STUDY OF FREE-RANGING SHARKS 



429 



DATA 

 FRAME 



DATA 

 FRAME 



DATA 

 FRAME 



I 1 II --- 



N 



\ 



\ 



N 



\ 



B 



Ch 3 

 DATA 



APPROX 2-3 • 



1 



REFERENCE 



CHANNEL 



10 s 



Ch 1 

 DATA 



80 • 



Figure 5 Data formats for the eight-channel CSULB ultrasonic transmitters: (A) Rapid 

 multiplexing (one pulse interval per sensor), with three data frames shown. (B) Enlarge- 

 ment of one data frame from A, shows one reference channel and seven data channels. 

 (C) Slow multiplexing (fixed time per sensor), in this case shown as 10 s per channel, 

 thereby 80 s per full data sequence. 



free-ranging shark. One such function is transponding, in which interrogation 

 by the boat unit causes reply by the shark unit. The interrogate and reply 

 frequencies are normally different to prevent the shark unit's responding to 

 its own output. The primary advantage of transponding is that it permits 

 accurate measurement of shark-to-boat distance, thus facilitating more pre- 

 cise determination of the shark's position. Receiving equipment in the boat 

 provides this distance as a function of transponder reply time, based on the 

 speed of sound in seawater (1535 m/s at 25° C, 35 parts per thousand 

 salinity, and surface pressure). 



A simple transponder for this purpose can be made by adding a small 

 receiver /thresh old detector circuit to any existing UST. Reception of an 

 adequate interrogate signal triggers an additional normal transmitter pulse 

 (the reply pulse). Recognition of this reply pulse and measurement of its 



