116 



RADAR SCOPES 



covering" exposure: as the cumulative area 

 gets bigger there is an increasing probability 

 of a unit area's being looked at. 



Other Factors Influencing Pip Size 



Several factors influence effective pip size 

 besides pulse length and heamwidth. Some 

 of these are as follows. 



Sweep Time or Range Scale. On a radar 

 set, the range scale is the control for sweep 

 transit time, i.e., the time required for the 

 deflection of the electron beam from the 

 center to the periphery of the scope. Since 



RADIAL POSITION IN INCHES 

 Fig. 7. Visibility and range 

 Range of target is defined by position on 3- 

 inch radius. Data are for a typical 7BP7 tube, 

 no noise. Large pip is 3 jus x iO°; small pip, | 

 ns, X 1°. Low bias is 10 volts less (brighter) than 

 visual reference; optimal bias is 5.5 volts less. 



scope radius is constant, a one microsecond 

 pulse will occupy a greater proportion of the 

 radial sweep on a short distance range scale 

 than on a long. It will appear, in fact, 10 

 times greater in radial length on a 20-mile 

 scale than on a 200-mile scale. This rela- 

 tion has been investigated and the expected 

 effect on visibility obtained (4) . 



Range Position. As a pip closes in range it 

 moves closer to the center of the PPI; and 

 though its angular beamwidth remains con- 

 stant, the same degree of modulation 

 is placed on the electron beam at all range 

 positions. This means that, although the 



pip is becoming smaller at close range, it is 

 also becoming brighter, unless limited by 

 phosphor saturation. Experimental tests 

 (cf. next section) have not consistently 

 shown any effect of range. Data sometimes 

 show a drop-off in visibiHty at the extreme 

 periphery or very near the center, but these 

 effects usually may be attributed to poor 

 phosphor qualities, i.e., "burning" at the 

 center and thinness at the edge. There is, 

 however, good reason to believe that visual 

 size may be a factor, under certain condi- 

 tions. The argument is as follows: there is 

 an approximate compensation of area loss by 

 brightness, i.e., the A X I = C (Ricco's law) 

 statement is roughly true for small objects. 

 Apparently what happens in the usual PPI 

 scope is that the increasing brightness of the 

 pip, as it approaches the center, approxi- 

 mately compensates for the decreasing 

 beamwidth. This is to be expected under 

 favorable brightness conditions, but if the 

 phosphor is nearly saturated the brightness 

 will not be able to increase proportionately, 

 the total light increment will be reduced, 

 and visibility will be more impaired. This 

 effect should be greatest with very small pips. 

 An experiment to test this expectation more 

 or less confirmed it. The data are sho^vn in 

 Fig. 7. 



Distance of Observer from Scope. As the 

 distance of the eye from the scope increases, 

 the retinal subtense of the pip decreases and 

 therefore visibility is expected to become 

 poorer. This is true, as shown by Bartlett 

 and Williams (4), for noise-free scopes but 

 there was no advantage of the closer view 

 when the scope was so heavily cluttered by 

 large noise pips as to require considerable 

 discrimination among them. The maximal 

 advantage of a 6-inch over a 24-inch view 

 was 8 db. 



Scope Size. Within wide Umits, the scope 

 diameter will probably not affect the visi- 

 bility of large pips, but for very small and 

 weak pips, the larger scope should furnish 

 slightly better visibility, in accordance with 

 the laws governing size effects. No direct 



