346 GENERIC TYPES OF RADAR SYSTEMS AND TECHNIQUES 



controls are activated (as in Fig. 6-36b) to return the target image to the 

 center of the circular scan. Smoother tracking and less tendency to hunt 

 result from a system in which the error signal varies in magnitude with the 

 ofF-course position of the target, and this is usually a feature of actual 

 tracking systems. 



Detection Performance. The actual capabilities achievable with 

 present-day infrared systems can be estimated from target intensities, 

 background intensities, and detector sensitivities. As an example, consider 

 a radiometer having a filter limiting the sensitivity of the radiometer to the 

 region 1.7 to 2.7 microns and a lead sulfide detector at the focus of a 

 collector mirror 1 ft in diameter. With a readily available detector (say an 

 Eastman Kodak Ektron cell) of practical size (say 1 mm square), an output 

 signal just equal to the rms noise from the detector can be achieved under 

 tactical conditions when about 10~^^ watt/cm^ falls on the collector mirror 

 and is focused on the detector. This would represent a signal-to-noise ratio 

 of 1, here arbitrarily construed as a necessary criterion for detectability. 

 If the target is the exhaust port of a typical jet engine, the irradiance 

 (watts /cm^ in the 1.7 to 2.7-micron region) at the collector mirror will be 

 about 400 /i?2, where R is the target range. This results from assuming the 

 exhaust port to be a 24-inch-diameter blackbody of emissivity unity and to 

 have a temperature of 600°C. Through a completely clear atmosphere, 

 then, and with no background interference this jet exhaust port could be 

 seen from a distance of 2 X 10^ cm or 125 miles, at which distance it would 

 irradiate the collector mirror with the necessary 10"^^ watt/cm^. Atmos- 

 pheric attenuation, which is severe in the lower atmosphere, and back- 

 ground interference may under average conditions degrade this range to 

 less than a third of this number. 



Further, the target we are considering, a single-engine jet aircraft, will be 

 a much fainter target at any other than tail aspect where the exhaust port 

 is visible. In side aspect the radiation emanates from the hot exhaust gases 

 which, while of extended size and quite hot, emit only the wavelengths of 

 the characteristic infrared bands of the gases. If the fuel is a hydrocarbon 

 these are the bands of water vapor and carbon dioxide. Atmospheric 

 attenuation is most severe in this case, since the cold water vapor and 

 carbon dioxide in the intervening air path absorb most of what is emitted. 

 In side aspect a jet will be less than one-tenth as intense a target as in tail 

 aspect and will therefore be detectable at less than one-third the range 

 realizable when looking at the exhaust port. In nose aspect, it will be 

 considerably worse than this, since here most of the exhaust gases are 

 hidden by the aircraft and the hot parts of the engine are not visible. 



