MAGNETIC METHODS 



239 



stationary. The magnetometer, altimeter and ground photographs are 

 marked periodically with fiducials to indicate corresponding points on the 

 records. The strip photographs are compared to photo-mosaics of the 

 area flown and the flight path is transferred from the flight strip photo- 

 graphs to the mosaic. 



The photographic method of positioning is not usable over water, desert 

 or other areas of featureless terrain. In such cases radio methods which 



Fig. 118. — JNIagiietometer installation in DC-3 plane. (Left) — Shoran navigation 

 equipment. (Right) — magnetometer electronics and continuous recorder. (Courtesy of 

 Gulf Research and Development Company.) 



involve the measurement of distances of the plane from two or more fixed 

 ground stations are used. What is measured is essentially the transit time 

 of a radio-pulse from the aircraft to a ground station and back to the 

 plane. In determining distance in this way, the instrument time-lags must 

 be taken into account. One radio system of this kind is known as Shoran. 

 In this system the Shoran dials are photographed, which provides the posi- 

 tion information when properly correlated with the magnetometer and 

 altimeter records. 



The Shoran method can often be used advantageously for position deter- 

 mination in areas in which the photographic method is perfectly feasible. 

 Exceptions are where the flying is done at low levels, in which case the dis- 

 tance range of Shoran would be too limited, and in cases where suitable 

 sites for ground Shoran stations are not accessible. 



It is quite obvious that, to take full advantage of the magnetometer 

 record, the corresponding positions must be quite accurately known. In 



