PIPOLOGY 



127 



interesting interpretation of this kind of "fa- 

 tigue," based on concepts of conditioning, 

 the reader is referred to the original article. 



Perceptual Problems 

 Discrimination of Separate Pips 



Almost all of the preceding discussion has 

 been devoted to the visibiUty of a single 

 pip. In most of the studies, indeed, the 

 pip was stationary. In actual operation, 

 pips appear at different locations on succes- 

 sive antenna scans. If the scan is fast 

 enough, the successive pip locations form 

 a target course. They may cross the course 

 of another target. As a pip closes in range 

 it may also break up into two pips or three 

 pips, that is, become evident as a cluster of 

 separate ships or airplanes. At such times 

 discriminabihty becomes important. Dis- 

 criminabihty, of course, is basically a func- 

 tion of the resolving power of the CRT gun 

 and screen. It may also be a function of the 

 operator's visual acuity. With a given CRT 

 and operator, pip discriminabihty would be 

 a function of beamwidth and pulse length. 

 The only pubUshed study directly relevant 

 to this problem is that of Garner and Ham- 

 burger (15). They investigated pulse 

 lengths from one to five microseconds in 

 length on both a projection PPI and a stand- 

 ard PPI with a P7 screen, and found that 

 above three microseconds of pulse length 

 there was a minimum discriminable space 

 which was proportional to the pulse length. 

 Below three microseconds the P7 screen was 

 sHghtly superior to the projection PPI. 

 Their results are reported in terms of the 

 minimum detectable range difference in 

 yards between targets — a very practical unit. 

 Extrapolating their data, it would appear 

 that ultimate discriminabihty with short 

 pulse lengths would be greater for the P7 

 screen than for the projection screen. 



Visual Illusions 



Special mention should be made of illu- 

 sions, partly because of their inherent in- 

 terest and partly because they tend to be 

 ignored by non-psychological scientists. Il- 



lusions are real perceptual events caused by 

 physical stimuh but they differ from other 

 perceptions in their Ukehhood of misinter- 

 pretation. Two illusions have thus far been 

 discovered in radar work. 



The first, described by Sweet and Bartlett 

 (49), is a subjective "second" sweep hne on 

 a PPI. It appears "real" enough, even 

 though it is not as bright as the electronic 

 sweep and assumes a different center of 

 rotation, depending on the point of visual 

 fixation. The "ghost" sweep has appeared 

 thus far only at moderate-to-dim back- 

 ground brightnesses and at rotation rates 

 above 15 to 20 rpm, although very high rates 

 have not been investigated. It is believed 

 to be a special case of the after-image known 

 as Bidwell's ghost. It probably does not 

 distort pip detection but does serve to dis- 

 tract the observer. The second is the hnear- 

 perspective illusion on the scopes used in 

 OCA (Ground Controlled Approach) radar 

 which will be referred to in Chapter 7. It 

 is to be expected that more examples of 

 illusions will crop up from time to time as 

 radar undergoes technical development. 

 Some may prove genuinely bothersome to 

 the operator. 



"Pipology" 



In evaluating pips as to their probable 

 target source, an operator uses cues of bright- 

 ness, size, number, and location and espe- 

 cially the temporal variations in these. Add 

 to these the operator's intelligence and ex- 

 perience and the result is "pipology,'' the fine 

 art of target evaluation. In a sense, the 

 whole knowledge of the radar system is part 

 of it. The operator's available cues are ably 

 discussd from the practical standpoint by 

 Orman (42), Orman uses terminology fa- 

 miliar to radar operators and tells what to 

 do and when to do it, using scopes current 

 in 1947. Included in his article are hints on 

 estimating the number and composition of 

 sea and air targets from (1) size, (2) shape, 

 (3) fluctuation and (4) movement of pips. 



In another article, Orman (43) discusses 

 the training of radar operators and strongly 



