Page 454 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

FIGURE 16.19
Vessel-referenced positioning system.
vessel being inspected. With either method, operation in and around vessels and enclosed structures may not be conducive to acoustic positioning.
One acoustic ship hull inspection system uses a series of assumptions in order to gain accurate position measurements near ship hulls despite blockage and multipath problems.
A series of acoustic transponders are hung over the side of a vessel to be inspected (Figure 16.19). The transponders are positioned below the level of the keel so that all are “in sight” once the submersible is under the vessel. Each transponder has a separate frequency corresponding to its position on the vessel (i.e., forward port, aft port, forward starboard, aft starboard). Once the submersible travels toward the surface on one side of the vessel, it loses sight of the transponders on the opposite side of the vessel (an example would be traveling up toward the surface on the star- board side, losing sight of the port side transponders). Positional accuracy is then maintained with two transponders. The “baseline ambiguity” is resolved by knowing that the submersible cannot be “inside of” the vessel being inspected, thus putting the submersible in only one of the two possible positions. If only one transponder is received, it can be assumed that the submersible is in contact with the hull in the quadrant of the receiving transponder.
16.10 Position referencing
16.10.1 Geo-referencing of position
The basic resolution of an acoustic positioning system is range and bearing. Input of the offset from the primary reference transducer to the center rotation point of the vessel will produce a refer- ence point for the extended centerline of the vessel (Figure 16.20). From this, convert relative bear- ing from the vessel’s axis into magnetic bearing. By comparing the relative bearing (resolved by triangulation) from the vessel to the submersible, with the magnetic heading of the vessel, a resolu- tion is then possible for magnetic bearing to the submersible. Then add the offset of the center pivot point of the vessel to the GPS antenna, and resolution of the range and magnetic bearing to
16.10 Position referencing 447