Page 437 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

430 CHAPTER 16 Acoustic Positioning
For determining the distance from a transducer, the resolved equidistant line of offset is known as a “line of position.” This is defined as a line of known distance from a point (i.e., the transpon- der). The easy part of this equation is resolving the range question. The remainder of this chapter (and the difficult part of this technology) now focuses on resolving the bearing.
With two transducers, the lines of position will correctly resolve to two different locations (Figure 16.3). This has been experienced in the field, giving a resolution to an in-water position that showed the vehicle to be on land! The distance from each transducer is known, but the LOPs cannot resolve the different locations. This is termed “baseline ambiguity.” A third transducer is needed in order to resolve the exact point. The software that comes with some of the positioning systems cur- rently on the market allows the operator to select/assume a side of the baseline for operations. This allows disregarding readings on one side of the baseline, thus allowing a two-transducer arrangement.
In the scenarios shown in Figures 16.3 and 16.4, certain assumptions can be made. If it is deter- mined that the submersible will be operated only on one side of the baseline (as shown in Figure 16.4, it is impossible to operate the system on land), all positional resolutions on one side of the baseline can be disregarded, producing accurate navigation with only two surface stations.
With underwater acoustic navigation and positioning, there are various types of underwater markers functioning as transmitters, receivers, or both. The six main classifications (Milne, 1983) are:
1. Interrogator: A transmitter/receiver that sends out an interrogation signal on one frequency and receives a reply on a second frequency. The channel spacing for these transmit/receive signals is often 500 Hz (0.5 kHz) apart.
S h i p
Dock
False position
Line of position
Transducer
Transducer
Line of position
Vehicle
Harbor
FIGURE 16.4
False position resolution.