Radar System Engineering

SEC, 17.4] METHODS OF COMBATING IN TERFEREA’CE 685
method is satisfactory only for transmitting continuous rotation at
a fairly constant rate.
2. Theangle itself can betranstitted titerms of therelative phasesof
two sets of periodic signals which are usually either sinusoids or
pulse trains (Sec. 17.5). Since this isasingle-valued method, no
zero-phase adjustment need be made. It can therefore be used for
sector scanning, or forint,errupted scanning. Thedatacan be used
to position a scan converter by means of a phase-sensitive mechanism,
or they can be used directly to provide electrical information
for such an indicator
as a B-scope.
3. The values of the sine and the cosine of the scan angle can be
transmitted in any of several ways (Sec. 17.7). Transmission of
sine and cosine is also a single-valued method. The data can be
used to control a scan converter or directly in electronic PP1’s or
related
indicators.
17.4. Methods of Combating Interference.-Aside from providing
the best possible signal-to-interference intensity ratio at the receiver
input, the principal method of minimizing the effects of interference is to
take advantage of differences between the desired and the undesired
signals. The~fferences, \vhichshould deliberately bemadelarge, canbe
exploited both by using them as a basis for excluding the undesired signals
from the operating device and by making that device as insensitive as
possible to interfering signals that are not excluded. Unwanted signals
can be rejected by frequency discrimination; the bulk of the interference
is excluded in this way. How-ever, since it is always necessary to have a
finite bandwidth to admit the necessary information, some interference is
likely to get through to the analyzer.
In the case of the scanning data, it is often possible to protect against
transient interference (or absence of signals) by exploiting electrical or
mechanical inertia. Care must be taken that the inertia does not
appreciably inhibit satisfactory response to scanning accelerations, a
serious restriction if sector scanning is involved.
A second, and more promising, method of approach is to take advantage
of approximate knowledge of what the real signals should do by
excluding completely all information that does not closely agree with
expect ation, just as tuned circuits or filters reject signals outside their pass
band. The knowledge on which to base the selection may be available
a priori, or it may depend partly on a memory of what has happened in the
immediate past.
As an example of such methods consider an information-bearing pulse.
Very similar pulses with the same frequency components as the signals
are likely to be present in the interference and of course cannot be