200 



ECHOES AND TARGETS 



creases the signal thresliold power. There is a real 

 difference between the video bandwidth and the i-f 

 banilwiilth in tlic following respect. Decreasing or 

 increasing the i-f bandwidth causes the components of 

 low-frequency noise in the video to change proportion- 

 ally. Video narrowing, however, does not change the 

 low-frequency video noise components. Therefore, the 

 reduction in signal visibility with video narrowing is 

 less pronounced than with i-f narrowing. 



The human eye cannot distinguish between two ob- 

 jects which are closer together than about 1 minute of 

 arc. If the light intensity contrast is limited, two ob- 

 jects cannot be resolved even at a much greater angular 

 separation. When the separation approaches approxi- 

 mately Vi of a degree, the best visibility will be 0I3- 

 tained for the smallest contrast. Thus, the action of 

 the human eye can be regarded as that of a filter which 

 pi-eferentially selects those frequencies having a period 

 of the order of V^ of a degree on the scope. At normal 

 viewing distances this value of angular separation is 

 of the order of 1 mm linear separation. Since the 

 screen behaves like a linear transformation between 

 the video signal and the light transmitted to the eye, 

 this filter action of the eye is exactly equivalent to a 

 video filter whose maximum pass frequency corre- 

 sponds to 1 mm divided l)y the sweep speed s. For most 

 presentations, where the pulse length is considerably 

 shorter than 1 mm on the scope, this effective video 

 narrowing action of the eye is usually much more im- 

 portant than the effect of video bandwidth in the re- 

 ceiver. It is to be noted, however, that video bandwidth 

 effects in the receiver can be observed when the sweep 

 speed is sufficiently fast for proper delineation of the 

 pulse. There is now a considerable amount of experi- 

 mental evidence to support this rather simple picture 

 of the combined effect of video bandwidth and the 

 resolution properties of the eye. 



Because of this property of the eye, if the viewing 

 distance is maintained constant, a large diameter PPI 

 will be more sensitive in the detection of signals than 

 a small one. A magnifying glass will produce an effec- 

 tive increase in sensitivity on the small scope at the 

 expense, however, of a restricted searching area. 



The focus on the PPI or A scope also acts like a 

 video narrowing deface. If the tube is defocused along 

 the range scale, equivalent video narrowing will take 

 place by an amount which is dependent upon the spot 

 size. However, because of the effect on the human eye 

 a loss in signal visibility will not occur until the de- 

 focused spot is larger than apjiroximately 1 nnn. 



Del'ocusing to this extent is certainly disadvantageous 

 in the ultimate discrimination of two close radar tar- 

 gets, and for this reason good spot focus must be 

 maintained. 



In signal detection it is clearly necessary that the 

 average signal deflection voltage be as large as the 

 average noise fluctuation in the absence of signal. This 

 is a purely statistical problem susceptible to theoretical 

 analysis. Calculations show that the quantities which 

 determine signal visibility, apart from the geometrical 

 factors just described, are the total number of sweeps 

 on which the signal is visil^le and the total number of 

 sweeps on which only noise is visible. It is assumed 

 that for these sweeps integration or averaging takes 

 place. This result is confirmed experimentally with 

 two restrictions. The total number of signal pulses is 

 given by T X PBF (pulse repetition frequency), and 

 the signal threshold power is found to vary inversely 

 with both PRF and T. While this holds for all values 

 of PBF under investigation (13.5 to 3,200 c) it holds 

 only for a limited region in T (approximately 0.05 to 

 3 sec ) . The reason why the integration is not satis- 

 factory outside these limits of T are related to the 

 maximum flicker frecpiency detectable by the eye. Por 

 times shorter than perhaps 0.05 sec additional sweeps 

 containing only noise will be integrated. Likewise, for 

 2' greater than 3 sec the eye and brain do not appear 

 to integrate properly all the individual voltages. In 

 other words, the system has incomplete memory. It 

 has been found that the maximum system integration 

 time (usually of the order of 6 sec) can be increased 

 ajipreciably by operator practice. With a consideralile 

 amount of experience a good radar operator can effec- 

 tively integrate for times as long as 1/2 min. It is to 

 be noticed that because of this integration in the eye 

 and brain of a radar operator other methods for pro- 

 viding integration, such as P-7 screens or photographic 

 integration, will fail to provide substantial benefit 

 unless their effective integration time exceeds several 

 seconds. This conclusion is borne out experimentally. 



In the radar scanning problem the same factors 

 must be considered as have already been discussed, 

 but, in addition, one must investigate factors peculiar 

 to scanning. Among these are the rotation speed of the 

 antenna, the beam width of the set, etc. It has been 

 found, however, that the statisical problem met with 

 in scanning is quite similar to that encountered in the 

 absence of scanning. The complete system integration 

 depends on two factors : the number of pulses inter- 

 cepted by the radar beam during one traversal of the 



