Stationkeeping near the ocean bottom requires the capability to accurately sense a vehicle's position and movements relative to ocean floor, nearby objects, or a fixed coordinate system. Some of this information can be obtained using inertial navigation systems, pressure sensors, and various ranging devices based on sonar and acoustic techniques. Reliable horizontal positioning, however, is rather difficult to achieve. For example, Doppler acoustic methods for position sensing by dead reckoning are sensitive to drift. Other active methods require beacons to be positioned accurately at the work site and recovered upon completion of the task. This is costly, tedious, and impractical where active sensing must be avoided. Passive optical sensing, which exploits the bottom surface texture as beacons, is proposed as an alternative solution. This has many advantages, particularly in applications where active sensing must be avoided. In addition to stabilizing position and orientation for stationkeeping, visual sensing is useful in other applications which involve locating, recognizing, and manipulating objects in the environment. The methods for depth, orientation, and motion sensing are described, and examples of experiments with real images are given.
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