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Aircraft Radar Basic Principle
The basic principle used by radar is portrayed in Figure 3.5. The energy radiating from a radar transmitter propagates in a similar fashion to the way ripples spread from an object dropped in water. If the radiated energy strikes an object – such as an aircraft – a small proportion of that energy is reflected back towards the radar. The transmitted energy effectively has a double journey: out to the target and back again. Radar uses this principle to measure the distance to the target; knowing that the speed of light is ~3 x 108 m=s, and by measuring the time taken for the reflection to arrive, makes it possible to calculate the target range:
where R is the range of the target, c is the speed of light (3 x 108 m/s ) and A t is the time taken for the radar energy to perform the round trip.
Radar energy may also be transmitted in a number of ways. Figure 3.6 shows two situations; one where the RF energy is sent in pulses and the other where RF energy is radiated continuously – also known as a continuous wave.
Pulsed radar transmission is useful when information is required regarding the range of a target. Clearly, by transmitting a pulse of radar energy it is easy to measure when the reflected pulse returns and hence determine the target range using the formula given above.
Using a continuous wave transmission allows the closing (or receding) velocity of the target to be determined. This is achieved by using the Doppler effect. The Doppler effect is one by which the frequency of radiation is affected if a target is moving in the radial direction between radar and target (see Figure 3.7 which depicts a point radiating source travelling with a velocity from left to right).
If a radiating (or reflecting) target is receding from an observer, the frequency will appear to reduce as far as the observer is concerned. Conversely, if the target is approaching then the frequency will appear to increase. The classic illustration is of a train approaching, passing and receding from a stationary observer: as the train approaches, the sound pitch will be higher than when it recedes. As will be seen, the Doppler effect is a very useful property that is extensively used in various radar applications.
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