Page 434 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

16.4 Basics of acoustic positioning 427
16.4 Basics of acoustic positioning
The basic underwater “speaker” is a transducer. This device changes electrical energy into mechan- ical energy to generate a sound pulse in water. For transducers used in underwater positioning, the typical transducer produces an omnidirectional sound beam capable of being picked up by other transducers in all directions from the signal source.
Acoustic positioning is a basic sound propagation and triangulation problem. The technology itself is simple, but the inherent physical errors require understanding and consideration in order to gain an accurate positional resolution.
As discussed in Chapter 2, water density is affected by water temperature, pressure, and salinity. This density also directly affects the speed of sound transmission in water. If an accurate round-trip time/speed can be calculated, the distance to a vehicle from a reference point can be ascertained. Therefore, the simple formula R 3 T 5 D (rate 3 time 5 distance) can be used. The time function is easily measurable. The rate question is dependent upon the medium through which the sound tra- vels. The speed of sound (or “sonic” speed) through various media is listed in Table 16.1.
As shown in Table 16.1, pure water and seawater have different sound propagation speeds. For underwater port security tasks, varying degrees of water temperature and salinity conditions will be experienced. The industry-accepted default value for sound speed in water is 4921 ft/s (or 1500 m/ s). If the extreme speed of pure water (4724 ft/s or 1440 m/s) to the median (4921 ft/s or 1500 m/s) is experienced, the difference of 197 ft/s (60 m/s) is approximately a 4% error (or 4 ft over a 100- foot distance). If this level of maximum error is acceptable, use the default sonic speed setting. Otherwise, consult the temperature/salinity tables for your specific conditions. Make speed adjust- ments within the software of the positioning system based upon those conditions.
The range to an acoustic beacon/transponder is a simple calculation: R512vt;
where R, range, is half the round-trip time, t, multiplied by the velocity, v.
If there is any latency time for a transponder/responder to process the signal, subtract that out of
the time equation to produce a clean range to the beacon.
Table 16.1 Speed of Sound in Various Materials at 68F/20C at 1 Atmosphere
Material Speed (ft/s) Speed (m/s)
Air
Air (32F/0C) Helium Hydrogen Pure water Seawater
Iron and steel Glass Aluminum Hard wood
1125 343 1086 331 3297 1005 4265 1300 4724 1440 5118 1560 16,404 5000 14,764 4500 16,732 5100 13,123 4000
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