Page 438 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

16.5 Sound propagation, threshold, and multipath 431
2. Transponder: A receiver/transmitter installed on the seabed or a submersible (relay), which, on receipt of an interrogation signal from the interrogator (command) on one frequency, sends out a reply signal on a second frequency.
3. Beacon/pinger: A transmitter attached to the seabed or a submersible, which continually sends out a pulse on a particular frequency, that is, free running.
4. Hydrophone: An underwater “microphone” used to receive acoustic signals. This term also sometimes refers to a directional or omnidirectional receiver system (hydrophone plus receiver electronics), hull mounted, which is capable of receiving a reply from either a transponder or a beacon/pinger.
5. Transducer: A sonar transducer is the “antenna” of a transponder or interrogator. Either connected by a cable to the electronics package or hard mounted to it, a transducer can both transmit and receive acoustic signals. This is in contrast to a hydrophone, which is specified for receive operation only.
6. Responder: A transmitter fitted to a submersible or on the seabed, which can be triggered by a hard-wired external control signal (command) to transmit an interrogation signal for receipt by a receiver (hydrophone).
The particular arrangement of these items as well as their signal sequence determines the physical parameters of the overall positional accuracy.
16.5 Sound propagation, threshold, and multipath
Sound propagated in water as well as in air possesses many of the same characteristics. A shout in a canyon or in a theater can be echoed several times to the receiver (in this case, your ear). A con- cert played in an enclosed auditorium sounds much different from when it is played in an open-air location. Picking out the sound of one instrument in a full symphony is often a difficult task. Thus, trying to pick up a positioning beacon within the cacophony of underwater sounds within a busy harbor is also a challenge.
Range measurements are made by measuring the time it takes an acoustic signal (a “ping”) to travel between the endpoints of the range of interest. In order for the range measurement (and hence the position determination) to be successful, the acoustic signal must be detected. A signal is “detected” if a pressure wave, of the proper frequency, has amplitude greater than a set threshold. All signal detection really means is hearing the “ping.” The “ping” must be of the proper frequency and loud enough to hear (i.e., a signal-to-noise ratio of greater than 1).
Figure 16.5 shows an example of the reception of a signal from above and below the noise and reception threshold level; the signal is accepted in “1” and rejected in “2.”
The performance of an acoustic positioning system can be predicted using a sonar equation that expresses the relationships between signal received and surrounding in-band noise. If a signal pulse results in a negative signal-to-noise ratio (or less than 5 dB), then most acoustic positioning systems will fail to detect the incoming signal (Wernli, 1998).
Signal-to-noise ratioðdBÞ5ðE
NÞ;