antenna attached to the vehicle's bow which 

 activates an audible signal when it is point- 

 ing in the direction of the pinger. A commer- 

 cially available model of Helle's Pinger-Re- 

 ceiver (Model 6550) is attached to DS-2000 

 in Figure 10.33. The Helle unit is tunable 

 through 25 to 40 kHz, it is lightweight and 

 can be either self- or vehicle-powered. 



Transponders —Transponders offer more ex- 

 acting information, but they carry a price 

 several orders of magnitude higher than 

 that of a pinger. The cost is sufficiently high 

 (about $5,000) to warrant their retrieval, 

 whereas the pinger in the free-fall marker is 



considered expendable. Consequently, when 

 using transponders one must include a re- 

 lease mechanism (to sever the transponder 

 from its anchor) and budget some time for 

 subsequent retrieval. Also, the use of a tran- 

 sponder requires a transmitter and a re- 

 ceiver, whereas the pinger needs only a re- 

 ceiver. For such reasons the use of transpon- 

 ders as simple homing beacons has been 

 spotty. The advantages of a transponder are 

 longer operating life, longer-range reception 

 and range and bearing to the target instead 

 of bearing only. In addition to being a hom- 

 ing beacon, the transponder can also be used 



Rg. 10 33 DS-2000 with a Helle Engineering pinger-receiver on its brow for acquiring and homing in on a beacon transmitting at any frequency between 25 and 40 KHz. An 



extendable light boom is above the receiver. 



518 



