Radar System Engineering

548 THE RECEIVING SYSTEM—INDICATORS [SEC. 13.20
by the same azimuth voltage. The horizontal sweep is unmodulated but
arranged to permit delay in discrete steps. Height is read from an
engraved scale as explained in Sec. 6.12. If several scales are to be used
in connection with delayed sweeps, the reflection method of Fig. 7.4 is
used.
SIGNAL DISCRIMINATION, RESOLUTION AND CONTRAST
The factors that influence the dkcernibility of a simple radar echo
signal against thermal noise have been discussed in Chap. 2 and in the
early sections of Chap. 12. We have seen that the characteristics of
the receiving equipment which influence this matter most profoundly
are the noise figure and bandwidth of the receiver, the integrating
properties of the cathode-ray tube screen, and the type of display. To a
lesser extent the cathode-ray-tube spot size and the scale-factor of the
display also play a role. Section 12.8 points out methods of increasing
the signal discernibility in the presence of various forms of radiant
interference by taking advantage of differences between the desired
and the undesired signals, and shows that to some extent these same
techniques aid in distinguishing a point target from more diffuse ones.
Chapter 16 describes an extremely elegant method by means of which
a given target can be readily distinguished from its surroundings, providing
it is in motion with respect to those surroundings and that they
are at rest or have a uniform mass motion.
The following paragraphs will dk.cuss another aspect of the discrimination
problem—that of the presentation of a multiple or complex
picture when it is not desired to suppress any of the information but
rather to present it in m detailed a manner as possible.
13.20. Resolution and Contrast.—In order to distinguish between
objects or among different parts of the same object it is necessary that
they be resolved. The characteristics of the receiving equipment which
influence resolution are the bandwidth of the signal channel, which
affects range resolution, and the spot size and scale factor of the display,
which also affect range resolution and to a lesser extent angular resolution
as well. Since factors concerned with the display are least under control,
and since their limitations often fix the limit of useful bandwidth, they
will be discussed first.
The spot size of a cathode-ray tube depends upon the tube type,
the performance of the individual tube, the voltage employed, the
design of focus and deflecting coils, and the intensity level at which
the tube is operated. In the series of magnetic tubes used for radar,
spot size is roughly proportional to the tube radius and is usually such
that between 150 and 200 spots can be resolved along the display radius.
In this discussion, the number of resolvable spots in a radius will