aboard DS-2000 . Almost all are adopted 

 from aircraft designs because of their light 

 weight and small size. One of the smallest is 

 R. C. Allen's Electronic Direction Indicator 

 used aboard PC-14 and PC-9 (TS-1) and 

 shown in Figure 10.21. This unit is about the 

 size of a tennis ball can and may be mounted 

 directly on an instrument panel. Preliminary 

 tests, however, showed a drift rate of some 8 

 to 9 degrees in 2 hours. It is not clear 

 whether the instrument finally stabilized or 

 would have continued its precession, or 

 whether it was being affected by the sub- 

 mersible's {PC-9) electronics. Regardless, 

 once the steady-state drift rate has been 

 definitely established, it is possible to com- 

 pensate for it and maneuver accordingly. 



A few vehicles use both a magnetic com- 

 pass and a gyrocompass. Both SEA CLIFF 

 and TURTLE use a gyrocompass as the pri- 

 mary direction indicator and a Magnesyn 

 compass as a backup indicator. To reduce the 

 influence of the submersible's magnetic hull, 

 the master compass (called a "transmitter") 

 is mounted outside the hull in a pressure 

 compensated container. The repeater (indica- 



*^^ 



E^-Sl^.-^ 



"i^MlMIES 



Fig. 10.22 To reduce Ihe interfering eflects of the tiull on its magnetic compass, 



SHELF DIVER'S compass is mounted aft of tfie conning tower in tfie bubble-like 



protrusion 



Fig 10 21 An R C Allen electronic direction indicator aboard PC-14 



tor) is mounted on the operator's panel and 

 is shielded to prevent magnetic interference. 

 Locating the transmitter external to the 

 pressure hull is not original to the Navy's 

 submersibles; Perry's SHELF DIVER (Fig. 

 10.22) and others follow the same procedure. 



Also adopted from the aircraft industry is 

 the directional gyro used aboard SDL-1 . The 

 directional gyro is essentially a gyroscope 

 pointed in a desired direction which it main- 

 tains for some period of time. Its primary 

 purpose is to permit the operator to steer a 

 relatively straight line. The drift rate of 

 SDL-l''s directional gyro is approximately 1 

 degree per hour and, hence, it is not intended 

 for extended dead reckoning. 



Generally, a dead reckoning position is ob- 

 tained by starting at some known point (de- 

 termined by the surface ship) and carrying 

 this point along a course, the direction of 

 which is given by the compass or gyrocom- 

 pass, the distance derived from speed x time. 

 The potential for error in this approach has 

 been discussed. Currents, instrument errors, 



507 



