SCIENCE AND THE SEA 



ECHO-SOUNDING 



Submarine topography is becoming increasingly important to 

 the mariner as a means of navigation. With the development of the 

 modern sonic sounding equipment found on most naval vessels and 

 many merchant ships, it is possible to record depths up to 6,000 

 fathoms with an error of approximately one fathom. These sounding 

 devices have made the profile of the ocean floor potentially the most 

 universally accessible aid to navigation yet envisioned. Recent 

 hydrographic surveys have given special prominence to this work 

 and, as adequate bathymetric charts become available, navigation 

 by underwater features may become as common as coastal piloting. 

 Few bathymetric charts have been developed, however, for full reliance 

 in navigation. 



The standard velocity of sound waves as calibrated for all 

 American-made equipment is 4,800 feet per second. Although the 

 true velocity varies wit a local values of temperature, water pressure, 

 and salinity, the difference is not considered important except in 

 highly technical research work. It is the policy of the Oceanographic 

 Office to chart all soundings on the basis of this standard value. 

 Soundings obtained by equipment not calibrated to the American 

 standard will not agree with the depths shown on H. 0. Charts. 



Echo sounding equipment, like any aid, is subject to errors if the 

 navigator is not fully familiar with equipment operating character- 

 istics and limitations. The routine checks outlined in the instruction 

 manual should be carefully conducted at least once a watch. 



The phenomenon known as "phantom bottom" has caused 

 considerable confusion among many navigators. The phantom bot- 

 tom appears on the trace as a bank between 125 and 375 fathoms 

 below the surface and is only detected during daylight hours. The 

 exact reasons for the occurrence of this phantom bottom return are 

 not definitely known, but it has been experienced in most parts of 

 the world. One theory offered is that concentrations of marine life 

 descend to this area during daylight hours and then rise nearer the 

 surface during the night. The navigator can often rule out these 

 false returns by carefully checking them against known charted 

 depths. 



Excessive underwater turbulence which aerates the water can 

 distort the outgoing signal (sound waves) to the point of preventing 

 any echo from being received. Usually, this condition is only a prob- 

 lem when the vessel is rolling or pitching in heavy seas, backing down, 

 or steaming in column formation. 



Another cause of significant error is fluctuation of the current 

 supply driving the depth-indicator motor. The accuracy of 

 soundings is directly related to the revolutions per minute of this 

 motor which normally operates on a 60-cycle supply. A change of 

 one cycle, say 61 cycles, would cause an error of about 33 fathoms in a 

 recorded depth of 2,000 fathoms. The navigator should be alert for 

 this problem at all times. 



NIGHT VISION HORIZON 



During World War II, with our submarine forces operating along 

 hostile shores for prolonged periods of time, an urgent need arose for 

 fixing position without revealing presence to the enemy. The use of 

 electronic aids was too risky in most cases and had to be forsaken in 

 favor of celestial observations taken late at night. 



Confronted with this situation, the submariners soon developed 

 a highly reliable skill of observing stars against a night-vision hori- 

 zon. The technique requires some preparation which at first may 

 seem somewhat foreign to the surface mariner, but its usefulness 

 should not be overlooked. 



The observer's eyes must be completely "dark adapted." Proper 

 dark adaptation can best be accomplished by wearing red goggles for 

 at least 30 minutes prior to going on the bridge for observations. 

 Once on the bridge, and in complete darkness, the observer must 

 spend an additional 20 minutes further adapting his eyes to the sky 

 and horizon. Great care should be taken not to look at any light or 

 to use binoculars, because, by so doing, dark adaptation can be 

 instantly lost and the entire time-consuming procedure would have 

 to be repeated. 



When the observer can see the horizon, he should send for a 

 reliable assistant. The assistant brings the sextant, hack chronom- 

 eter, and a flashlight fitted with a red lens emitting only a very dim 

 light. It is also advisable for the assistant to dark adapt his eyes. 



Once on the bridge, the assistant hands the observer the sextant 

 set at the approximate altitude of the first star to be observed. He 

 then stations himself behind the observer, back to back, illuminates 

 his hack chronometer and waits for the "Mark!" 



Navigator and assistant, having completely "dark adapted" their 

 eyes, prepare to fake round of sights against a Night Vision Horizon. Dark 

 adaption can be instantly lost by looking at any artificial light source. 



The observer holds the sextant upside down, pointed at the star, 

 and brings the horizon up to the star. Next, the sextant is reversed 

 and the star is adjusted to the horizon in the normal manner. During 

 the observation it is extremely important that the observer does not 

 look directly at the horizon. Instead, he should look up about 20°, 

 keeping both eyes open and dim the star with a filter until it can 

 scarcely be seen. When the observer is ready to "Mark", both eyes 

 are closed for about 5 seconds. The eyes are then opened wide and 

 the sight taken when in focus. 



After the first sight is taken, the observer must be careful not 

 to look at any light source until he has taken all the other sights he 

 needs. The sights may then be worked by any method suitable to 

 the observer. 



SIGHT ERROR COMPENSATION 



When possible, the navigator should take star sights both north 

 and south of the zenith as this will tend to eliminate all systematic 

 errors from the results. For example, one navigator might consist- 

 ently bring his stars down too low, while another might tend to keep 

 his too high ; the horizon might be abnormally elevated or depressed ; 

 the actual refraction might be somewhat different than tabulated; 

 or the sextant error allowed for incorrectly. 



If the total effect of these errors makes the altitude too high, a 

 northern star will give a latitude too far north and a southern star 

 too far south. 



STAR 



SOUTH 



OF ZENITH 



STAR 



NORTH 



OF ZENITH 



^ 



i^ 



SHIP N' 



Figure!. Sum of navigator's systematic errors result in observed altitudes 

 that are too high. 



In figure 1, the sum of the systematic errors, in each case, gives 

 altitudes that are too high which result in apparent positions for the 

 ship at N' in the case of the northern star and at S' for the southern 

 star. The actual position lies about halfway between N' and 

 S', at Ap. 



In figure 2, the sum of the systematic errors gives altitudes that 

 are too low. The actual position, however, still lies at Ap, about half- 

 way between the apparent positions, N' and S'. 



STAR 



SOUTH 



OF ZENITH 



STAR 



NORTH 



OF ZENITH 



1^ 



ik 



Ap 



SHIP S 



Figure 2. Sum of navigator's systematic errors result in observed altitudes 

 that are too low. 



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