Fighter Combat

40 FIGHTER WEAPONSsensor systems, which normally require that the strength of the targetsignal exceed that of the background (i.e., the signal-to-noise ratio must begreater than one).Background IR radiation is generated by the sun, by reflections offwater, snow, etc., and also by clouds and hot terrain such as deserts. If thetemperature of the background is within the band of sensitivity of thesensor material and is of sufficient intensity, it will be detected along withthe target heat. When sensors are made sensitive to cooler targets forimproved detection, the seeker becomes more susceptible to backgroundnoise also. This problem is partially resolved by designing the seeker totrack only small, "point-source" radiations, usually associated with aircrafttargets, rather than the broader areas of IR energy common to manybackground sources. In general, the seeker tends to track the most intensepoint-source target within its band of temperature sensitivity. The greaterthe background radiation within the band of temperature sensitivity of theseeker, the stronger the IR signal received from the target must be if it is tobe detected and tracked. This fact may limit the detection range for a targetof given IR intensity.A hot object emits IR energy in a rather wide band of frequencies. As theobject becomes hotter the radiated power increases very sharply (proportionalto the fourth power of absolute temperature), and the frequencyof the most intense IR radiation is shifted higher. The hot metal of jettailpipes can be expected to emit IR energy of greater intensity and higherfrequency than that of the hot exhaust gases, which begin to cool rapidly.Depending on the sensor material used, a heat-seeking missile may detectonly the tailpipe, or it may also be sensitive to the cooler exhaust gas andeven to the heat generated by air friction on a very fast aircraft. Onedisadvantage of tailpipe guidance is the likelihood that the hot metal mayin some views be shielded by part of the aircraft structure. Hot exhaust gasis 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 reducesubstantially the IR signature of his exhaust gases easier and faster (bypower reduction) than he can his metal tailpipe, which tends to retain heatlonger. The physical size of an exhaust plume may also cause problems forhot-gas seekers, as they may become "saturated" at close range. Rathersophisticated guidance techniques are required to cause such missiles toaim forward of the heat source in order to hit the target. Discriminationbetween this cooler target and the background radiation may also be aproblem, as explained previously.IR energy is absorbed and dissipated by water vapor, making heat seekersall but useless in clouds or rain. Even in relatively dry air this energy isattenuated more quickly than many other types of radiation, with the ratelargely dependent on altitude and humidity. This characteristic makesheat 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. Besidesweapons guidance, radars are also valuable for providing fighters with theinformation necessary to detect enemy aircraft at long range, at night, and