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AP3302 Pt3 Contents

AP3302 Pt3 Section 1 Contents

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AP 3302 Pt. 3

Section 1

CHAPTER 5

Basic outline of CW radar

when it arrives back at the aircraft the transmitter frequency has in the meantime changed to, say, 410.2 Mc/s. The reflected wave of course has its original frequency of 410 Mc/s.

A portion of the transmitter output is fed directly to the receiver where it combines with the reflected input to produce a difference frequency, in this case 0.2 Mc/s. The greater the altitude of the aircraft the greater is the difference in frequency between the direct and reflected inputs. This difference frequency is automatically measured in discriminator circuits in the receiver, the output from which operates a simple meter display as shown in Fig. 3.

Since the transmitter frequency is changing linearly by 40 Mc/s every 100 us, a change of 0.2 Mc/s in the transmitter frequency represents a time interval of:

(100 x 0.2)/40 = 0.5 us.

What range, or altitude, does a time interval of 0.5 us represent? We know that one radar mile (5,280 feet) is equivalent in time to 10.75 us. A time interval of 0.5 us therefore represents an altitude of:

(5,280 x 0.5)/10.75 =250 feet approximately.

This is merely one application of f.m.c.w. radar and it will be considered in more detail in Section 7. In general we can say that f.m.c.w. radar can be used to detect an object-indicated by the production of a difference frequency (a beat frequency) in the receiver discriminator circuits; it can measure the range of a target by measuring the beat frequency; and it can provide information on the bearing in azimuth and elevation of the target by using beamed radiation in the same way as pulsed radar.

The Doppler Effect

If we transmit a continuous wave at a fixed frequency, when the beam strikes an aircraft some of the r.f. energy is reflected (Fig 4). From the reflected signal, information about the presence of a target and the target's angular position relative to the transmitting aerial can be obtained.

There is however another phenomenon associated with all wave propagation which is used in unmodulated c.w. radar.

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