SECT. 3] APPLICATIONS OF THE GYROPENDULUM 331 



calibrate adjustment of the lower trunions (these should lie parallel to the level 

 east-west line maintained by the meridian gyro ; squaring-on is accomplished 

 by means of a long-radius ball-and-socket mounted directly on the servo-driven 

 meridian gyrocompass housing), (2) the altitude circle of the lower theodolite to 

 give the altitude of the polar axis or latitude, (3) the azimuth circle of the upper 

 theodolite to serve as an hour circle, and, finally, (4) the altitude circle of the upper 

 theodolite to serve as a declination circle. The following observational procedure 

 proved effective : 



(i) set the declination of the celestial body on circle 4, 



(ii) set the approximate ship's latitude on circle 2, 



(iii) set the approximate L.H.A. of the body on circle 3. 



Then, having sighted the body, make the fine adjustments of the altitude of 

 the polar axis required to bring the object to the equatorial cross-hair, lock 

 the polar axis, and, having set the hour angle cross-hair slightly west of the 

 position of the object in the field, note the G.M.T. of its crossing the given 

 L.H.A. The latitude of the sight position is given directly upon reading circle 2. 

 The longitude is easily reckoned from a knowledge of the G.H.A. of the body 

 and its observed L.H.A. 



The precision and reliability of the assembly is such that once the lower 

 theodolite has been squared on the gyro head (by taking sights on the horizon 

 in daylight) and the azimuth of the lower trunions adjusted (by taking sights on 

 celestial objects low in the eastern and western skies), it is possible to determine 

 the ship's position to an accuracy of ± 1 nautical mile (Figs. 6, 7 and 8) during 

 the course of the ensuing 36 h. 1 



To keep small the effects of atmospheric refraction and hunting of the gyro- 

 compass north point it is desirable to restrict sights to objects near the zenith. 2 

 This is easily accomplished at night, when the usual navigation stars are 

 accessible along with many other celestial objects, but less so by day. During 

 mid-day the sun is the obvious target, but in the early morning or late afternoon 

 it is preferable to choose a bright star or planet higher in the sky. Favoring the 

 visibility of such objects during the early and late hours of daylight is the fact 

 that the brightness of the sky tends to fall off rapidly with increasing angular 

 distances from the solar disc. Under favorable conditions it is possible to 



1 This accuracy was achieved in 1960-61. During 1962 improvements in gyro per- 

 formance and in observing technique have resulted in a decrease in the average error of 

 position fixing to about ±0.5 nautical mile, provided the ship has been underway at 

 constant speed and on a relatively straight course for two hours or more. In recent experi- 

 ments, the errors in fixing position have been assessed through a comparison of the 

 charted range and bearing to points on nearby islands with the range and bearing deter- 

 mined by ship's radar. 



2 When a suitable object is not to be found near the zenith in daylight, the polar axis 

 can be elevated to the zenith and the altitudes and azimuths of two or more objects 

 measured (without reference to the horizon) to determine lines of position in the con- 

 ventional way. Since both altitude and azimuth can be preset to lie within the field of 

 view of the telescope, the problem of finding celestial objects in full daylight is eased. 



