INTRODUCTION TO SONAR 



BEARINGS 



In establishing a bearing, we invariably must 

 have a reference point to ensure that the direction 

 is always the same. True bearing is referenced 

 to true north regardless of the direction or 

 motion of the ship. Relative bearing is always 

 referenced to the ship's bow. In all instances, 

 bearings are reported in degrees and are read 

 clockwise. 



Sonar bearings are reported in three figures, 

 and you say "zero" instead of "oh." A sonar 

 contact due east of your ship is reported as 

 "Sonau" contact, bearing zero niner zero." If 

 the contact were due west, it would be reported 

 as "Sonar contact, bearing two seven zero. 



Notice in the foregoing examples that the 

 word "true" is not used. Unless stated other- 

 wise, always assume that bearings are reported 

 as true bearings. If a gyro failure occurs, 

 relative bearings must be used, and the sonar 

 operator will add the word "relative" to his 

 bearing report. If he wishes, the operator can 

 avoid saying "relative" after each report by 

 stating "All bearings will be relative until 

 further notice." 



TRUE BEARINGS 



To illustrate that true bearings are independent 

 of the ship's heading, figure 3-1 shows three 

 different bearings. The ship and the contact have 

 been drawn in to show that, although the ship 

 has a different course, the true bearing of the 

 contact remains the same. In the illustration, 

 compare the examples on the left with the 

 examples on the right. 



All shipboard and submarine sonar sets utilize 

 a gyrocompass repeater to provide true target 

 bearing by reading the sonar bearing marker 

 against the dial. Figure 3-2 shows this dial as 

 it is in a standard shipboard sonar set. The 

 section enclosed within the dotted lines is the 

 area visible to the sonar operator through the 

 glass window. The marker at the top, which 

 can be seen through the transparent bearing 

 dial, indicates the bearing to which the operator 

 has trained the cursor on the scope. Because 

 the cursor normally is positioned in the middle 

 of the target presentation, the dial marker 

 indicates the center bearing of the contact. 



RELATIVE BEARINGS 



True bearings are the ones of principal 

 concern to Sonar Technicians because standard 



operating procedures are based on true bearings. 

 You need to understand relative bearings, how- 

 ever, inasmuch as casualties to the gyrocompass 

 necessitate shifting to relative bearing pro- 

 cedures. 



Relative bearings ai'e read in degrees clock- 

 wise from the ship's bow, which is always 000°. 

 If a contact is broad on your ship's port quarter, 

 the relative bearing is 225°. If a contact is on 

 your starboard beam, the relative bearing is 

 090°. Figure 3-3 diagrams these two examples, 

 illustrating how relative bearings are determined. 



When the ship changes course, the relative 

 bearing of a target changes. In figure 3-4 the 

 ship changes course 60° to the right. This course 

 change causes the relative bearing of the target 

 to change from 090° relative to 030° relative. 



Relative bearings can be read on the sonar 

 console from the same dial that gives true 

 bearings, although not at the same time. If a 

 complete gyro failure should occur, this dial 

 automatically indicates relative bearing. Occa- 

 sionally, the gyro may act erratically for a 

 few moments, causing the picture on the scope 

 to jump, making it difficult for the operator to 

 track the target. To obtain a presentation with 

 more stability, the operator can control the 

 equipment so that a relative picture is pre- 

 sented on the scope, the dial indicating relative 

 bearing. 



Figure 3-5 typifies a bearing indicator from 

 which true and relative bearings may be read 

 simultaneously. This type of indicator, used in 

 earlier sonars, is a good example for showing 

 the comparison of true and relative bearings. 



The outer dial, which is fixed, indicates 

 relative bearing. The inner dial is free to 

 rotate. When connected electrically to the ship's 

 master gyrocompass, it acts as a gyro repeater 

 and shows ship's course and true bearing. The 

 diamond- shaped pointer between the two dials 

 indicates the direction in which the sound re- 

 ceiver is trained. Both relative bearing and 

 true bearing are read by observing the position 

 of the pointer. 



True course is read on the inner dial opposite 

 000°R on the outer dial. As shown in figure 3-5, 

 ship's course is 045°, and a contact broad on 

 the starboard bow (045°R) has a true bearing 

 of 090°. 



STERN LINE INDICATOR 



With modern scanning sonars, the sonar 

 operator has an ideal pictui-e of antisubmarine 

 action. He not only receives the audio response, 



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