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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

if it is flying away from the radar. Thus the sign of the discriminator output indicates whether the target is approaching or moving away. The magnitude of the discriminator output depends upon the frequency deviation of the reflected signal in relation to the transmitter frequency, and this in turn is proportional to the target's radial velocity.

An installation using c.w. Doppler radar will provide the following information about a target:

a. The presence of a moving target is indicated by the production of a Doppler frequency. Stationary objects provide no change in frequency.
b. The bearing and elevation of the target is determined by using narrow beams.
c. The radial velocity of the target is determined by measuring the Doppler shift in frequency.
d. The direction of travel of the target is determined by noting the sign of the Doppler shift.

Note that c.w. Doppler does not measure the range of a target. For this we use either pulse-modulated radar or f.m.c.w. radar.

Use of the Doppler Effect in Airborne Radar

If the radar transmitter is located in an aircraft the signals reflected from the ground ahead of the aircraft will also be subject to the Doppler effect. Use is made of this property in aircraft navigation. To navigate accurately one important factor which must be known by the navigator is the ground speed of the aircraft. This may be quite different from the air speed. Let us see how the Doppler effect helps here.

Since the ground ahead of the aircraft is being illuminated by the radar beam from the airborne transmitter it will reflect energy back towards the aircraft. The aircraft is always moving towards the apparent source of radiation and so the received frequency fr is higher than the transmitted frequency ft by the Doppler shift fd, i.e. fr = ft + fd. The Doppler shift is deter-mined by the radial velocity of the aircraft and is given as before by the expression fd = (2v/c) ft.

Special circuits in the receiver automatically measure the Doppler shift and the receiver output can be displayed on a simple meter calibrated in m.p.h. A practical equipment which uses this system transmits not one radar beam but four at different points around the aircraft. The information which is received from all four points on the ground is used to eliminate errors which would otherwise arise when the aircraft is climbing, diving or banking. The four beams also enable the drift of the aircraft to be calculated. A typical meter display is illustrated in Fig 10.

Summary

In this chapter a broad outline of the ideas involved in the use of c.w. radar has been given. Many factors have not yet been considered, nor can they be until we have learned something


 

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