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The flux gate compass has proven less shiggish and has given 

 fairly accurate and reliable results. In high latitudes the gyro- 

 compass performs satisfactorily if properly adjusted but, since it 

 is always subject to mechanical failure, two gyrocompasses should 

 be installed in addition to the magnetic compasses. 



During the voyage no opportunity should be missed to observe 

 the errors of the compasses, particularly by azimuths of the sun. 

 An azimuth attachment for a telescopic alidade is recommended; 

 it may be of value in obtaining accurate azinmths for determining 

 gyro error when the sun is not brilliant enough to obtain an azimuth 

 by the use of an azimuth circle. The present azimuth tables for 

 high latitudes can be used only during a certain portion of the day 

 but computed azimuths for any time may be taken from H. O. 

 Pub. No. 214. 



DEAD RECKONING 



The Dead Reckoning Analyzing Indicator (DRAI) is designed 

 for use below latitude 70° and above this will not function. For 

 ease in navigation it is suggested that a ship planning to operate 

 above latitude 70° have its DRAI factory-adjusted to perform in 

 latitude up to 85°. On Operation NAXOOK the Afvie was 

 equipped with a DRAI which, although not adjusted for high lati- 

 tude operation, was made to work satisfactorily by the application 

 of a few well thought out corrections. 



Two systems were devised for using the DRAI above 70°. In 

 both systems it was set back some number of degrees of latitude. 

 Between 70° and 75°, for instance, it was set back 5°. This made 

 the DRAI read 65° when the latitude was actually 70°; (56° was 

 71°, etc. The DRAI latitude was then corrected by adding 5° to 

 it, but the longitude was in error because the distance between 

 meridians is a function of the cosine of the latitude, and the DRAI 

 latitude was 5° in error. The problem therefore became that of 

 determining the correct longitude. 



One method of correcting longitude is to determine the mean lati- 

 tude between the last DRAI latitude and the present DRAI latitude. 

 These are the actual readings of the DRAI, not corrected by adding 

 5°. The mean DR latitude equals the mean DRAI latitude plus 5° 

 in this case. The correction to the DRAI longitude can then be 

 computed by using the following formula : 



^ ^ , (longi — long.) cos (mean DRAI lat.) , , 



Corr. to long.=-^ ^ ~^, Woi-r^ where longi 



*= Cos (mean DR lat.) 



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