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AP3302 Pt3 Contents

AP3302 Pt3 Section 1 Contents

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AP 3302 Pt. 3

Section 1



How Bearing is Obtained Using Sound Waves

Let us return to the ship sailing through fog and imagine that the captain has used the fog-horn to detect an iceberg somewhere ahead of the ship and has measured its range. Earlier we assumed that the fog-horn would beam the sound in a generally forward direction ahead of the ship. An iceberg situated anywhere within the sound beam will cause an echo but, as shown in Fig 12, the captain cannot be sure of its exact position.

To take the necessary avoiding action the captain also needs to know the relative bearing of the iceberg, i.e. whether it is dead ahead or to one side of the direction in which the ship is heading.

In order to state the bearing clearly we imagine that the sea ahead of the ship is marked out as in Fig 13. The captain can obtain a more accurate bearing by using a fog-horn which concentrates the sound waves into a very narrow beam. Echoes are then heard only when the beam is aimed directly at the iceberg and the direction in which the horn is pointing gives the relative bearing.

Notice that while the narrow beam of sound is aimed at iceberg A there is no echo from iceberg B which is much nearer to the heading of the ship. To ensure that all icebergs ahead are detected the horn must first be pointed well to one side and sounded while the crew listen for echoes, then turned slightly towards the heading and sounded again and the process repeated until the horn is pointing well to the other side. This procedure is known as scanning (Fig 14) and it should be a continuous process.

Radar Bearings

The bearing of a target relative to a radar installation is obtained in a similar manner. To find the bearing the pulses of r.f. energy produced by the transmitter are concentrated into a very narrow beam by means of a directional aerial array.

To scan the whole of the area from which the aircraft might approach, the aerial assembly is systematically turned from side to side (Fig 15). Echoes are received only when the beam is aimed directly at the target and, since we know the direction in which the aerial is pointing at any given time, we can find the bearing of any aircraft which reflects the r.f. pulses.

For many applications the radar aerial is required to rotate through a full 3600 so that the complete area surrounding the radar installation may be examined. In such cases it is usual to


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