operator errors and time errors are the 

 greatest adversaries. Even if we assume that 

 all of these errors are zero and that no 

 currents are present, the speed of most sub- 

 mersibles is rarely precisely known and this 

 leads to a larger error which accumulates 

 with distance traveled. No point is served in 

 further belaboring the inadequacies of this 

 approach. In the final analysis it is so full of 

 unknowns that to consider it as more than a 

 gross position approximation would be a mis- 

 take. Two other DR systems, which do not 

 rely on vehicular speed, are the Unigator 

 and Doppler Sonar. The former has limited 

 application; the latter is rapidly becoming a 

 quite useful tool. 



The Unigator — The Unigator (unicycle-navi- 

 gator) is merely a weighted bicycle wheel 

 suspended on a rod which the submersible 

 tows over the bottom (Fig. 10.23). An odome- 

 ter cable is attached to the wheel and the 

 odometer display is mounted externally on a 

 viewport. Used in conjunction with the vehi- 

 cle's compass or gyrocompass, the odometer 

 measures distance traveled along a particu- 



■v«^ 



Fig 10 23 The Unigator 



lar bearing. The Unigator has a major short- 

 coming: It measures every undulation of the 

 bottom. Hence, on all but a flat bottom, the 

 wheel provides a distance in excess of that 

 actually traveled on a straight path. There 

 are quite a few flat ocean areas where dis- 

 tance measurements by the wheel would be 

 quite accurate. If, for example, a submersible 

 were inspecting a submarine cable, the 

 length of cable inspected, as measured by the 

 wheel, would be as accurate as any method 

 in existence (providing the bottom was flat 

 or gently rolling). Additionally, the Unigator 

 serves a function similar to that of the bathy- 

 scaph's guide chain. Once the vehicle is 

 trimmed to a point where it is being held to 

 the bottom only by the weight of the wheel, 

 no further adjustments are required to keep 

 it at a constant altitude. The designers of 

 this method relate the details of its construc- 

 tion and operation in reference (25). Signifi- 

 cantly, they report a test run along a trian- 

 gle of 0.1 nm length on each side which 

 resulted in the submersible (PC-3B) being 

 offset by only 25 feet from its starting posi- 

 tion (a buoy). The error was attributed to 

 currents exceeding 1 knot which swept the 

 test area and displaced the submersible. As a 

 geographic positioner, the Unigator offers 

 far less errors than "fortune cookies" and is 

 probably better than dead reckoning by 

 speed-time estimates. Under the proper con- 

 ditions it is probably as good as any in exist- 

 ence, and undoubtedly one of the least ex- 

 pensive to measure bottom distance. 



Doppler Sonar — The Doppler principle of 

 speed measurement is based on the fact that 

 a signal transmitted from a moving object 

 and reflected from a stable surface is shifted 

 in frequency proportional to the speed of the 

 object in relation to the surface. In Doppler 

 Navigators, sonic beams from a moving sub- 

 mersible are directed at the bottom and are 

 reflected back at a frequency which differs 

 from the transmitted frequency. The Doppler 

 unit measures the speed directly by detect- 

 ing and quantifying this frequency shift. On 

 surface ships pitch, roll and heave, however, 

 add another apparent motion with respect to 

 the bottom. To cancel out this effect, present 

 Doppler sonar models employ two pairs of 

 beams instead of one. A pair of beams is 

 angled fore and aft, and another pair angled 



508 



