues. Asked to design this technological component, RTsys created a system that uses real-time acoustic data processing, which enables the drone to quickly adjust its position during the various approach phases. In effect, the station transmits acous- tics pings, then, using triangulation algorithms, the drone can locate the docking station. It modifies its trajectory to enter the station with a precision on the order of a few centimeters. At the heart of this docking station is the implemen- tation of two algorithms based on underwater acoustics, one for bidirectional positioning and the other for long- range, low-speed communication. In this way, the AUV can position itself relative to the station and also trans- mit positioning information over a distance greater than 5 km, requiring power amounting to only a few tens of watts. This enables a multifunctional, smaller system that facilitates integration into various drone models, particu- larly the most compact. Contrary to the majority of existing underwater posi- tioning systems, which have a latency time, the homing kit gives the drone a reaction time on the order of 10 milliseconds. The drone, in the station approach phase, can adapt its altitude and course in real time, for exam- ple, to compensate for the current. On return, the drone also transmits an acoustic signal that informs the user connected to the station of the drone's approach. This positioning system allows the station and the drone to know their relative positions in real time. To complete the system, Naval Group has developed, with the Prolexia company, a 3D modeling system so that the user can display the drone's movements in real time. Decision makers will be able, if necessary, to abort the mission by sending a mission cancellation acoustic command. Underwater Wi-Fi Another challenge to be met was to transmit data between the drone and the docking station without a connecting cable. Underwater, electromagnetic waves are highly attenuated. To overcome this problem, Naval Group and RTsys developed a system similar to Wi-Fi. Based on electromagnetic waves and lossless integration techniques, this communication between the station and the drone allows the station to retrieve the data from the mission. Next, the exchanges between the docking station and the user, on land, are made in a more clas- sic manner: by wire. This underwater Wi-Fi also allows transmission to the drones of the information required for programming the next mission. Contactless Recharging In order to recharge the drones without contact, Naval Group has developed a technology based on a thin elec- Drone Docking Underwater Acoustics-Based System Extends Drone Performance By Marc Richard • Raphaël Bourdon • Alice Vettoretti

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