Page 347 equipment and instruments 4442 



at distances where it might be of use the results are not sufRciently precise. Any error 

 is an angular one, increasing the linear error proportionally to the distance. Radio 

 bearings may be useful as a last resort to fix the end of an astronomically controlled 

 sounding line, when fog or other conditions prevent the use of more accurate methods. 



Radio bearings are useful in navigation in clear weather as well as in fog. Collisions 

 with passing vessels can be avoided in fog by observing their radio bearings and 

 whether they change. 



Certain radiobeacons operated by the United States Coast Guard have their radio 

 signals synchronized with sound signals, so that, within audible range of the sound signal, 

 a distance as well as a bearing from the station may be determined. 



Unless the instrument is equipped with a gyro repeater, the helmsman or an 

 observer must note the ship's heading at each bearing and each bearing must be cor- 

 rected for the actual heading just as bearings by pelorus are corrected (see 4434). If 

 the minimum is indistinct or if the ship is yawing badly owing to heavy weather, the 

 mean of several readings should be used. 



4442. Radio Direction Finder Deviation 



Every part of a vessel and every object on board, capable of conducting electricity, 

 has an electromagnetic field which affects the direction of the received signal in a 

 manner similar to the effect of the ship^s magnetic field on a magnetic compass. This 

 error will differ for each bearing of the loop antenna with reference to the ship's heading. 



Each instrument should be adjusted to compensate for most of the eiTors and the 

 residual error should be determined for each position of the loop. This error is called 

 the deviation and a deviation curve can be plotted similar to one for a ship magnetic 

 compass. 



The deviation is determined from a ship swing within sight of a radio transmitting 

 station by taking several simultaneous visual and radio bearings on each heading. 

 The transmitting station should be at the optimum distance for the visual and radio 

 bearings, but never less than 1 mile. There should be little or no land between the 

 sMp and the station. Radiobeacons operated by the Coast Guard will broadcast 

 specially for use in calibration if requested. 



The differences between the visual bearings and the respective radio bearings, 

 referred to the centerline of the ship, are the deviations of the radio direction finder for 

 the various positions of the loop antenna. The visual bearings must be observed with 

 great care since on them depends the accuracy of all future radio bearings. The ship's 

 heading at each observation must be noted. The swing should be similar to that 

 required for a magnetic compass. 



The calibration of the radio direction fmder should be made with the vessel in 

 normal condition for sea, with all boats hoisted and movable objects in place. It must 

 be borne in mind that changes in the positions of large metal masses, particularly if 

 near the loop, may change the deviations of the instrument. 



The values of the deviations should be verified occasionally by simultaneous 

 visual and radio bearings. 



Usually the maximum deviation will be where the plane of the loop is at a 45° 

 angle to the fore-and-aft line of the vessel, and the minimum will be where it is parallel 

 or perpendicular to the fore-and-aft line. 



