Cooperating systems

Cooperating systems is a rising topic in the field of underwater intelligent vehicles providing many potential benefits. The systems can all be AUVs, or be a mixture of underwater and surface vehicles. Some vehicles may be autonomous while other may be remotely controlled or manned. Below are some examples of cooperation. The mission time to survey a given area may be significantly reduced by using several autonomous vehicles. The level of communication between these vehicles depends on the level of planning involved (and thus on the control architecture). If each vehicle is given a part of the area to explore independently, no intervehicle communication is necessary. However, it is also possible for the vehicles to interact, for instance by transmitting their positions and sensor findings as guidelines for the other vehicles. This is of particular interest for adaptive data collection, where the combined efforts of all the vehicles can be focused through either directions from a master AUV or control algorithms inspired by swarm intelligence (Bonabeau et al., 1999). The use of several AUVs moving in formation may be advantageous when tracking sensor data gradients, as calculating the spatial gradient based on data from a single vehicle requires intensive manoeuvring. Two AUVs with different sensor suites can cooperate in a mission where the objective is to detect and examine certain objects or features on the seafloor. The first vehicle is equipped with a sensor for long-range detection and covers the complete area using an efficient survey pattern (e.g. lawnmower). Sensor data is automatically processed in the vehicle, and results transmitted over acoustic link. The second vehicle is equipped with close-range inspection sensors and visits only the detection locations transmitted by the first vehicle.

An AUV can cooperate with an accompanying surface ship in several ways. The surface ship (manned or unmanned) may provide an accurate position update to the AUV, using GPS and USBL as described in Section 2.6. The AUV can also acoustically transfer key parameters and data subsets to an unmanned surface vessel, which can use wireless communication to receivers located on land or in air, thus substantially increasing the effective communication range for the AUV. Another mode of cooperation is a bistatic sonar configuration. Typically the surface ship will transmit with an active sonar, with passive receiver antennas located on (or towed behind) one or more AUVs.

The cooperation between an AUV and a manned military submarine also offers interesting possibilities. The AUV may be used as a forward sensor platform to avoid exposure of the submarine. As another example, the AUV may act as a decoy, using sonar to imitate a real submarine's acoustic signature, thus deceiving the opponent.

Planning of cooperative missions can primarily be divided in two areas. If the mission can be split into non-overlapping tasks each involving only a single vehicle, then these tasks can be planned separately. Each vehicle in the system will have one or more of these tasks, and will execute them independently of the others. The only procedure involving actual cooperation will be the distribution of tasks. However, if the mission can not be split into non-overlapping tasks, the planning will become very complex. Further, a high level of active cooperation between the vehicles will require a high level of communication. Longrange, high-bandwidth, networkable underwater communication is technically difficult to achieve, and also undesirable if the vehicles should avoid being detected. The level of cooperation is thus highly dependent on the mission, but it is evident that several missions can greatly benefit from using cooperating systems.

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