Page 445 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

438 CHAPTER 16 Acoustic Positioning
(a)
(b)
(c)
FIGURE 16.10
Typical USBL arrangement (a) profile view, (b) plan view and (c) graphical depiction.
acoustic signal, which the transponder detects, causing it to transmit an acoustic signal in response on a discrete channel. The time from the transmission of the first signal to the reception of the second is measured. As sound travels through the water at a known speed, the distance between the transducer and the beacon can be estimated. The process is repeated for the remaining beacons and the position of the vessel relative to the array of beacons is then calculated or estimated.
In principle, navigation can be achieved using just two transponder beacons, but in that case there is a possible ambiguity as to which side of the baseline (a line drawn between the beacons) the vessel may be on (the so-called baseline ambiguity—see previous section of this chapter). In addition, the depth or height of the transducer has to be assumed, unless there is an embedded depth transducer that encodes depth measurements with the transponder response. Three transpon- der beacons is the minimum required for unambiguous navigation in three dimensions; four is the minimum required for some degree of redundancy. This is useful for checks on the quality of navigation.
The term “long baseline” is used because, in general, the baseline distances are much greater for LBL than for SBL and certainly for USBL. Because the baselines are much larger, an LBL