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Permanent Ocean Presence With Autonomous Sailing Robots

By José C. Alves • Nuno A. Cruz

The graphical user interface used to program, simulate, supervise and play back a sailboat mission. The track presented was recorded during a 2.5-hour trial in Cardiff Bay, Wales, and shows five laps around three virtual buoys.
The demand for accurate ocean sampling is continuously growing to provide a better understanding of the complex sea environment. Current economic and social activity is strongly dictated by knowledge built on data collected from thousands of sensors around the world, ranging from space-borne remote sensors to underwater devices transported by profilers. Although the most widespread data are weather forecasts and early warnings for hurricanes or tsunamis, another sensible domain that relies on data gathered from the ocean environment is the security of maritime frontiers against illegal activities such as trafficking drugs and people, and terrorism threats.

Potential Benefits of Autonomous Sailboats
Recent advances in the comprehension of several oceanographic phenomena have been fostered by the development of key technologies, including modeling capabilities and the ability to feed those models with real field data. Such data have been obtained using several types of innovative sensors, both on traditional science cruises and integrated in more complex systems, such as drifting buoys, ASVs or AUVs. For a given application or challenge in data acquisition, the combination of assets is chosen according to the tradeoff between estimated performance and operational cost. With the expansion of the suite of available robotic devices, other scenarios are envisaged for their operation, with an increasing demand for a longer mission duration. In recent years, there has been a strong effort to develop devices that target a permanent presence in the ocean, with notable achievements by long-term AUVs, underwater gliders and wave gliders. However, their limited speed precludes the ability to beat the ocean currents and limits the scope of utilization.

Autonomous sailboats are a relatively new robotic technology that may complement existing efforts toward a permanent, sustainable presence in the ocean. They rely on wind to provide propulsion and only need electrical energy for onboard electronics and rudder/sail adjustments, which only require a few watts. If we combine this low consumption with the energy provided by current solar panels and the energy densities of existing batteries, it is feasible to devise an energy management system that provides a continuous supply of power to the onboard electronics. The absence of electrical propulsion has an additional benefit: the silent nature of operations, which is extremely important in all sorts of missions involving underwater acoustic detection.

Autonomous sailboats can transport a wide variety of sensors in the hull and mast, or towed/winched several meters below the sea surface. Data from these sensors can be stored on board or transmitted to shore via radio or satellite. The exact position of the device can be tracked so that all data can be geolocated. All this information can be interpreted by a mission supervisor in real time, changing mission parameters, sensor settings or taking other actions. The cost of operating a fleet of autonomous sailboats is associated with the support infrastructure, such as communications, backing personnel and hypothetical emergency rescue equipment.

Autonomous sailboats have great potential to gather long-term data to understand multiple aspects of the ocean environment. In terms of oceanography, they can be used to study many processes occurring at the surface, like the energy exchange between the ocean and the atmosphere and how it affects the climate. They can also be a valuable tool to understand the dynamics of episodic events that evolve on a timescale of weeks or months, like harmful algae blooms or the evolution of pollution plumes. Even though these incidents can already be tracked by satellite, the ability to capture in-situ data for the full cycle can provide valuable data about the phenomena.

The silent nature of the operation of sailboats makes them an ideal platform for acoustic measurements in the ocean, such as the detection of mammal sounds or the localization of moving vessels with specific acoustic signatures. Ocean surveillance can also be complemented with more conventional technologies like imaging or radar, taking advantage of the mast height for increased range of sight. The dynamic reconfiguration of a coordinated fleet of such crafts can be arranged for tackling specific surveillance objectives.

Technology Status
One important point to address regarding permanent ocean presence is the ability to withstand the harsh conditions at sea during long periods of time. Up to the present, there have been some successful demonstrations of high-endurance capability with small autonomous wind-propelled crafts, with different proposals resembling conventional sailing boats or more customized and sophisticated designs. Success stories include the French Vaimos from ENSTA Bretagne (Brest, France), the Norwegian Sailbuoy vessel from CMR Instrumentation (Bergen, Norway) and the Saildrone project, a 19-foot-long trimaran rigging a wing sail that recently completed a 34-day trip from San Francisco, California, to Hawaii.

In some applications, other features are needed to complement the endurance capabilities, such as maneuverability, accuracy of course and station keeping. Examples include monitoring pollution plumes, performing scans of the seafloor and continuously sampling the near-surface environment.

All these developments have benefited from the active sharing of knowledge and field experience among various groups from universities, research and development centers, and industry. Since 2006, two annual events, one mainly in Europe and another in North America, have offered opportunities for joining the international community active in robotic sailing to discuss achievements through technical meetings and performance evaluations with field exercises. These are excellent opportunities to assess the results of practical developments, while attempting to accomplish increasingly complex tasks, thus encouraging the community to invest in new improvements. Such challenges are planned to evaluate the accuracy of course control, speed along different points of sailing, the ability to perform a station-keeping navigation pattern and an endurance task to verify the aptitude for long-term missions.

The FASt Sailboat
The FASt autonomous sailboat is a 2.5-meter-long, 50-kilogram unmanned mono hull capable of fully autonomous navigation under sail through a set of waypoints. The sailboat was designed and built during 2007 in the school of engineering of the University of Porto, Portugal, and since then has been evolving to include additional features. The latest developments have focused on complementing endurance with trajectory accuracy and the precise execution of sailing maneuvers like gibing and tacking.

A small solar panel is the only source of electric energy that proved to be sufficient for long-term missions due to a low-power computing system and the ability to adapt the frequency of sail and rudder actuation for adjusting the navigation performance as a tradeoff of accuracy of route, speed and usage of the available power for the electric motors. To continue this article please click here.

José C. Alves is a researcher in the robotics unit at INESC TEC and professor at the University of Porto, faculty of engineering. His research interests include the design of high-performance embedded computing systems for autonomous robotic applications. He supervises the operation of FASt.

Nuno Cruz is a researcher at INESC TEC and a lecturer at the University of Porto. During the last 15 years, he has been involved in the design, development and operation of small-size AUVs and ASVs. His current research interests include the implementation of efficient strategies for ocean sampling

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