surface and inextricably bound to the sur- 

 face positions' inaccuracies. 



Bottom-Mounted Acoustic Systems: 



The most discussed and promising schemes 

 for underwater navigation are systems 

 which employ bottom-mounted acoustic bea- 

 cons of precisely known position to provide 

 near-continuous range and bearing data to 

 the submersible. From this information an 

 onboard processor displays and/or records 

 the vehicle's relative position in real-time. 

 The instruments and techniques used in this 

 approach are varied, but the concept is quite 

 simple and employs either pingers or trans- 

 ponders. 



Any number of bottom markers may be 

 used, but three is the preferred minimal 

 amount because the intersection of three 

 range vectors from three known positions 

 provides a triangle of error by which the 

 accuracy of the fix may be measured. A 

 strong attraction of this system is that re- 

 peatability can be theoretically within the 

 underwater range of viewing by the human 

 eye (approximately 30 feet) and the operator 

 can return time and again to the same loca- 

 tion as long as the marker network remains 

 functional. Either pingers or transponders 

 may be used. Rather than use hypothetical 

 examples of this system, two actual opera- 

 tions have been documented and serve as 

 good examples of the variations of this tech- 

 nique. 



Timed-Pingers —In 1967 ALVIN discovered an 

 F6F aircraft at 5,543 feet; an expedition was 

 undertaken to relocate the aircraft the fol- 

 lowing year using timed pingers for on-site 

 navigation. 



ALVIN^g initial position at the aircraft rel- 

 ative to the surface ship (LULU) was ob- 

 tained by using a tracking system. LULU 

 obtained its own position (directly over ALr- 

 VIN) with Loran A to a geographic accuracy 

 of ±1 mile. Hence, a circle of 2 miles in 

 diameter was established at the search area. 

 ALVIN also obtained a depth reading at the 

 wreckage site accurate to within ±20 feet. 

 (The source of this data is reference (28) 

 which relates in detail the many considera- 

 tions of the relocation approach, most of 

 which are not repeated here.) 



The operation plan envisioned dropping 



two timed-pingers upslope of the aircraft on 

 the 5,250-foot contour and navigate relative 

 to them. 



The heart of the timed-pinger system is a 

 master clock on the submersible which is 

 synchronized with acoustic transmitters on 

 the beacons before deployment and thence- 

 forth serves as the time standard. The two 

 beacons transmit a pulse (one at 4 kHz; the 

 other at 5 kHz) at the same instant and the 

 arrival times of these pulses are measured 

 by ALVIN, processed and displayed digitally 

 and graphically as slant ranges from sub- 

 mersible-to-pinger. A knowledge of the water 

 sound velocity is required to obtain the opti- 

 mum accuracy, as well as an extremely accu- 

 rate master clock. 



The first beacon was dropped by LULU at 

 positions obtained by Loran A and echo 

 sounder depths. The second beacon was 

 dropped at a pre-determined slant range ob- 

 tained from the first and a surface-obtained 

 echo sounder depth. The resultant base line 

 distance between the two pingers was 5,180 

 feet. ALVIN subsequently dived and obtained 

 a depth reading on each and, alternately, a 

 range from one to the other. The submersible 

 then conducted its search along a depth con- 

 tour (1,709 decibars), obtaining its position 

 from the pingers. A reconstruction of AL- 

 V/iV's track during this operation is pre- 

 sented in Figure 10.25. Adding to the sys- 

 tem's accuracy, a sound velocity meter was 

 carried by ALVIN to measure the local area 

 structure. 



Comparing the timed-pingers to transpon- 

 ders (discussed in the following section) 

 Rainnie (19) lists these advantages of this 

 system: Any number of vehicles may use it 

 concurrently; the one way acoustic path de- 

 creases inaccuracies due to sound speed vari- 

 ations; electronic "decision making" proce- 

 dures in the pinger are not required; am- 

 bient noise has no effect and the circuitry is 

 simple. Rainnie also lists such disadvantages 

 as higher energy (power) requirements and 

 degradation of accuracy with time. 



Transponders — This system follows the same 

 format as the foregoing, the important ex- 

 ception being that the submersible must in- 

 terrogate (command) the transponder to ob- 

 tain a range. The primary equipments are an 

 interrogator (generally a CTFM sonar), 



510 



