782 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



is usually so defined that two spots separated by a spot diameter are just 

 resolvable. In terms of resolvable lines this represents about 10 lines per 

 millimeter or more. Usable diameters for tubes which achieve spot sizes of 

 this magnitude are about 3 to 5 inches. For a usable tube diameter of say 

 3.5 inches and a spot size of 0.1 mm the available number of elements or 

 lines would be about 890 along the tube diameter. For a sweep range of 

 10 n.mi. this would represent a minimum resolvable range element of about 

 69 ft. In terms of system requirements the maximum resolution in range 

 required for a pulse width of 0.25 /xsec would be about 123 ft; for a pulse 

 width of 0.1 Msec this would be reduced to about 50 ft. In azimuth the 

 minimum required resolution as determined by the beamwidth varies with 

 slant range. For a beamwidth of 0.25° the arc subtended by the beam on 

 the ground would vary from approximately 27 ft at a slant range of 1 n.mi. 

 to 265 ft at a slant range of 10 n.mi. For a spot size of 0.1 mm the CRT 

 would limit the minimum resolvable distance in azimuth at slant ranges 

 less that about 2.6 n.mi. Thus spot size may or may not limit system 

 resolution, depending on various system parameters. 



Display contrast depends on the characteristics of the cathode ray tube 

 screen and the manner in which the tube is excited. The dynamic range 

 obtainable with present cathode ray tubes is of the order of 10 db. This 

 represents even more of a limitation than does the range of the receiver. 

 One technique that has been used with some success to retain detail is the 

 three tone method which enhances low-level signals while retaining to some 

 extent the detail in strong signals. ^^ Where the display is viewed directly 

 from the scope by an observer, resolution will be affected also by physiolog- 

 ical considerations — viewing distance, levels of luminance, gradients 

 between areas of high and low luminance, and background levels of illumi- 

 nation. 



The order of resolution that can be obtained with typical photographic 

 systems is such that this should not represent a limiting factor on overall 

 system resolution. This is considered in more detail in Paragraph 14-20. 



14-19 MODIFYING THE RADAR RANGE EQUATION FOR 

 THE ACTIVE GROUND MAPPING PROBLEM 



The radar range equation is conveniently considered in terms of the 

 signal-to-noise ratio for mapping systems as 



(14-33) 



|a^| [2(4x)y.A3A^T5(NF)J[acsc3^cose J 

 where Pavg = average transmitted power (= peak power -^ /vt) 



^^Cathode Ray Tube Displays, Radiation Laboratory Series, Vol. 22, p.38, McGraw-Hill Book 

 Co., Inc., New York, 1948. 



