4443 HYDROGRAPHIC MANUAL PaGE 348 



4443. Radio Bearings 



A radio direction finder is generally used to determine the bearing of a radio signal 

 transmitted from a station whose geographic position is known. There are four general 

 classes of stations whose bearings may be observed: 



(a) Radiobeacons operated as aids to navigation bj- the Coast Guard. These operate con- 

 tinuously during fog or low visibility, and in clear weather during certain specified intervals. Each 

 station emits a characteristic radio signal by which it may be identified by anyone without knowledge 

 of the radio code. They are installed on lightships and near lighthouses where bearings to them will, in 

 most cases, be entirely over water. The positions and characteristics of these stations and other 

 valuable information are contained in the Light Lists of the United States Coast Guard, and in H. O. 

 Publication No. 205, Radio Navigational Aids; their characteristics are also given on small reference 

 radiobeacon charts published by the Coastguard. All are charted on the nautical charts. 



(b) The transmitters of direction-finder stations operated by the Coast Guard. These stations 

 are primarily for furnishing bearings to vessels not equipped with radiocompasses, but bearings of 

 their transmitters may be observed. A list of these with their call letters and characteristics and with 

 the geographic positions of the transmitters is contained in H. O. Publication No. 205; other operational 

 information is included. Similar information is in the Light Lists, except for the positions of the 

 transmitters. All are charted on the nautical charts. 



(c) Other federal and commercial shore radio stations near the coast, whose positions are known. 



(d) Ships at sea. 



For bearings on stations in (b), (c), and {d), except broadcasting stations, it is 

 necessary to call the station by its radio call letters for identification and perhaps to 

 get it to transmit. 



It should be borne in mind that the published positions of stations in (c) are only 

 approximate in many cases. Also such stations are likely to be inland from the coast 

 and any intervening land may affect the direction of the received signal. It is best 

 not to use stations in (c) unless they are known to be very near the shore. 



4444. Accuracy of Radio Bearings 



In navigation, if three radio bearings intersect at a point, or nearly so, it may be 

 generally assumed that a reasonably accurate position of the ship has been obtained. 

 Less trust should be placed in radio bearings taken when conditions appear to be unfa- 

 vorable, as for example, when they may be affected by excessive static, or by night 

 ejfect; at such times they should be checked by repetitions and by any other means 

 available. Radio bearings obtained with a radio direction finder on board ship may 

 generally be considered to be correct within 2° if the following precautions are obsei"ved: 



(a) Radio bearings should be observed by an experienced operator. 



(b) The received signal should be sufficiently strong for the minimum to be clear cut and well 

 defined. 



(c) Radio bearings on stations more than 150 miles distant should be considered approximate. 



(d) On some instruments the operator cannot tell whether he is measuring the bearing or its 

 reciprocal. 



(e) The deviation of the radio direction finder should be known accurately and applied. 



(/) Unless a gyro repeater is used, the ship's heading should be noted at the instant each bearing 

 is observed, and any correction applied. It should be noted that the accuracy of a radio bearing 

 depends on the accuracy with which the ship's heading at the instant of observation is known. The 

 same care is required as for bearings by pelorus or azimuth circle. If the heading depends on a mag- 

 netic compass, the accuracy with which its deviation and the variation are known also enters into the 

 • result. 



(g) The bearing of a station, whose radio signal passes over land in reaching the ship, should be 

 considered approximate. 



