AP 3302 Pt. 3
Section 1
CHAPTER 5
Basic outline of CW radar
Use of the Doppler Effect in Ground Radar
With pulse-modulated ground radar equipment reflections from large fixed objects cause permanent echoes on the indicator, and random reflections from small objects close to the radar cause clutter at the centre of the p.p.i. (Fig 8).
For most applications the receiver should ignore reflections from fixed objects and respond only to moving targets. This can be achieved by using the Doppler effect because a Doppler frequency is produced only by the radial velocity of a moving target. For a stationary object the reflected signal has the same frequency as the transmitted signal.
Fig 9 illustrates a typical arrange-ment for the indication of a moving target. The frequency fr of the reflected signal differs from that of the trans-mitted signal ft by the Doppler shift fd. The reflected signal is mixed with the output of a local oscillator to produce an i.f. signal (fif±fd) where fd is the Doppler shift. This signal is amplified and fed to a discriminator whose output is either a positive-going or a negative-going d.c. voltage depending upon whether the frequency of the reflected signal is above or below that of the transmitter. Remember that the frequency of the reflected signal increases if the target is approaching and decreases
