Radar System Engineering

150 C-W RADAR SYSTEMS [SEC. 5.11
cycle, and turns the transmitter off and the receiver on during the following
cycle. This permits the use of a single antenna and also reduces
the clutter intensity by making it possible to gate out the strong returns
from near-by objects.
Difficulties with systems of this type are as follows: (1) determination
of range by variation of jr takes much more time than is justified by t,he
receiver bandwidth; (2) the distance measurement is unambiguous
ordy over a 2-to-1 range; (3) proportions must be such that jD < j,—this
means long wavelengths and/or slow targets; (4) it is difficult to get the
modulation cycles to repeat well enough for suppression of really serious
clutter.
6.11. Pulse-modulated Doppler System.—The last system to be
described works against the heaviest ground clutter, and, in principle,
on a plurality of targets and with no restriction on the relative values
of doppler and recurrence frequencies. Practically, there are limits
on the last two factors, the limits being set by questions of apparatus
complexity.
A block diagram is shown in Fig. 5“15 and the operation may be
described as follows. If, as we shall assume for purposes of explanation,
only a single target is to be observed, the transmitter is turned on for
half the keying cycle and off during the other half by means of the
modulator, square-wave generator, TR and ATR tubes. If a multiplicity
of targets must be handled, the transmitter pulse is made
correspondingly shorter. The transmitter power then follows the Curve
a of Fig. 5“16, and the receiver input follows Curve b with the time delay
depending on the range and with the dotted part of the received signal
rejected because the receiver is off. 1 This received signal is amplified
by a conventional superheterodyne receiver whose only special feature
is the derivation of the local-oscillator frequencY from the transmitter
frequency by the addition of 30 Me/see. Thk 30 Me/see is then subtracted
in a second detector so that the received signal is translated
down to zero frequency.
Consider the voltage at point 1 in Fig. 5“15 just after the second
detector. If the returns are from a stationary target the voltage is of
the form shown in Graph a of Fig. 5.17. This voltage is periodic and
has magnitude depending on the target cross section and on the exact
phase of the returned signal—a change of range of h/4 resulting in a
reversal of sign. If the target moves, the change of range cauees a
periodic oscillation of amplitude, the resulting signal being like that of
Graph b Fig. 5“17 and having an envelope which is a sine wave of
doppler frequency.
1Actually, the receivergate is not opened until several microsecondsafter the
transmitteris off. In this way groundclutterdue to nearby objects is eliminated.