Radar System Engineering

SEC. 15.4] THE RANGE EQUATION 595
not unduly difficult to fit by beam-shaping. Even greater coverage both
in range and height was known to be desirable, but it appeared to be
attainable only by excessive antenna size or by an excessive multiplicity
of separate radar beams, each with its own transmitter and receiver
system.
A microwave set with the coverage shown in Fig. 15.4 presented a
very difficult problem. Because of uncertainty in measurements and
in the outcome of component development, there was no firm assurance
that it could be met. The success of the undertaking, however, was not
dependent on meeting this coverage fully, since maximum range and
altitude values within 20 per cent of the values of Fig. 15.4 would have
represented a major advance over any previous equipment. Failure to
detect one heavy plane above 30,000 ft would have limited the utility of
the equipment for special problems, but, because of the practice of group
flying, the normal problem was much less severe.
Azimuth Beamwidth.—The usefulness of the radar set for surveillance
was expected to improve in traffic-handling capacity, in ability to
resolve closely spaced aircraft, in accuracy of data, and in reduction of
ground clutter, as the azimuth beamwidth was decreased. The desirable
lower limit was expected to be set only from considerations of a practical
upper limit to antenna size and requirements of mechanical accuracy.
Results obtained fully confirmed these conclusions for bearnwidths down
to 1° at half power, but in a later section of this chapter we shall see that
qualifications may be introduced if future development is extended to
beams much narrower.
Miscellaneous Requirements. —Although limitations on size, complexity,
weight, and power consumption could be subordinated to the requirement
for excellence in radar performance, the designers were fully aware
of the importance of practical limitations on these characteristics. It
was also well understood that without great care and effort to achieve
the utmost in simplicity, dependability, convenient facilities for test,
and ease of maintenance, the equipment might fail to give satisfactory
field service.
In computations leading to the choice of system parameters the
designers did not have the advantage of much information that is now
available, yet in practically every case their decisions were excellent.
The discussion of design now given will make use of up-to-date information
not available to the designers. This procedure is helpful for clarity
of presentation and will make the analysis more generally applicable.
15.4. The Range Equation.-There will now be undertaken a detailed
discussion of considerations leading to the choice of values for the following
system parameters: (1) pulse length, (2) pulse recurrence frequency,
(3) azimuth scanning, rate, (4) azimuth beamwidth, (5) beam shape in