786 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



The presence of sidelobes can also have a degrading effect on azimuth 

 resolution. For targets of sufficient strength sidelobes could result in an 

 ultimate resolution of 4 beamwidths^^. 



The same type of analysis can be used to study range resolution if the 

 modulator pulse form can be suitably described. In general the slope of the 

 modulator pulse will not be as steep as that radiated from the antenna; the 

 minimum resolvable separation in range will be more affected by differences 

 in target size. 



The other area of consideration regarding resolution is the receiver and 

 display system. The importance of the dynamic range of the receiver and 

 display and of the spot size of the CRT has been pointed out. In general the 

 resolution of the display will be limited by the minimum spot size. 



Modern mapping systems usually employ a camera to photograph the 

 display. This can also affect the overall system resolution. The resolution 

 of the overall photographic process can be determined by an equation of the 

 form 



where the A^'s refer to the number of resolvable lines arising from each 

 independent element of the system^^. There are four major independent 

 factors which combine to determine the photographic system resolution. 

 These are: 



1 . Lens Resolution — The resolution of the lens is determined by such 

 factors as lens diameter,/ number, and image size. For a flux response 

 factor of 0.5 a typical value of A^ for high-quality lenses is about 70 lines per 

 millimeter. 



2. Depth of Field — The image formed by the lens will be in focus only 

 for objects at a given distance. Objects at a slightly greater or lesser 

 distance will not be sharply focused upon the film. However, because of 

 lens aberrations even a point in the focal plane will be imaged as a small 

 disk, called the disk or circle oj confusion. The circles of confusion for points 

 at other distances will be larger. Depending upon the sharpness of focus 

 required, there is a certain range of object distances, called the depth of fields 

 within which all objects are in satisfactory focus. The depth of field can be 

 expressed in terms of resolvable lines and for a typical system could be of 

 the order of 100 lines per millimeter. 



3. Fihn Resolution — The resolving power of photographic films depends 

 upon such factors as grain size and emulsion thickness. A conservative 



'^Freedman, loc. cit. 



'''O. H. Schade, "Electro-Optical Characteristics of Television Systems," Parts I through IV, 

 RCA Review^ (1949). 



