428 MECHANICAL AND ACOUSTICAL SENSES 



to be used with sensors (variable-resistor types) and thus has good pulse-rate 

 stability. 



Other examples of circuits usable as either pingers or single-channel USTs 

 are given by Mackay (1970) and by various contributors to the "Underwater 

 Telemetry Newsletter." Circuits for miniature depth-sensing transmitters 

 using the strain-guage, bridge-type pressure sensors are given by Luke et al. 

 (1973) and by Pincock and Luke (1975). 



Multichannel— There are various methods by which data from several 

 sensors can be simultaneously telemetered from a single transmitter. One 

 way would be to encode Sensor 1 as pulse rate, Sensor 2 as pulse length, 

 Sensor 3 as frequency, and so forth, but this method has not been used 

 because of the problems associated with reception of pulse length and fre- 

 quency. Another method would involve transmission of each sensor on a 

 separate frequency channel, but this would involve the added complication 

 of a multifrequency or FM/FM (frequency-multiplexed) transmitting and 

 receiving system. To the author's knowledge, a system of this type has never 

 been used on free-ranging marine animals. The preferred method appears to 

 be time multiplexing, in which only one output frequency is used, but the 

 information from the sensors is sequentially switched onto that one fre- 

 quency channel. 



An example of a multiplexed UST is the eight-channel transmitter devel- 

 oped at CSULB for monitoring the behavior of sharks at sea, which has been 

 described in detail by Ferrel et al. (1974). An updated version of the circuit 

 is shown in Figure 4. Up to seven sensors can be incorporated, the remaining 

 channel being used as a reference for sequence identification. In the rapid- 

 multiplexing format, each sensor controls pulse rate for just one pulse inter- 

 val, then the next sensor is switched in. The resulting data (Figure 5) are in 

 the form of a series of eight-interval data frames, each frame covering about 

 2-3 s. In the slow-multiplexing format, the channels are switched at a pre- 

 selected clock rate, e.g., once each 10 s, allowing decoding of data by manual 

 stopwatch timing. As in the previous example, a reference channel identifies 

 the sequence of the other seven channels. This channel is set at a fixed pulse 

 rate beyond the range of the data channels, e.g., reference interval, 150 ms; 

 data intervals, 250-500 ms. 



A different method of distinguishing between multiplexed channels was 

 used in the two-channel temperature transmitter used by Carey and Lawson 

 (1973) for comparing surface and deep-body temperatures in free-swimming 

 tuna and sharks. This unit switched between sensors at the rate of once each 

 minute, and channel identification was accomplished by slight frequency 

 shifting (about 300 Hz) of the 21-kHz output frequency for one of the 

 channels. Thus, without changing the receiver tuning, tracking personnel 

 could determine the channel by the distinct shift in tone of the received 

 50-ms pulses. 



Acoustic-Command Function (e.g., Transponding)— There are several 

 advantages in having an acoustic -command capability that allows the tracker 

 to control certain functions of a transmitter while it is being carried by a 



