Radar System Engineering

128 C-W RADAR SYSTEMS [SEC. 5.1
target. It is assumed that the target is moving radially, thus supplying
the needed modulation.
Inconsequence of theradial motion of the target, which we suppose
to be at constant velocity v,, the phase of the returned signal, relative
Target
f
+
-
FI~. 6.1.—Simplest possible radar system, using single transmitted frequency, with
modulationderived from target motion.
to the outgoing one, shifts continuously. In other words, the returned
frequency is different from the outgoing frequency, the difference being
(2w/c)f with v, the radial velocity and c the velocity of light.
Numerically, with v, in mph and j defined by a wavelength x in cm,
the frequency” difference, which is called the ‘‘ doppler frequency, ” is
given by
L_lf.f-
j:
;:
Zz
ilJ.—- /-
FIG. 5,2.—The upper diagram
showsthe singleFourier
componentfo radiated hy the
transmitter; the lower shows
the componentspresentin the
receiver, these being fo from
the transmitter and fo * fD
from the target.
~. = 89.4 ~ A’
(1)
or, at 10-cm wavelength, f. amounts roughly
to 9 cps/mph. The doppler-shifted return
signal, when added to the transmitter voltage
and rectified, gives rise to a voltage with
small pulsations recurring at the doppler frequency.
The steady component is removed
by the highpass filter, which may be simply
a transformer or a series condenser. The
fluctuations are amplified and used to actuate
the indicator. Thus the presence of a (moving)
target is detected.
Although the system just described is hardly
practical, apparatus working on exactly this
principle but with slight te~hnical modifications can be useful.
In Fig. 5.2 such a system is described diagrammatically in terms of
type of modulation and use made of the returned signal. The upper
part of the diagram shows the amplitude of the Fourier components
radiated by the transmitter; the lower part shows the amplitude of the