Fighter Combat - Tactics and Maneuvering

42 FIGHTER WEAPONS
Since CW radars have no pulses that can be timed for range determination,
another method is necessary. This is generally accomplished through
a frequency-modulation (FM) technique. If the transmitter frequency is
varied continuously up and down, the reflected wave will vary in the same
manner. The peaks of the reflected wave, however, will be delayed (phase
shifted) by a length of time proportional to the range between the receiver
and the target. The accuracy of FM ranging is usually inversely proportional
to target range (i.e., accuracy improves as range decreases), unlike
pulse-ranging accuracy, which is fairly independent of range. So, although
FM ranging can be very accurate over short distances, its accuracy is
usually inferior to that of pulse technique at greater ranges.
The great advantage of CW over pulse radar is its much higher average
transmitted power, since the transmitter does not have to turn off and wait
for an echo. The pulse-ranging technique requires long listening periods
between each pulse because of the time necessary for the pulse to reach a
distant target and return. Such a radar is classified as having a low pulserepetition
frequency (low PRF). Low PRF results in less average power and
fewer pulses of energy reaching the target per second, reducing range
performance. Another method, known as high PRF, allows many pulses to
be in the air at a given time and substitutes FM-ranging techniques for
conventional pulse ranging. This results in greater average power and the
long-range benefits of CW, while allowing the double use of a single
antenna, as with pulse.
Pulse-Doppler radars are commonly of this high-PRF variety. They send
out pulses of a very finely tuned (coherent) frequency and listen for returns
of a different frequency, which would indicate Doppler effect from bouncing
off a moving object. This technique offers the great advantage of being
able to distinguish moving targets from stationary ones, such as the
ground. Again, FM ranging normally is employed.
One of the most severe limitations of pulse radars is ground clutter, or
reflections off the earth's surface. These reflections may be returns of the
radar's main beam, or of any of the many weaker side lobes of energy
radiated in all directions because of antenna imperfections and other
factors. Clutter is seen by a receiver as noise, and the strength of the target
return must exceed that of the noise by a given amount for target detection.
When a target is close to the ground its return may lie within the mainbeam
clutter (MBC) of an illuminating radar. In this case the target will
most likely be obscured by the noise created by the ground. Likewise,
when the radar platform is near the ground, reflections from the side lobes
generate noise in the receiver, even when the radar is looking up, requiring
increased power in the target return before detection is possible, and
reducing maximum range.
Doppler radars in moving aircraft also have problems with clutter, since
returns off the ground reflect the host aircraft's own airspeed. Because this
speed is known, however, MBC can be eliminated by "blanking out"
returns of the approximate frequency associated with this closing velocity,
so that the intensity of the clutter return will not overpower the receiver.