316 Lecture 16 
A somewhat similar acoustic system working at 132 kcps has been used by 
Trefethen, Dudley, and Smith [15] to follow the movements of salmon. In their 
system, the complete transistorized transmitter is only 2.4 in. long and 0.9 in. 
in diameter, which is so compact that it can be fixed to the fish as a "tag" and 
followed from a boat using a 4-element receiving array and a servomechanism 
which keeps the receiver pointing at the salmon by a phase-comparison system. 
By controlling the repetition rate of a Pinger to precisely one per second, 
signals can be received and recorded on the precision echo-sounder receivers 
already described in Section 16.2.1. Edgerton and Cousteau [16] have used such 
a system for measuring the distance of a deep-sea camera from the sea floor 
by measuring the separation of the direct pulse from that reflected from the 
sea floor. 
Laughton [17] with his camera arranges for the pulse repetition rate ofa 
Pinger to change when the trigger-weight hits the sea bed and a photograph is 
taken. The signals are received by a hydrophone hung from the ship, and as soon 
as the change of rate of pinging is heard, the camera is raised a short distance, 
the ship is allowed to drift, and the camera is then relowered and another photo- 
graph taken. 
16.4.3. Acoustic Telemeters with Continuous Transmission 
A few telemetering systems have been built inwhich the information is trans- 
mitted over a continuous carrier. Perhaps the type of telemeter most in demand 
is one which transmits the depth of a piece of equipment, usually by measuring 
the external water pressure. Such an instrument is in demand, for example, by 
commercial fishermen who are becoming increasingly interested in catching 
shoals of fish in midwater using midwater trawl nets. Their echo-sounders tell 
them the depth of the shoal as they go over it and they want to make sure that the 
nets they aretowing are at the samedepth. The same problem is also encountered 
by biologists studying the scattering layers (see Section 16.3.2), and in other 
applications. 
A successful device of this type has been described by Stephens and Shea 
[18]; it is based on an instrument designed by W. J. Dow. In this instrument, the 
pressure is made to vary the frequency of a supersonic carrier, and the tem- 
perature is also measured and varies an audio-frequency which is transmitted 
as an amplitude modulation on the carrier. 
The physical arrangement of the system is shownin Fig. 16.16. The acoustic 
transducers have to have fairly wide beams, but even some directionality greatly 
<— SHIP'S WAKE 
TELEMETER 
TRANSMITTER 
DIRECTIONAL 
RECEIVING / Zk 
TRANSDUCER Se; 
TRAWL CABLE 
MIDWATER 
Fig. 16.16. The arrangement in use of an acoustic telemetering depth-of-net meter. NET 
