Radar System Engineering

SEC. 15.5]
CHOICE OF PULSE LENGTH
597
where
N. =
“r =
e=
~=
the number of pulses per scan on the target,
pulse recurrence frequency,
azimuth beamwidth in degrees between half-power points, and
scan rate in degrees per second,
we obtain the relation
Sti, . “
J- G,”
From the preceding section we have
(2)
(3)
where 7 is the pulse length.
From Eqs. (1), (2), and (3) we obtain
R m.
~ P$~T54v,~,
holding other parameters constant,
or
R~nx = Prvji. (4)
The average power output is given by Pr v,.
It is desirable to drive the transmitter of a high-performance groundbased
system at the highest pulse power and the highest average power
that it will safely withstand. The value of R- can then be increased by
an increase in pulse length and a corresponding reduction in the pulse
recurrence rate, leaving average power and pulse power fixed at their
highest safe values.
Limitations on pulse length are imposed by the following considerations:
1.
2.
3.
4.
The minimum range difference at which two targets can be resolved
vanes directly with pulse length. With a l-~sec pulse, aircraft
inseparable in azimuth can be resolved if they differ in range by
more than 164 yd plus a small distance which depends on the
characteristics of the receiver and the indicator. With a 5-psec
pulse, this limit for resolution becomes 820 yd plus the same
additive constant.
The intensity of cloud return relative to signal return from planes
varies directly with pulse length (Sec. 3.10).
The coverage area obscured by ground clutter will increase as pulse
length is increased.
Magnetron behavior has been found to become more critical, and
requirements on the voltage pulse from the modulator to become
more exacting, with increasing pulse length.