Sea Technology

MAY 2013

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

Issue link: http://sea-technology.epubxp.com/i/128560

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Navigation

Page 37 of 87

sensors, and all data can be stored onboard and transmitted in real time via radio. These vehicles operate with two independent thrusters, capable of holding position or navigating up to 4 knots. Zarco and Gama have been in operation since 2006, either independently (for bathymetry, for example) or coordinated, to provide a moving baseline for AUV navigation. The NIBs are auxiliary equipment commonly used as static beacons responding to acoustic pings, using a two-way travel time scheme, or transmitting synchronized pings for one-way travel time positioning, therefore providing ranging capabilities to underwater vehicles. They are equipped with GPS receivers and radio-communication devices. Whenever communication with AUVs is available, the geolocation of the NIBs are fed into the vehicles for more precise localization. The Ocean Systems Group has tackled the technical challenges associated with the operation of these marine systems, such as relative and absolute positioning systems for multiple vehicles; sensor fusion for precise navigation, including underwater navigation in a moving acoustic network; and guidance and control of individual vehicles, both ASVs and AUVs. The current research builds upon earlier results to address the challenges associated with coordinated operations, mainly formation control under the constraints of communications and networking. These are particularly severe when using the underwater channel, with high latency, low data rate and high probability of transmission error. The goal is for the communications network to be adaptive according to the instantaneous conditions of the acoustic channel and to the capabilities of the moving nodes. In particular, some of the nodes may have to move to increase the overall throughput, while, at the same time, the network routing will deal with the transmission conditions between the available nodes to ensure a proper transmission of information. (Top) Trajectory of the Zarco ASV, following the position of the MARES AUV. (Middle) Trajectories of the Zarco and Gama ASVs in a coordinated mission, three meters apart. (Bottom) Relative distance of the vehicles. 38 st / May 2013 Motion Coordination The capability of vehicles to follow a given trajectory and maintain reliable data exchange are among the most relevant topics when it comes to coordination of marine robots. First, the vehicles need robust and versatile controllers to allow coherent motion of the formation. This may be quite challenging in an unpredictable environment with dynamic disturbances. Therefore, robust control laws must ensure position stability for all the vehicles. Of course, control performances can be improved at the cost of more navigation sensors, although this may be unaffordable for a feet of robots. Second, data exchange via communications suffers from several constraints in marine environments. Robot formations should be fexible enough to accommodate intrinsic delays of radio or acoustic communications. Further to the delays, and mainly due to the compromise between frequency and range of operation, acoustic communications have very low data rates as compared to radio or optical communications. Gray areas originated by multipath, oc- www.sea-technology.com

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