STUDY OF FREE-RANGING SHARKS 



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Figure 3 Basic signal parameters for pulsed ultrasonic transmitters. Oscillographic 

 representation of signal pulses (tone bursts) of frequency, 40 kHz; pulse length, 10 ms; 

 and pulse rate, 4/s. 



A. Pulse interval (0.25 s). Pulse rate is the reciprocal of pulse interval, 1/0.25 = 4.0/s 

 pulse rate. 



B. Pulse length (pulse width), e.g., 10 ms. 



C. Length of one cycle (0.025 ms). Frequency is the reciprocal of cycle length, 

 1/0.025 = 40 kHz frequency. 



D. Pulse amplitude (peak-to-peak), corresponding to sound pressure in the medium or 

 to voltage in the hydrophone, receiver, or oscilloscope. Volts (root mean 

 square) = volts (peak-to-peak)/2.83. 



Location-Only (Pinger)— The simplest types of USTs are those with- 

 out sensors— often referred to as "pingers"— which transmit pulses at a 

 relatively fixed rate. Units of this type were used by Bass and Rascovich 

 (1965), who were the first to show the feasibility of tracking large, fast- 

 swimming fishes (tuna, sharks) from a moving vessel at sea. Since the general 

 location of the tagged individual is the only information obtainable from a 

 pinger, instabilities in the frequency or pulse rate are of no consequence 

 unless they adversely affect signal reception or the ability to distinguish 

 between units. 



Simultaneous tracking of several recognizable individuals requires that 

 each pinger be set at a different frequency and /or pulse rate; these should be 

 reasonably stable as temperature, pressure, and battery voltage vary. Con- 

 stant frequency is particularly important because drifting frequency can lead 

 to confusion in distinguishing individual units. In addition, if the frequency 

 drifts away from the expected, especially after a prolonged tracking inter- 

 mission, the trackers may fail to regain the signal because they are not 



