unrealistic. Instead, the following discussion 

 proceeds from the simplest to the more so- 

 phisticated. 



Visible Markers: 



During the THRESHER search TRIESTE 

 I experienced great difficulty in obtaining 

 reliable underwater positions to assure that 

 a particular area had been searched and to 

 prepare photographic montages. A method 

 was evolved that consisted of laying individ- 

 ually numbered and color-coded plastic 

 markers at intervals along the ocean floor to 

 serve as visible landmarks (24). The markers 

 were 17- x 21-inch plastic sheets which were 

 rolled up and secured by rubber bands with 

 ends held by magnesium wire. The markers 

 were tied to 10-pound sash weights and 

 dropped by a surface craft along designated 

 tracks at 6-second intervals. Several factors 

 limited the effectiveness of these "fortune 

 cookies" (so called because they had to unroll 

 in order to be read): The tracks and spacing 

 for dropping could not be held constant, 

 many "cookies" failed to unroll, and the ef- 

 fects of variable currents considerably 

 changed the planned from actual landing 

 spot. Although some 1,441 markers were 

 dropped at about every 58 feet on an 11-line 

 grid, as much as 2 hours might pass between 

 TRIESTE''s marker sightings. Admittedly an 

 approach born from the lack of alternatives, 

 after the THRESHER search the "fortune 

 cookie" returned to its time-honored role of 

 providing a chuckle instead of a fix. 



Dead Reckoning: 



Dead reckoning (DR) is the determination 

 of position by advancing a known position 

 from a knowledge of heading, speed, time 

 and drift. It is reckoning relative to some- 

 thing stationary or "dead" in the water, and 

 hence applies to courses and speeds through 

 the water. Because of inadequate allowances 

 for compass error, imperfect steering and/or 

 error in measuring speed, the actual motion 

 through the water is seldom determined with 

 complete accuracy. In addition, if the water 

 itself is in motion, the course and speed over 

 the bottom differ from that through the 

 water. Geographically, a dead reckoning po- 

 sition is an approximate one which is cor- 

 rected from time to time as the opportunity 

 is presented. 



Dead reckoning systems used in submers- 

 ibles all rely on a magnetic compass or a 

 gyrocompass for course direction. Distance 

 has been measured with a wheel, by esti- 

 mates of speed/unit time and by Doppler 

 sonar. 



Every contemporary submersible lists 

 either a magnetic compass or a gyrocompass 

 (or both) in its onboard inventory. The for- 

 mer, in its simplest form, allows a free swing- 

 ing and dipping magnet to align itself with 

 the earth's magnetic field; the latter depends 

 upon one or more north-seeking gyroscopes 

 as the directive element(s) to indicate head- 

 ing relative to true north. The construction 

 and workings of the magnetic compass and 

 the gyrocompass are discussed in Bowditch 

 (15) in great detail and nothing more can be 

 added to clarify the subject herein. The use 

 of either compass to follow a specific course 

 is inordinately simple: The vehicle is turned 

 until the appropriate bearing matches a lub- 

 ber line (a mark on the inside surface of the 

 compass bowl which indicates foi-ward direc- 

 tion parallel to the longitudinal centerline) 

 and proceeds foi-ward on this course. A num- 

 ber of submersibles have repeaters which 

 are a part of a remote indicating system that 

 repeats the indications of the master com- 

 pass or gyrocompass. Magnesin repeaters 

 are quite frequently used, and the entire 

 system (master and slave) is referred to as a 

 Magnesin compass. These require an AC 

 power source and generally include a small 

 DC-AC inverter. 



Magnetic Compass — The magnetic compass is 

 simple, rugged, reliable and requires no elec- 

 tric power, but it has serious limitations in 

 submersibles. Since it responds to the net 

 local magnetic field, a steel hull, a wrench, 

 iron ballast, bars or shot, magnetic tape re- 

 corder, external instruments, internal re- 

 corders, pocket knives, keys or electrical con- 

 ductors near the compass can influence its 

 reading. In most submersibles it is difficult, 

 if not impossible, not to be near the compass. 

 For this and a host of other reasons, navigat- 

 ing by magnetic compass is fraught with the 

 potential for error. Because it is so widely 

 employed, one would expect that its applica- 

 tion in submersibles would have been thor- 

 oughly researched and studied, but, like the 

 lead-acid battery, reports or articles dealing 



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