Radar System Engineering

SEC.16.2] BASIC PRINCIPLES OF M TI 631
This can be done by allowing a sufficient amount of power from the transmitter
to enter the resonant cavity of the oscillator, which is then forced
into step with the transmitter. This process is called “locking” the
phase of the oscillator, or making it “coherent.” The appearance of
the A-scope will be the same as that shown in Fig. 16”5.
Two methods of presenting
the doppler information have been
‘“:’;~
mentioned: the aura 1 method
(Fig. 16.3), and the A-scope
method (Fig. 16.6). These indicators
are useful only with a
stationary or very slowly rotating
antenna. At norm a 1 rates of
scanning, it is necessary to display
“’ ‘~
“ ‘~
Cancelled signals
2 minus 1 A
signals on a PPI in such a way
that only moving targets appear
33s2
w
4 “
on the scope. This can be done
‘~
by the method of pulse-to-pulse FIG. 16.7.—Pulse-to-pulse cancellation.
cancellation illustrated in Fig. The first four traces represent successive
sweeps of A-scope. The last three traces
16.7, where the first four traces show the canceled signals.
represent successive sweeps on
the A-scope of Fig. 165. The scheme is to delay the signals of
Sweep 1 for a whole repetition period and then subtract them from the
signals of Sweep 2. In the same way Sweep 2 is delayed and subtracted
from Sweep 3, and so on. The results are shown in the last three traces
of Fig. 16.7. This process can be
Supersomc
Subtractmn carried out continuously by the
[
delay line
I
circuit
I arrangement of Fig. 16.8, in which
the signals are split into two channels,
one of which contains a superlfi’1
Race&er PPI sonic delay line, and then brought
together a gain f o r cancellation.
F1~.16.S.—Cmcellationof signals of con- An important practical point in the
stant pulse-to-pulse phase.
use of this scheme is the degree to
which the output signal of the delay line simulates the input signal,’ for
the fidelity of reproduction of signals by the delay line influences the
maximum clutter cancellation possible.
Ib is also possible to delay the signals by means of a “storage tube, ”
which works in a manner related to that of the Iconoscope used in
television. The supersonic delay line was used as a delay device in the
MTI systems that have had the most thorough testing; its use is therefore
assumed in what follows. Certain advantages attend the use of a
storage tube, notably the ability to apply MTI to a system whose