10.28. In this example the array is ship- 

 deployed and consists of an anchor, a release 

 mechanism, a sonar reflector, a pinger or a 

 transponder, a connecting cable and a buoy- 

 ancy element to hold the array at a desired 

 level off the bottom. To use this method (Fig. 

 10.28) as a homing device, the submersible 

 must have a transmitter/receiver (trans- 

 ceiver) for the transponder, a receiver for 

 using only the pinger or a transducer for the 

 sonar reflector. The information required by 

 the operator to close with the array can be in 

 several forms: Range and/or bearing dis- 

 played as a "blip" on a CRT with which the 

 operator visually closes; an audible tone 

 emitted by a speaker and which is loudest 

 when the vehicle is oriented directly toward 

 the sound source; or a pair of sonic-activated 

 lights, one of which blinks to indicate direc- 

 tion to the sound source. On CTFM sonars 

 and some others, both audible and visible 

 displays are provided. The audible display is 



BLINKING 

 LIGHT \ 



BUOYANCY 

 ELEMENT 



X?=L 



PINGER 



OR 



TRANSPONDER 



SONAR 

 REFLECTOR 



PULL 

 RELEASE 



- (AO 



Fig 10 28 Elements o( a hypothetical homing array. [From Ret (30)] 



handy, in that the operator need not take his 

 eyes from the viewport or TV monitor to 

 obtain direction. To release the array from 

 its anchor for subsequent surface retrieval a 

 submersible would require a manipulator or 

 some grasping/pulling device to release the 

 pull-pin in Figure 10.28. A further refine- 

 ment, quite helpful in visual detection, is a 

 flashing light which can be detected for some 

 distance in the absence of natural sunlight 

 or the submersible's artificial light sources. 



The sources and nature of these compo- 

 nents are quite numerous and varied. The 

 trade-offs inherent in the choice of different 

 operating frequencies are the same as those 

 encountered in the selection of an underwa- 

 ter telephone: Low frequency provides 

 greater range with decreased resolution; 

 higher frequency provides shorter ranges 

 with increased resolution. 



Illustrative of the devices used in homing 

 (and navigation as well) are those developed 

 by the ALVIN Group at Woods Hole during 

 the late sixties. These are thoroughly de- 

 scribed in reference (30), and the following 

 examples are taken from it and reference 

 (23). 



Acoustic Reflectors —A Tri-plane acoustic re- 

 flector (Fig. 10.29) was developed by WHOI 

 for ranging and homing out to 600 yards on a 

 frequency of 72 to 82 kHz. The steel plates 

 are Vs-inch thick and the geometric design of 

 the Tri-plane provides sufficient flat surfaces 

 to reflect a major portion of the acoustic 

 pulse. The disadvantages of reflectors are 

 that line-of-sight conditions must exist and 

 back-scatter from hilly terrain in the area 

 may completely mask the reflector's return. 

 Field tests with ALVII^ in 1966 produced 

 usable reflections from 1 ft^ reflectors of this 

 type to ranges of 230 yards with the sub- 

 mersible 10 feet off the bottom. Visual range 

 to the reflectors (under artificial light) was 

 from 10 to 20 yards. In some cases reflecting 

 tape on the Tri-planes produced visual 

 ranges of 30 to 50 yards. 



The Benthos Corp. supplies a hollow glass 

 sphere capable of withstanding virtually any 

 ocean depth. Such spheres are better sonic 

 reflectors than are steel plates, and a bat- 

 tery-powered flashing light inside the sphere 

 offers better visual contact than does reflect- 

 ing tape. Benthos' spheres and lights were 



514 



