Page 440 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

FIGURE 16.6
Multipath signals.
Source
2
1
Receiver
16.5 Sound propagation, threshold, and multipath 433
air (boat hull, fish, sea grass, etc.). Blockage will make it appear that the signal is too quiet and it may not be detected. Blockage can also be by air bubbles, kelp, rock outcroppings, mud, etc. The best way to combat signal blockage is to avoid it.
Other causes of a weak signal include damaged equipment (broken station or transducer), using the system at too great a range, the presence of thermoclines, surface effects, using a system in a liquid with a greater sound adsorption, etc. There are a large number of possible causes for a weak- er than expected signal and often it will be difficult to determine the cause.
16.5.2 Noise
Noise is essentially the detection of an unwanted signal, thus drowning out the wanted signal. In order for a signal to be detected, there must be a method of separating the signal from any noise present. Essentially, the signal must be louder than the noise. Most hardware performs signal extraction through filtering and thresholding. “Filtering” is the operation of passing a signal at spe- cific frequencies. “Thresholding” is the operation of classifying a signal based on amplitude. Basically, the system listens on a specific channel (frequency) for a signal above a certain ampli- tude level (threshold). If a signal of the appropriate frequency is received and it has amplitude above the threshold it is a valid signal, not noise, and will be allowed to pass.
Sound propagation in water is subject to a number of challenging environmental factors. Over the shorter distances covered by most observation-class ROV positioning systems, many of the physical factors affecting sonic speed (i.e., speed of sound in water) are insignificant. The signifi- cant physical factors over these shorter distances are sound threshold considerations and the multi- path phenomenon.
16.5.3 Multipath
In Figure 16.6, all of the sounds shown originated from the same source (e.g., an acoustic beacon)
but arrive at the receiver at different times. If each of these receptions appeared as a beacon on a
Hull
3