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BEARINGS: RELATIVE AND TRUE

A bearing is a number which tells the direction of another ship or object. A lookout gets bearings by sight, a conning officer by looking through the periscope, a sonar operator by listening through his gear. After contact is made with a target, the sonar operator keeps reporting bearings continually. Therefore, he has to know what bearings are, and how to read and report them.

 

Relative Bearings

Usually a sonar operator reports relative bearings because they tell direction in relation to own ship. Imagine two lines drawn from the center of the submarine: the first through the bow dead ahead, and the second to the target. The angle between these two lines, measured clockwise from the first line, is the relative bearing. A target dead ahead is at 000 degrees relative. Since there are 360 degrees in a circle, dead astern is at 180 degrees relative. These three diagrams show relative bearings of 50 degrees, 160 degrees, and 240 degrees.

  Diagram of 50 degrees relative bearing.
 
Diagram of 160 degrees relative bearing.   Diagram of 240 degrees relative bearing.
 
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True Bearings

The sonar operator may also be ordered to report a target's true bearing. Imagine two lines drawn from the center of the submarine: the first to the North Pole, the second to the target. The angle between them, measured clockwise from the first line, is the true bearing. A target due North of the submarine is at 000 degrees true, due East at 090 degrees true, due South at 180 degrees true, due West at 270 degrees true no matter where the submarine is heading. Only when own ship is heading due North, will a target's relative bearing and its true bearing be the same. Note how the true bearings are measured in the figures below.

 

Diagram of 60 degrees true bearing.   Diagram of 250 degrees true bearing.
 
Diagram of 110 degrees true bearing.   Diagram of 300 degrees true bearing.
 
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Reading a bearing indicator

Below is a picture of a bearing indicator. It has three parts:

1. The outer scale represents relative bearings. Zero always points to your own submarine's bow.

2. The inner scale represents true bearings. Zero always points to true north, no matter where your submarine is heading.

3. The diamond-shaped bug has two tips which point out on both scales the direction in which the projector is facing.

Photo of bearing indicator.  It shows the relative bearing of 30 degrees, own ship's course of 40 degrees and a true bearing of 70 degrees.

This is how to read the bearings

1. Read the relative bearing at the outer tip of the bug.

2. Read the true bearing at the inner tip of the bug.

NOTE: You can also read your own submarine's course on the true (inner) scale by noting the degrees directly below zero relative.

 
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When own ship changes course

The diagrams below show that the relative bearing changes when own ship's heading changes. But the true bearing, measured from the motionless North Pole, is fixed. Therefore keeping the true bearing in mind helps the operator maintain contact with a target, no matter where his submarine is heading.

Drawings showing 70 degrees relative bearing, 50 degrees true.  Drawings showing 30 degrees relative bearing, 50 degrees true.  Drawings showing 0 degrees relative bearing, 50 degrees true, now the submarine points to the target.

Know these general terms

The direction of a target may also be described in a more general way, as shown in the diagram at the right. Ahead, astern... port and starboard...bow, beam, and quarter...these are familiar words. But you must learn to know these eight combinations by heart, so that the instant you hear any one of them, you get a picture of the general direction of the target in relation to your submarine.

  Drawings showing bearing names. Dead Ahead, Stb Bow, Stbd Beam, Stbd Quarter, Dead Astern, Port Quarter, Port Beam, Port Bow
 
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Version 1.10, 22 Oct 04