Chapter 6 — PRINCIPLES OF SONAR 



Sonobuoys are used primarily by ASW units, 

 but they also are employed in harbor defense 

 Installations. The three methods of detecting 

 underwater targets are echo ranging, directional 

 listening, and nondirectional listening. 



The echo ranging type of sonobuoy uses a 

 miniature transducer for transmitting sound 

 pulses and receiving echoes. Operating depth 

 is down to 200 feet. Received echoes are trans- 

 mitted to the aircraft or ship in the form of 

 range information only. 



The directional type of sonobuoy provides 

 bearing information on detected underwater 

 sounds. The submerged unit rotates at a slow 

 rate. Sounds detected by the hydrophone are 

 amplified and frequency- modulated for radio 

 transmission. The transmitting frequency is 

 determined by the heading of a magnetic com- 

 pass at the instant of sound reception. 



The nondirectional type provides only detec- 

 tion information. Range and bearing of the sound 

 source are unknown. 



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