Page 450 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition
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16.8 Capabilities and limitations of acoustic positioning 443
• Good positioning accuracy is possible when operating from larger vessels or a dock (good transducer separation)
SBL disadvantages:
• Poor positioning accuracy when operating from very small vessels (limited transducer separation)
• Requires deployment of three or more surface station transducers (more wiring than USBL) LBL advantages:
• Very good position accuracy independent of water depth
• Redundancy
• Wide area of coverage
• Single, simple deployment pole
LBL disadvantages:
• Complex systems requiring more competent operators
• Large arrays required
• Longer deployment/recovery time
• Calibration time required at each location
16.8 Capabilities and limitations of acoustic positioning
Acoustic positioning is capable of high-precision positioning in a number of frames of reference. Once the local sonic speed is computed, range is capable of being accurately measured. The larger the spacing between the transducers forming the baselines, the higher the possible bearing accuracy. The basic range/bearing resolution accuracy with LBL positioning is generally of a higher quality than the SBL and USBL techniques due to the inherent greater angular offset reception time spacing of the baselines. Range in all methods is quite accurate, assuming line of sight is maintained. Bearing accuracy of 13 for SBL and USBL is considered acceptable, while bearings of ,1 are possible in LBL configuration.
Acoustic positioning relies upon a line-of-sight sound signal from a transducer (generally mounted aboard the submersible) to a receiver, so that an accurate range and bearing can be resolved to give a position in some frame of reference.
Just as picking out a voice in a crowd is difficult, so is picking out an acoustic positioning bea- con in a noisy harbor environment. The challenge is balancing the reception sound threshold to pick up the line-of-sight transmission and not the false signal from noise or from multipath sound reflections.
If the sound threshold level is placed too high, the possibility exists that the true line-of-sight signal from the beacon to the receiver is rejected (Figure 16.16). If the sound threshold reception level is set too low, false-positive readings from either noise or from multipath reception will show erroneous positions for your vehicle.
In Figure 16.17, sound source 1 is below the noise level and below the reception threshold—the signal is not received. In signal source 2, the source level is above the reception threshold and
