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AP3302 Pt3 Contents

AP3302 Pt3 Section 1 Contents

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AP 3302 Pt. 3

Section 1

CHAPTER 1

PULSE-MODULATED RADAR

as the aircraft moves from one position to another the amplitude of the combined input varies as the two signals come into, and go out of, phase. This makes the picture flutter.

The fact that aircraft reflect e.m. waves to give such a noticeable effect on the screen of a cathode ray tube is important because enemy aircraft will also reflect waves which can then be used to detect their approach and determine their position so that fighters or missiles can be guided to intercept them.

How a Target is Detected

Imagine that you are on board a ship, which is sailing in dense fog through a region where there are many icebergs. How can an iceberg ahead of the ship be detected?

One well-known method is to sound a short blast on a foghorn, which directs the sound ahead of the ship, and listen for echoes (Fig 4). As the ship moves forward the horn is sounded at regular intervals and the crew listens for echoes in the pauses between the pulses of sound. Reception of an echo indicates an object ahead of the ship.

 

 

 Pulse-modulated radar detects aircraft in a similar manner using radio waves instead of sound waves (Fig 5). The transmitter is switched on for a very short period only to give a pulse of r.f. energy which is radiated from a directional aerial. If the e.m. wave encounters an aircraft, some of the energy is reflected back as an echo towards the transmitter-receiver. The output from the receiver is displayed on an indicator, which shows the operator that there is an aircraft within range.

Several hundred pulses per second are normally transmitted and the receiver 'listens' for echoes in the intervals between pulses.

How Range is Measured

Let us think again of the ship sailing in fog. When the presence of an iceberg somewhere ahead of the ship is indicated by the reception of an echo the captain must find out how far ahead it is, i.e. its range, in order to take the necessary avoiding action.

As the speed of sound waves is known (approximately 1,000 feet per second) the range may be found by using a stopwatch to measure the time interval between the sounding of the fog-horn and the reception of an echo. Let us assume that the horn is sounded and an echo heard ten seconds later (Fig 6). In this time the sound travels 10,000 feet. This is the total distance from the ship to the iceberg and back to the ship. The range of the iceberg is thus:

10,000/2 = 5 000 feet.

This gives a simple relationship:


 

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