Radar System Engineering

124 LIMITATIONS OF PULSE RADAR [SEC. 4.4
l/v, long, which previously separated successive pulses. At the same
time the average power is kept constant by a reduction in pulse power
by the factor rv,. The receiver bandwidth, meanwhile, is reduced to
about U,cps. The result is essentially a c-w system in so far as power and
sensitivity are concerned, with a bandwidth of v,. It can see as far as
the previous radar system, but it cannot see as much in the same time.
Unless it is provided with multiple channels it can collect information
only at the rate v,.
The reader must be cautioned against taking too literally the result
of the above argument, for the hypothetical variation of system parameters
cannot be duplicated in practice. A change in pulse power, by a
large factor, entails a change in the type of tube used as a transmitter,
and perhaps in other components as well. The radar designer, in other
words, cannot adjust pulse duration arbitrarily, keeping average power
constant; nor can the receiver pass band be made arbitrarily narrow
as ~ is increased, for effects such as fluctuations of the echo eventually
impose a limit. This problem will be discussed in detail in the following
chapter which is concerned specifically with c-w radar. The general
conclusion, however, remains valid—an essential advantage of high
pulse power, as used in pulse radar, is that it permits information to be
obtained rapidly.
For certain tasks, no very high information rate is required, and for
such applications the various c-w methods (Chap. 5) deserve consideration.
In the problem of the radio altimeter, for example, what is required
is merely the range of a single, large, ever-present target—the earth below.
It is significant that the most important practical application of c-w
radar has been made in this field.
4.4. Clutter.-One of the most formidable limitations to the usefulness
of pulse radar until recently was that imposed by “clutter.” Often
a radar system sees too much, rather than too little; the picture is confused
by unwanted echoes, or clutter. This can be made up of echoes
from surrounding objects on the ground (ground clutter), of echoes from
the irregular surface of the sea (sea clutter), or even of echoes from storm
clouds. The problem is to find the desired echo in the midst of the
clutter. Although various palliative have been invented (Chap. 12)
which accentuate a fairly strong echo relative to a diffuse background of
clutter, this difficulty is plainly fundamental as long as there is no essential
difference between the echoes that make up the clutter and the echoes
from what we may choose to call the true target. How can the radar
system distinguish between the echoes from boulders, ridges, trees, and a
multitude of irregularities on the side of a hill, and the echo from a tank
moving down the hill? How can the periscope of a submarine be seen
against the background of echoes from a considerable area of rough water?