Radar System Engineering

SEC. 212] I’RO1’AGA TIO.V 01’ER .1 lil?l~Lh’CTI.VG ,$’171tl’ACE 47
in a manner consistent with the inherently statistical nature of the
problem. That R.., is not rendered so uncertain by these statistical
effects as to lose any usefulness is largely due to the inverse-fourth-power
law expressed by the radar equation, which makes the range relatively
insensitive to moderate changes in the system parameters.
It should not be forgotten that in these latter pages we have been
wholly concerned with that obscure marginal region between strong
signals and utterly undetectable signals. This margin is not, after all,
very broad. A glance at Table 21, for instance, sho~vs how very rapidly
w diminishes as the signal becomes strong. }Iost of the useful signals in
any radar set are strong enough to be practically unmistakable.
MICROWAVE PROPAGATION
In the first part of this chapter the radar equation was derived under
the assumption of free-space propagation. The assumption is frequently
not justified, and we must now turn to some of the important cases that
fail to fulfill one or more of the requirements laid down in Sec. 2.1. We
shall try, where possible, to modify the radar equation to suit the new
circumstances, but it will be our broader purpose to describe, if only
qualitatively, certain propagation phenomena peculiar to the microwave
region, This is a vast subject. It includes some exceedingly difficult
problems in mathematical physics, not yet completely solved; it includes
topics in meteorology; above all, since it involves the weather and the
variegated features of landscape and seascape never susceptible of exact
mathematical description, it includes a large collection of observations
and experience, rarely easy to interpret. We could not here treat such
a subject. comprehensive y, but certain aspects with which the radar
engineer should be familiar are not hard to explain. Their influence on
radar planning and design is felt, or should be, at a very early stage,
2.12. Propagation over a Reflecting Surface. -If the transmission
path lies near a reflecting surface it may be possible for energy to reach
the target, and hence also for scattered energy to return to the radar
antenna, by way of the surface as well as directly. The result of combining
the direct and the reflected wave at the target will depend on the
relative intensity and phase of the reflected wave, which in turn will
depend not only on the difference in the length of the two paths but upon
changes of phase or intensity int reduced in the process of reflection.
The analysis is very easy in the case of a jut, perfectly rejecting, surface.
Let us consider a nondirective transmitting antenna A located at a
height h, above a flat reflecting surface S, as in Fig. 2.9. The field
strength at some other point B of height h~ can be described by giving
the ratio of the field strength at that point to the field strength which
would have been observed in the absence of the reflecting surfacein