RESTRICTED 

 FIXING POSITIONS 



High latitude Mercator plotting sheets should be used by vessels 

 operating in polar waters for convenience in laying out courses even 

 though distortion is great. The polar plotting charts are generally 

 too awkward to handle in actual navigational problems although of 

 great value as a geographical guide and for planning purposes. Never- 

 theless, the advantages and disadvantages which each projection offer? 

 should be understood, and an analysis of these factors as related to 

 the particular problem at hand made as the best guide to the kind of 

 projection to use. 



When navigating in ice, it is necessary to get astronomical or land 

 fixes as frequently as possible in order to check the course and speed 

 made good. The resultant takes into account the ship's course and 

 speed and the drift of the ice. During summer in the polar regions, 

 the long days and short nights limit the number of stellar fixes. 

 Kunning fixes from sun sights offer the best means of determining 

 the vessel's position. The large amounts of cloud cover and of fog 

 add to the difficulty; therefore, no opportunity to take observations 

 should be missed. A routine schedule for navigation is out of the 

 question after entering an area where bergs, growlers, pack ice, fog, 

 and overcast are the rule rather than the exception. As the sun fre- 

 quently appears through the fog only for a short time, sextants should 

 be kept ready. Electronic aids to navigation, where they exist, should 

 be utilized as fully as possible. 



Sights must be taken with great care, for while in the pack false 

 horizons may frequently be observed. If the horizon is covered with 

 ice, it may still be used for astronomical observations by subtracting 

 the height of the ice above the water from the actual height of eye. 

 The possible error due to this cause is less than 4 minutes. It may 

 be preferable to make astronomical observations by using a sextant 

 with an artificial horizon. Excellent results have been obtained with 

 the bubble sextant by vessels operating in pack ice when there was no 

 excessive motion. 



Navigatoi-s usually avoid observations of bodies within 15° of the 

 horizon because of the significant variations in refraction in this band 

 of the sky. In polar regions the only available body may not exceed 

 an altitude of 10° for several weeks; in practice, therefore, there is no 

 lower limit to observations. Because of the low temperature in polar 

 regions the refraction correction for sextant altitudes should be ad- 

 justed for temperature (Table 25, Bowditch). 



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