INTRODUCTION TO SONAR 



RtV>TIVE BEARin^ 



BEARING 

 CURSOR 



TARGET 

 ECHO 



200 , ' ' ' ' ' 160 

 190 180 ITO 



45.29(71)K 

 Figure 3-5. — Bearing and course indicator. 



but is aided by a video presentation 360° in 

 azimuth. The presentation is a true picture of 

 the area surrounding the ship, and, (with excep- 

 tions noted later in the text) it does not change 

 with alterations in ship's course. 



Although the video presentation is independent 

 of ship's heading, the Sonar Technician needs 

 to be constantly aware of the ship's course for 

 purposes of conducting search arcs, performing 

 control manipulations, and reporting sonar in- 

 formation. Ship's course information is provided 

 in the form of a stern line on the face of the 

 scope. The stern line, illustrated in figure 3-6, 

 is presented as an illuminated broken line, wliich, 

 as its name implies, indicates the direction of 

 the ship's stern. If this line indicated the bow 

 of the ship, it would be much easier to interpret 

 ship's course. Because most sonar information 

 comes from forward, however, the addition of 

 the stern line indicator in that direction would 

 tend to clutter the scope unnecessarily. 



Normally, with the ship's gyro operating, 

 own ship's movement does not appreciably alter 

 the presentation on the scope. Only the stern 

 line indicator shows when the ship is turning 

 or if a course change has occurred. When 

 turning right, the stern line moves clockwise; 

 when turning left, it moves counterclockwise. 

 Movement of the stern line alerts the operator 



STERN 

 CURSOR 



EXPANDING 



CIRCULAR 



SWEEP 



NOISE 

 INDICATIONS 



Figure 3-6. 



45.29(71)J 

 -Stern line indicator. 



to course changes, so that he then can be 

 prepared to make adjustments to keep the bearing 

 cursor on target. 



When looking at the scope, it is best to 

 picture the ship as in the center of a small 

 segment of the ocean, with the ocean always 

 oriented to true north at the top of the scope. 

 The ship turns in the center, and its direction 

 is shown by the stern line to indicate the recipro- 

 cal of the true course. 



GYRO FAILURE 



When the gyro is operating, the top of the 

 scope is north (000° true). When the gyro is 

 inoperative, the top of the scope is 000° relative 

 (ship's head). 



The first indication of a gyro casualty is 

 the erratic movement of the stern line or the 

 illumination of the red GYRO OFF light on the 

 sonar console. Any failure of gyro input to the 

 sonar causes the stern line to swing to 180° 

 relative, where it remains as long as the gyro 

 inputs are cut off. If the ship is headed for a 

 target when the gyro fails, the target echo 

 moves to 000° relative. Thus, the ship may 



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