628 DISPLAY SYSTEM DESIGN PROBLEMS 



Film Strip 

 Direction 



Fig. 12-11 Intensity-Modulated Ground-Mapping Display. 



Example 2. As a second example, the AI radar discussed in Chapters 2 

 and 3 might be considered. This system employed a beamwidth of about 

 3.6°, a pulse length of 1 microsec, an azimuth coverage of 120°, and a total 

 surveillance range of about 25 miles. Thus, a B-scope search display (see 

 Fig. 12-6) must be capable of handling 33 angular elements and 300 range 

 elements. If the display height is 5 inches, then there are 60 range elements 

 per inch — a resolution capability that may be preserved by an indicator 

 with 120 resolvable elements per inch. In this case, however, it is important 

 to note that radar resolution is not a vital factor for mission accomplishment 

 - — the radar antenna size and pulse length were governed by other consider- 

 ations such as detection range, and the radar resolution capability need 

 only be great enough to resolve targets separated by several miles. Thus, 

 in contrast to the first example, the system performance is relatively 

 insensitive to indicator resolution, and this property of the indicator may 

 be traded for other properties more important to the AI radar detection 

 problem. 



Relative to requirements of this type, it is of interest to consider the 

 typical resolution characteristics of cathode ray tubes. These devices 

 provide the highest resolution of all the various types of display devices. 

 The spot size of the CRT is obviously a determining factor in the resolution 

 of the display. The smallest spot size obtainable at this time with careful 

 control of the electron-optics system and at a low beam current is approxi- 

 mately 0.8 mil at the center of the screen. Tubes having such small spot 

 size utilize magnetic focus. However, to reduce misalignment problems and 

 weight, electrostatic focus is used in most airborne cathode ray tubes. At 

 low beam currents and high anode potentials spot sizes of 1.5 mils have been 

 measured at the center of the scope. Usually, though, with the cascaded 

 phosphors which are used in most radar indicators, spot sizes of 10 to 40 

 mils are measured, depending on the beam current. The spot size is 

 degraded as the spot is deflected away from the center of the cathode ray 



