426 



MECHANICAL AND ACOUSTICAL SENSES 



listening at the right frequency. Furthermore, with directional receivers, a 

 relatively slow 360° scan of the "underwater horizon" is required to regain a 

 lost signal. If the frequency of the lost signal is uncertain, then the tracker 

 must make a separate scan at each of many different frequencies— a pro- 

 hibitively time-consuming process. This problem of missing the correct fre- 

 quency is most critical with the narrowband receivers that are usually pre- 

 ferred in tracking operations because of their greater noise-rejection capa- 

 bility. 



Serious consideration should therefore be given to the frequency stability 

 of any UST, even one intended only for general location tracking. While 

 some very simple, inexpensive pinger designs are available, they may suffer 

 from frequency variations in response to variations in temperature or battery 

 voltage. Extremely stable frequencies can now be obtained by quartz-crystal 

 control, such as in the CSULB Mark V transmitter diagrammed in Figure 4. 



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