790 



AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



smaller instantaneous fields. One to three milliradians is representative of 

 present capabilities, with requirements becoming more stringent for good 

 detail from very high altitudes. Insufficient detector sensitivity and the 

 Rayleigh limit are not working in favor of instantaneous fields of view 

 much below 0.5 milliradian with 4- or 6-inch optics. ^^ 



The radiation from the earth, considered as a black body, peaks at about 

 10 microns. In this respect the problem of ground mapping is significantly 

 different from that of detection of "hot" targets such as engines of jet 

 aircraft or high-speed missiles (see Paragraph 6-8) whose radiation peaks 



10 



2 3 4 5 6 



WAVELENGTH (microns) 

 Fig. 14-25 Spectral Response of Several Infrared Detectors. 



l^According to the Rayleigh criterion, two point sources are at the limit of resolvability when 

 the center of the diffraction pattern of one object coincides with the first minimum of the other. 

 The minimum point source angular separation (ffmin) for such resolvability with a circular 

 aperture is given by Omin = L22X/rt, where X is the wavelength of the radiation and a is the 

 aperture diameter. For a more complete description of resolution and methods of improving 

 resolution by means of apodization, see J. Strong, Concepts of Classifical Optics, W. H. Freeman 

 and Co., San Francisco, 1958. 



