Acoustics

The basic idea of positioning by acoustics is much the same as the telephone analogy used earlier, the direction of a sound can be determined by having spaced sensors receiving the sound (or revolving one sensor) like we can tell roughly the direction a telephone ring is coming from however whereas humans use how loud something is as an indication of how far away it is, this is not exact enough for positioning so we calculate distance by the time it takes for the sound to reach us. Similar to counting in seconds from the flash of lightning to hearing the thunder gives us roughly how far a storm is away in miles.

The simplest form of acoustic positioning, Ultrashort baseline, employs a pinger beacon, this transmits a sound pulse of set length, for a set period of time on a set frequency. This sound is received by a hydrophone on the ship, the hydrophone has three sensors on it arranged in an L shape.

The L shape arrangement of the sensors allows the angle of reception to be measured in both the longitudinal and transverse axis. 

By using phase comparison of the received pulse at each sensor we can calculate the horizontal and vertical angle of the signal, and by applying the depth the slant range and hence the position of the vessel relative to the pinger beacon.

If we advance a little and use a transponder beacon and a transducer instead of the pinger beacon and a hydrophone, we can also measure the slant range by measuring the time taken from the interrogation signal transmission to the reception of the reply signal. From this and knowing the speed of sound in water we calculate the range.

The transducer and transponder scenario is not necessary to obtain a position, but it does improve the performence

The big advantage of this system is that only one pinger and one hydrophone are required

Another type of acoustic positioning is Short Baseline.

Using this system we have 3 hydrophones on the vessel, again we use the "L" shape. We have one pinger on the seabed. Each hydrophone receives the sound pulse from the pinger and by comparing the time difference of arrival, the vertical and horizontal angles can again be calculated. Because the baseline (distance between sensors) is larger than USBL, the accuracy of this method is higher.

Unfortunately, it requires that a minimum of three hydrophones are fitted to the vessel.

Again the performance of this system can be increased by the use of transducers and transponders

The next system is the Long baseline system, here, the baseline is on the seabed, numerous (at least 3) Compatt (computing and telemetry transponders) transponders are placed in an array on the seabed, several hundred metres apart, a single transducer on the vessel sends a signal to a Compatt, once the signal is received by the Compatt its sends back a reply. The two way travel time of the signal is measured and the range from the transducer to the Compatt is calculated. This is repeated for each Compatt and a position is calculated using triangulation (ranges).

This is an extremely accurate form of position fixing, but it is costly to install, the equipment is expensive and you need to deploy a minimum of 3 Compatts a good distance apart which is not always practical.

In all forms of acoustic positioning it is normal to use more than the minimum requirements in the above descriptions. eg a long baseline array would have 4 or more Compatts, and an SBL equipped vessel would have at least 4 hydrophones.

Many of the developers and suppliers of acoustic positioning systems have designed variations on the above systems, either to improve accuracy, to improve reception or both. The major companies involved with acoustic positioning are Sonardyne, Kongsberg and Nautronix