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sumes the iceberg to be motionless in the water, an assumption not 

 believed to be justified in most instances. 



In depths where it is possible to sound, drift may be determined 

 with some accuracy. If the vessel is surrounded by ice, a hole can be 

 cut, the deep-sea lead dropped through it, and the trend of the drift of 

 the ice thus obtained. To determine the drift it is recommended to 

 make use of all enforced stops and even to stop the vessel especially to 

 carry out the necessary observations. 



Some navigators ignore such methods of reckoning because they 

 require too much time and are too detailed. They estimate by eye, 

 both the general course and the distance made good. As a result, 

 there have often been cases where a ship has found herself a consider- 

 able distance from the position estimated by the navigator. When 

 steaming in dangerous regions, the ship can easily run aground or 

 expose itself to other unexpected hazards. Because of insufficient in- 

 formation on tides as well as other factors, the most accurate dead 

 reckoning navigation will not result in giving the exact position of the 

 ship, but good and careful reckoning in accordance with the above 

 methods helps reduce errors. 



RADAR IN ICE 



The reliable detection of ice with radar is dependent upon the fol- 

 lowing factors : 



(a) Condition of the equipment. The importance of keeping the 

 radar at top operating efficiency for ships operating in ice areas cannot 

 be stressed too highly. It is possible, without adequately trained per- 

 sonnel, for a radar set to be 25 decibels down without the fact being 

 noticed. In calm to slight seas the detection of bergy bits or growlers 

 in time to avoid collision can safely be expected only when equipment 

 is operating at peak performance. 



( h ) State of the sea. As the state of the sea increases, so does the 

 minimum size of berg that can be detected. On very rough seas, bergs 

 as high as 50 feet cannot always be detected in the sea return. Only 

 in exceptionally smooth seas can radar be depended upon to pick up 

 growlers. 



(c) Weather conditions. For radar of both "S" and "X" bands, 

 the echo from certain weather features such as rain clouds may at 

 times obscure returns from ice. Meteorological conditions in certain 

 areas affect radar propagation in a manner that may under certain 

 conditions reduce range in fog where radar is most needed. 



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