CHAPTER 3 



BEARINGS AND MOTION 



In studying bearings and motion, you must 

 remember that they are of two types. The 

 two types of bearings are true bearing, which 

 uses true north as a reference, and relative 

 bearing, which uses ship's head as a reference. 

 True motion is the movement of an object across 

 the earth. Relative motion is the movement 

 of one object in relation to another object. 



This chapter discusses both types of bearings 

 and both types of motion. It shows how they 

 are applied in determining the location and 

 movement of an imderwater target, such as 

 a submarine. 



With modern sonar, it is possible to locate 

 and track a submarine with dependable accuracy. 

 In order to make full use of precision equipment, 

 it is necessary to evaluate bearings and motion 

 correctly and thus avoid setting erroneous values 

 into the equipment computers. 



REQUIREMENTS FOR LOCATING 

 SUBMARINES 



Many landmarks and familiar objects on 

 land can be utilized for establishing a position 

 and determining the location of a particular 

 point. In civilian life we were accustomed to 

 such descriptive terms as "The third house 

 on the right," or "Just a mile down the highway 

 at the big signboard." At sea, though, we must 

 adjust ourselves to doing without the convenience 

 of familiar objects and use other means at our 

 disposal for locating a target. Targets visible 

 on the surface or in the air can be pointed out, 

 but an unseen underwater object must be located 

 in a manner that is both clear and accurate. 

 This method of location is accomplished by 

 using sonar to determine the object's direction 

 and distance from the sound transmitting ship. 



In chapter 4 you will read about the numerous 

 variables affecting the travel of sound beams in 

 water. These variables greatly affect the pre- 

 cision with which the exact location of an under- 

 water target can be determined. With present-day 



equipment we are able to compensate for many 

 of the variables and obtain exact positioning 

 information. Electromechanical computers are 

 utilized in solving many of the problems of 

 target bearing and motion. It must be borne 

 in mind, however, that direction and distance 

 must be obtained first, then reported by the 

 operator before the computer can be set up 

 to produce a correct solution. Thus, the ability 

 to determine and report the correct bearing 

 and range as speedily as possible is of the 

 utmost importance to the entire attack problem. 



SONAR INFORMATION 



Sonar provides the two items of informa- 

 tion — direction and distance— from which we 

 derive all subsequent data. Direction and distance 

 are referred to as bearing and range. Correct 

 interpretation and transmittal of bearing and 

 range information are deciding factors in success- 

 fully conducting any attack. 



Bearing 



The direction of the echo from the sound 

 transmission source is called the bearing. Bear- 

 ing is measured clockwise in degrees of azimuth 

 in three figures from 000° through 360°. Azimuth 

 is defined as a horizontal arc of measurement 

 of the horizon in degrees. One quarter of a 

 circle is 90° and a full circle is 360°. 



Until a few years ago, an allowance of 2-1/2° 

 for error in bearing was considered within limits 

 of tolerance in antisubmarine attacks. Today, 

 this standard of tolerance has been reduced to 

 approximately 1°. A bearing error of 2-1/2° at 

 100 yards amounts to a position error of 4.36 

 yards; at 2000 yards this error is 87.2 yards. 

 With a bearing error of 1°, we are off target 

 only 1.75 yards at 100 yards, and 35 yards off 

 at 2000 yards. As you can see, modern equipment 

 has provided greater bearing accuracy. No doubt 

 future sonars will give even better accuracy. 



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