marker (Fig. 10.31). The marker consists of 

 an anchor, 125 feet of ^/is-inch nylon line, a 

 syntactic foam float, a 37-kHz pinger and a 

 stabilizing fin which is attached to the float 

 and encloses the pinger. The stabilizing fin 

 also serves as a passive sonar reflector as 

 well as restricting the free-fall velocity. A 

 cross-sectional drawing of the marker is 

 shown in Figure 10.32. As the marker de- 

 scends, increasing water pressure collapses 

 the bellows (restrained at one end by a re- 

 tainer clamp) and the detent plunger re- 

 tracts and slides free, allowing the retainer 

 balls to fall into the cavity left by the detent 

 pinger, thereupon unlocking the ball cage 

 from the float assembly. Once separated, the 



float assembly trails behind the nose cone. 

 The plunger pin serves as a backup for the 

 mechanical release of the ball detent separa- 

 tion assembly. On contact with the bottom 

 the pin contacts the bellows causing it to 

 collapse and release the ball detent lock. The 

 particular pinger in this assembly operates 

 on 37 kHz and emits a 25-millisecond pulse 

 every 700 milliseconds for 21 days. 



The effective range of such pingers varies 

 considerably. In a 1966 test (23) ALVIN was 

 able to acquire the 37-kHz signal at 4,500- 

 foot range; greater distance might be attain- 

 able under ideal conditions. 



Reception of the pinger's signal is by a 

 simple hydrophone-like device or directional 



Fig. 10.31 Free-lall bottom marker (WHOI) 



516 



