Radar System Engineering

630 MOVING-TARGET INDICATION [SEC. 162
To obtain range information with the scheme of Fig. 16”4, the phones
can be replaced by an A-scope synchronized with the modulator. The
appearance of the A-scope is shown in Fig. 16”5, where one moving target
C.w f Power
transmitter
amplifier
t
i
‘‘f
I
1’
Pulse
F1m 16,4,—Doppler effect with pulsed system suitable for low frequencies.
is seen among several stationary ones. The butterfly-like appearance
of the moving target is the result of the variation in pulse amplitudes
shown in Fig. 16.4. Stationary targets exhibit constant phase from pulse
to pulse, and therefore a steady
Modulator
amplitude.
The arrangement of Fig. 16.4 ~- —_______
can be used at frequencies of a few
t ‘;F::g TR switch
hundred megacycles per second but ~--—1 -- __,
Reference ,
is not practical at microwave fre- ,
---_l oscillator -__—l
quencies in the absence of suitable L-+—— Receiver
power amplifiers. However, the C-wreference 1
s)gnal
t
A-scope
FIG. 16.5.—Doppler beats on an & FIG. 16.6.—Doppler system for microscope.
waves. Dotted lines show what must
be added to ordinary radar set.
same effect 1 can be obtained by the circuit of Fig. 16.6, which shows the
basic diagram of a microwave pulsed radar, with the addition of a reference
oscillator. This reference oscillator provides a c-w signal with which
to beat the incoming echoes. Since the transmitter starts with random
phase from pulse to pulse, it is necessary to match the phase of the reference
oscillator to that of the transmitter at each transmitted pulse.
1(,second time around~) echoes—signalsreceived from the second preceding
pulse-will not be canceled by the arrangement of Fig. 166.