Fighter Combat - Tactics and Maneuvering

40 FIGHTER WEAPONS
sensor systems, which normally require that the strength of the target
signal exceed that of the background (i.e., the signal-to-noise ratio must be
greater than one).
Background IR radiation is generated by the sun, by reflections off
water, snow, etc., and also by clouds and hot terrain such as deserts. If the
temperature of the background is within the band of sensitivity of the
sensor material and is of sufficient intensity, it will be detected along with
the target heat. When sensors are made sensitive to cooler targets for
improved detection, the seeker becomes more susceptible to background
noise also. This problem is partially resolved by designing the seeker to
track only small, "point-source" radiations, usually associated with aircraft
targets, rather than the broader areas of IR energy common to many
background sources. In general, the seeker tends to track the most intense
point-source target within its band of temperature sensitivity. The greater
the background radiation within the band of temperature sensitivity of the
seeker, the stronger the IR signal received from the target must be if it is to
be detected and tracked. This fact may limit the detection range for a target
of given IR intensity.
A hot object emits IR energy in a rather wide band of frequencies. As the
object becomes hotter the radiated power increases very sharply (proportional
to the fourth power of absolute temperature), and the frequency
of the most intense IR radiation is shifted higher. The hot metal of jet
tailpipes can be expected to emit IR energy of greater intensity and higher
frequency than that of the hot exhaust gases, which begin to cool rapidly.
Depending on the sensor material used, a heat-seeking missile may detect
only the tailpipe, or it may also be sensitive to the cooler exhaust gas and
even to the heat generated by air friction on a very fast aircraft. One
disadvantage of tailpipe guidance is the likelihood that the hot metal may
in some views be shielded by part of the aircraft structure. Hot exhaust gas
is usually more difficult to shield, and this fact has led to heat seekers with
"all-aspect" capability. However, the pilot of the target aircraft can reduce
substantially the IR signature of his exhaust gases easier and faster (by
power reduction) than he can his metal tailpipe, which tends to retain heat
longer. The physical size of an exhaust plume may also cause problems for
hot-gas seekers, as they may become "saturated" at close range. Rather
sophisticated guidance techniques are required to cause such missiles to
aim forward of the heat source in order to hit the target. Discrimination
between this cooler target and the background radiation may also be a
problem, as explained previously.
IR energy is absorbed and dissipated by water vapor, making heat seekers
all but useless in clouds or rain. Even in relatively dry air this energy is
attenuated more quickly than many other types of radiation, with the rate
largely dependent on altitude and humidity. This characteristic makes
heat seekers most compatible with short-range weapons.
Radar-guided missiles, using many of the guidance techniques discussed,
are currently the most widely used all-weather AAMs. Besides
weapons guidance, radars are also valuable for providing fighters with the
information necessary to detect enemy aircraft at long range, at night, and