tist, and greater accuracy may not be con- 

 sistent with the requirements or time availa- 

 ble, or, most importantly, there may be no 

 alternative. For the submersible operator, 

 accuracy becomes an overriding considera- 

 tion when he asks: Where are we, or where is 

 it? because the answer must always be quali- 

 fied as: We're right here, plus or minus sev- 

 eral hundred yards, feet or several miles. 

 The inaccuracy in a position may have ex- 

 treme consequences underwater, because, 

 for example, precious battery power must be 

 spent maneuvering to find a particular site 

 or object. Searching for and acquiring an 

 object in the deep ocean is an extremely 

 frustrating experience. In the first place, one 

 only has the area of the vehicle's lights in 

 which to look. Secondly, once the object is 

 found the vehicle must stop, but stopping 

 takes some distance and the submersible 

 usually passes over or by the object. It then 

 must turn around and relocate it. This may 

 sound simple, but it isn't. For example, in 

 1970 under a contract with the Naval Ocean- 

 ographic Office off San Clemente Island, 

 DEEP QUEST passed over an unusual rock 

 pedestal at 2,820 feet deep. The operator 

 immediately bottomed the vehicle and 

 turned 180 degrees to return to the rock. 

 Some 2 hours later the search for the pedes- 

 tal was unsuccessful and terminated due to 

 the press of time. Numerous examples simi- 

 lar to this can be found in reports from other 

 vehicles. 



Let us hypothesize that the rock men- 

 tioned above had been located, and it became 

 necessary to surface and return to the rock 

 again. Generally, the procedure would be to 

 maneuver as close to the rock as possible 

 and, using whatever means available, obtain 

 one or more fixes (positions) on the rock's 

 location. Returning to the rock now intro- 

 duces another term describing a navigation 

 system's capabilities, "repeatability." Quite 

 simply, repeatability is a measure of how 

 closely the system can bring the vehicle back 

 to the rock, and in certain types of underwa- 

 ter work it can be as important as accuracy. 

 The geologist, for example, may be quite 

 content to know that the rock we mentioned 

 is approximately 5 to 6 miles south of San 

 Clemente; the exact geographic location (ac- 

 curacy) may be unimportant. What is impor- 



tant is that he can return to look at or 

 sample this rock without undertaking a ma- 

 jor search expedition. Likewise, the surveyor 

 who has found and mapped a suitable cable 

 route can accept the fact that the centerline 

 of the route is plus or minus a half mile from 

 some point on the earth's surface; his main 

 concern is that the centerline of this route 

 can be reacquired and the cable laid thereon. 

 Accuracy, therefore, is a measure of how 

 closely a navigation system can locate the 

 submersible on the earth's surface; repeata- 

 bility is a measure of how closely the system 

 can get it back to that spot. 



Surface Tracking 



In a few instances — especially in the early 

 years — the requirements for a geographic po- 

 sitioning system were rudimentary at best. 

 FNRS-2 and 3, TRIESTE I, SP-.350 and 

 ALVIN among others, asked nothing else of 

 the surface ship but to stand clear when they 

 surfaced. This was accomplished via the 

 underwater telephone, whereby the surface 

 ship, knowing that the vehicle was returning, 

 kept well clear. As a result, both were quite 

 far from each other and in 1965 ALVIIS re- 

 quired assistance from a Coast Guard aircraft 

 to reunite with its support craft (19). But, as 

 soon as technology made it possible, this 

 casual approach was replaced. What follows is 

 not an orderly chronological development of 

 submersible tracking systems because, quite 

 simply, there was and is no orderly develop- 

 ment. Some still use devices which were used 

 in early fifties, while others use systems of 

 the seventies. The discussion, therefore, shall 

 begin with the most basic and proceed to the 

 more sophisticated tracking systems. 



Marker Buoys: 



Simple in concept but quite difficult in 

 practice, one of the first attempts to ascer- 

 tain the whereabouts of the submerged vehi- 

 cle was the marker buoy. In this approach a 

 suitable length of line was attached to the 

 vehicle with a surface buoy on the other end. 

 The surface craft merely kept track of the 

 buoy and took compass bearings on it rela- 

 tive to itself. By keeping a continuous plot of 

 its own position, the surface ship needed 

 merely to draw a post-dive plot of its track 

 and that of the buoy to reconstruct the vehi- 



496 



