Page 442 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

FIGURE 16.7
Transducer #2
Transducer #1
Transducer #3
16.6 Types of positioning technologies 435
Baseline #2
Vehicle- mounted transducer
Basic arrangement of transducers into baselines for angular measurements.
16.6.2 Short baseline
A short baseline (SBL) system (Figure 16.9) is normally fitted to a vessel or fixed object with the vessel or fixed object used as the frame of reference. A number of (at least three but typically four) acoustic transducers are fitted in a triangle or a rectangle on the lower part of the vessel or fixed object. The distances between the transducers (the “baselines”) are made as large as practical given physical space limitations; typically they are at least 30 ft (10 m) long. The position of each trans- ducer within a coordinate frame fixed to the vessel or fixed object is determined by conventional survey techniques or from the “as built” survey.
The term “short” is used as a comparison with “long baseline” (LBL) techniques. If the dis- tances from the transducers to an acoustic beacon are measured as described for LBL, then the position of the beacon, within the coordinate reference frame, can be computed. Moreover, if redundant measurements are made, a best estimate can be determined that is more accurate than the basic position calculation by averaging several fixes.
SBL systems transmit from one but receive on all transducers. The result is one distance (or range) measurement and a number of range (or time) differences.
With an SBL system, the coordinate frame is typically fixed to the vessel—which is sub- ject to roll (change in list), pitch (change in trim), and yaw (change in heading) motion. This “disadvantage” can be overcome by using additional equipment such as a vertical reference unit (VRU) to measure roll and pitch and a gyro-compass to measure heading. The coordi- nates of the beacon can then be transformed mathematically to remove the effect of these rotational motions.
Baseline #1