14-23] BASIC PRINCIPLES CONCERNING IR GROUND MAPPING 



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to be the same as the angle subtended by the detector at the real or effective 

 optical element located a distance equal to the focal length from the 

 detector. Scanning the sensor across the desired mapping area in a direction 

 perpendicular to the flight path while the aircraft is in motion yields a 

 continuous line-by-line thermal map of the region below. Fig. 14-24 

 illustrates this type of scan pattern. 



The width of a strip map (w) as measured on the ground is given by 



w = 2h tan a (14-37) 



where w = object space width of map 



h = aircraft altitude 



a = total field half-angle. 



Table 14-2 shows approximate strip map widths for several altitudes for 

 60°, 90°, and 150° total field of view systems. As can be seen, total map 

 width increases rapidly as angular coverage is increased beyond 90°. 



Table 14-2 



The upper limit on angular coverage is generally governed by resolution 

 requirements. The size of the ground area defined by the instantaneous 

 field varies with the slant range. The ratio of the ground area filling the 

 instantaneous field when looking at an angle d as measured from the vertical 

 to the corresponding ground area when looking directly below the vehicle 

 is equal to sec ^9). Thus 



1^££%^ = sec^e (14-38) 



(area)e=o 

 Because of this resolution degradation, total fields of view are generally less 

 than 150°. 



The instantaneous angular field of view and electronic bandwidth 

 normally provide the limit to system resolution, given optics with better 

 resolution capability than is required. Therefore, the size of the smallest 

 anticipated targets exerts an influence on the chosen instantaneous field of 

 view. If detection alone is required, the instantaneous field need not be 

 small, and indeed may, depending on the target and background heat 

 characteristics, be substantially larger than the target itself. Reconnais- 

 sance usually demands identification of the type of target, thus requiring 



