Fighter Combat - Tactics and Maneuvering

FIGHTER WEAPONS 41
in bad weather, so that they might be intercepted and attacked on advantageous
terms. There are three types of radars which have application to
fighter weapons: pulse, continuous wave, and pulse Doppler.
Pulse radars work by transmitting a burst of radio energy (pulse) and
then receiving echoes of that pulse reflected off distant objects. If the
antenna is highly directional, aiming the energy pulse almost entirely
within a very narrow beam, the LOS to the target (azimuth and elevation)
can be accurately determined. This narrow beam can be formed mechanically
(parabolic-shaped antenna) or electronically (phased-array antenna).
Also, since radio waves travel at a known speed, the time elapsed between
transmittal of the pulse and receipt of the echo can be measured to derive
target range.
Radar electronics requires many compromises. Desirable features include
small size, light weight, long range, good range and angular accuracy
(resolution), and short minimum range. Unfortunately, improvement in
one area often leads to degradation in another. Light weight and small size
are important characteristics for aircraft radars, and obtaining them usually
requires relatively low-power, high-frequency units, which place
limitations on range. The small size of practical antennas also results in
wider beams, reducing angular resolution.
Range resolution is enhanced by shortening the duration of each pulse
(pulse width) so that the complete echo of a near target is received before
the first echo of a farther target arrives. Shortening the pulse width,
however, reduces the average transmitted power of the radar, thereby
lessening its maximum range. There are some electronic processing techniques
which can largely overcome this problem, allowing longer pulse
widths for greater range while maintaining range resolution, but minimum-range
performance, which is also proportional to pulse width, usually
must be sacrificed.
As the name implies, continuous-wave (CW) radars are not pulsed, they
transmit continuously. This means that the antenna used for transmission
cannot be used for reception, as with pulse radars, so multiple antennas are
required. CW is used quite often for semi-active and beam-rider missile
guidance, with the host platform transmitting and the missile seeker
receiving the transmission and/or the reflected energy. For long-range
shots the CW energy may be formed into a narrow beam and directed at the
target by the host tracking system. For short-range firings a fixed, wideangle
antenna may be used to illuminate targets within its field of view.
CW radars generally measure target closing velocity by the Doppler
principle, which most often is illustrated by the change in pitch (frequency)
of the whistle on a passing train. While the train is approaching,
one pitch is heard (higher than that actually produced by the whistle), and
as the train passes the pitch seems to decrease to a lower frequency.
Relative motion changes the frequency of sound waves or other waves
such that closing velocity between the source of the transmission and the
receiver causes an apparent frequency increase, while opening velocity
causes a decrease. This frequency shift is proportional to the closure and
offers a direct means of velocity measurement.