4437 HYDEOGRAPHIC MANUAL PaGE 346 



4437. Alinement of Pelorus 



A pelorus must be installed so that the line joining its index mark and the center 

 of its dummy compass is parallel with the lubber's line of the ship compass, to avoid 

 erroneous bearings. After the standard compass has been installed, the alinement of 

 a pelorus may be tested, or a new pelorus oriented most accurately by simultaneous 

 observations by pelorus and compass on a celestial body or distant object. The angle 

 measured by pelorus should equal the difference between the compass heading and 

 bearing. Every pelorus with which bearings for use in hydrography are to be observed 

 should be tested in this way at the beginning of each season. 



Any of the following methods may also be used, but with less accurate results, 

 and they generally presuppose that the lubber's line of the compass is correctly alined 

 with the centerline of the vessel: 



(a) By reciprocal bearings between the standard compass and the pelorus. if the lubber's line 

 of each instrument is parallel to the centerline of the vessel, the bearings shotild differ by exactly 180°. 



(b) By computing the angle at the jack staff between the centerline of the vessel and a line to 

 the pelorus. The pelorus is then set at this computed angle and the line of sight should pass through 

 the jack staff. To compute the angle at the jack staff, find where a perpendicular from the pelorus 

 intersects the centerline of the vessel, and measure the two distances. The distance from the point 

 of intersection to the pelorus divided by the distance from the point of intersection to the jack staff 

 will be the tangent of the desired angle. 



(c) By equal offsets from the centerline while the vessel is stationary alongside a wharf. A point 

 is located on the wharf or on shore which is on the prolongation of the centerline. The distance 

 between the centerline of the vessel and the pelorus is measured. This distance is laid off from the 

 point on the prolongation of the centerline. The index of the pelorus should pass through this point. 



444. Radio Direction Finder 



A loop antenna attached to a radio receiving set receives an incoming signal at 

 inaximum strength when the plane of the loop is parallel to the direction of the incoming 

 signal, and at minimum strength when its plane is perpendicular to the direction of the 

 incoming signal. The change in intensity for a given angular rotation of the loop is 

 more marked near the minimum than near the maximum. These characteristics are 

 utilized in the radio direction finder to determine the direction of an incoming radio 

 signal. 



A radio direction finder, sometimes also referred to as a radio compass or radio 

 pelorus, is basically a special type of radio receiver equipped with a loop antenna above 

 the top deck of the vessel. The loop is mounted on a vertical shaft to whose lower end 

 is attached a compass card. The loop can be rotated in azimuth. 



Bearings are read from the compass card referred to the magnetic compass, in which 

 case the ship's heading must be marked at each reading, or from a bearing repeater in 

 connection with a gyrocompass systerh. 



The radio receiver must give a relatively loud signal so that the minimum will be 

 more sharply defined and should eliminate other undesired signals or interference. 



Operating and maintenance instructions are supplied by the manufacturer with each 

 instrument and should be followed. 



4441. Use of Radio Direction Finder 



The radio direction finder is useful principally in navigation, and a bearing obtained 

 with it is utilized just as a visual bearing is, and may be combined with other data to 

 determine a ship's position. It is rarely of value in hydrographic surveying, for 



