Radar System Engineering

6 INTRODUCTION [SEC.1.3
pulse leaking into the receiver, and on the right, the echo signal from a
radar target. The target in the particular case of Fig. 1“3 is the earth’s
moon.
The measurement of range by means of radar is thus a straightforward
problem of time measurement. It is also desirable to be able to measure
the direction in which a target lies as viewed from a radar station. In
principle, this can be done on the basis of triangulation, using range
information on the same target from two or more separate radar locations.
Although this method permits of great accuracy and has occasionally
been used for special purposes, it is far more desirable from the standpoint
of simplicity and flexibility to measure direction, as well as range,
from a single radar station. Measurement of target bearing was made
possible by the development of radio techniques on wavelengths short
enough to permit the use of highly directional antennas, so that a more
or less sharp beam of radiation could be produced by an antenna of
reasonable physical size.
When the pulses are sent out in such a beam, echoes will be received
only from targets that lie in the direction the beam is pointing. If the
antenna, and hence the radar beam, is swept or scanned around the
horizon, the strongest echo will be received from each target when
the beam is pointing directly toward the target, weaker echoes when the
beam is pointed a little to one side or the other of the target, and no echo
at all when it is pointing in other directions. Thus, the bearing of a
target can be determined by noting the bearing of the radar antenna
when that target gives the strongest echo signal. This can be done in a
variety of ways, and more precise and convenient means for determining
target bearing by means of radar have been developed (Chap. 6), but the
method described here illustrates the basic principle.
It is convenient to arrange the radar display so that, instead of showing
target range only, as in Fig. 1.3, it shows the range and angular
disposition of all targets at all azimuths. The plan-position indicator,
or PPI, is the most common and convenient display of this type. Figure
1.1 is a photograph of a PPI-scope. The direction of each echo signal
from the center of the PPI shows its direction from the radar; its distance
from the center is proportional to target range. Many other forms of
indication are convenient for special purposes; the various types of indicator
are cataloged in Chap. 6.
1.3. Components of a Radar System.—A radar set can be considered
as separable, for the purposes of design and description, into several major
components concerned with different functions. Figure 1.4 is a block
diagram of a simple radar set broken up into the components ordinarily
distinguished from one another.
In the set illustrated in Fig. 1“4, a cycle of operation is begun by the