Radar System Engineering

CHAPTER 4
LIMITATIONS OF PULSE RADAR
BY E. M.
PURCELL
To a thoughtful observer, one of the most striking features of a
microwave radar screen is the quantity of information that is available
at a glance and continually being renewed. In some installations the
map of an area of many thousands of square miles is drawn every few
seconds. Such a map may comprise in effect some 105 to 106 separate
“elements” of information similar to the elements from which a television
picture or a half-tone cut is constructed. This in itself is no cause
for complacency; an ordinary photograph, recorded in a fraction of a
second, usually contains much more information. Indeed, the uninitiated,
comparing the rather fuzzy radar picture with the pin-point detail
of the photograph, may conclude that the obvious deficiencies of the
former merely betray the primitive state of the art, and that vast improvement
in distinctness of detail is to be expected in the normal course of
development. These conclusions are only partly true. In the first
place many of the unique capabilities of radar, such as direct range
measurement or detection of very remote objects despite cloud and darkness,
often deserve more emphasis in radar design than does the ability
of the set to produce a lifelike picture. In the second place, the pulse
radar process is subject to certain inherent limitations. These limitations
are of obvious origin. A few have been mentioned already in preceding
chapters, but since the interest there was merely in the detection of
energy reflected from a single target and not in over-all radar system
design, their implications were not pursued.
4.1. Range, Pulse-repetition Frequency, and Speed of Scan—It is
the function of most radar sets to search continually through some region
in space by scanning. Naturally the radar designer strives always to
enlarge the region which can thus be searched, to increase the rapidity
with which it can be completely explored, and to improve the ability to
distinguish detail within the region. In this endeavor he is made
acutely aware of two of the fundamental limitations of pulse radar which
can be blamed respectively on the finite velocity of light and the necessity
of funneling all information in sequence through a single electrical
channel. The effect of these two limitations and the close connection
between them can be seen in a simple example.
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