INTRODUCTION TO SONAR 



N 

 i 



SHIP'S HEAD 



029° RELATIVE 



'target 



/ 056' 



V- 



/;;:: 



45.29(71)F.2 

 Figure 3-8.— Target at 056° true. 



TARGET 



Figure 3-9. 



45.29(71)G.2 

 Target at 071° true. 



stated, the mathematical formula representing 

 the rule is given. The formulas are expressed 

 in fire control symbols. By means of these 

 symbols we avoid detailed explanations of the 

 values or measurements. Following are the 

 fire control symbols used in these formulas. 



By — True target bearing. 



B — Relative target bearing. 

 Co — Own ship's course. 



True target bearing equals relative target 

 bearing plus own ship's course. In formula, 

 this rule is expressed thus: 



By = B + Co 



If the true bearing, as computed, is greater 

 than 360°, that value must be subtracted from 

 the total. Our rule now is: True target bearing 

 equals relative target beai'ing plus own ship's 

 course minus 360°. 



By = B + Co - 360° 



If you know your course and the true target 

 bearing, the formula may be worked in reverse 

 to obtain the relative target bearing. Relative 

 target bearing equals true target bearing minus 

 own ship's course. 



B = By - Co 



If the true bearing is less than the ship's 

 course, 360° must be added to the true bearing 

 before subtracting ship's course. Relative target 

 bearing equals true target bearing plus 360° 

 minus own ship's course. 



B = By + 360° - Co 



If the true target bearing is 045°T and ship's 

 course is 270° true, for example, we find the 

 target's relative bearing thus: 



045°T + 360° = 405° - 270° = 135°R 



MOTION 



As mentioned at the beginning of this chapter, 

 the two types of motion are true and relative. 

 The Sonar Teclinician must have a thorough 

 understanding of both tj^es of motion in order 

 to interpret target movement correctly and to 

 give assistance in making an attack, 



TRUE MOTION 



True motion is the movement of an object 

 across the earth, using true (geographic) north 



as the reference point. 



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