122 



RADAR SCOPES 



for high. These observations are at least 

 consistent with expectations. 



Visibility of Pips 



The threshold values of a pip as a func- 

 tion of PRF and antenna rotation speed 

 have been investigated by Payne-Scott (44) 

 and the results have been mentioned above. 



Scott says. But this is true only for very 

 bright scopes or for one whose bias is ad- 

 justed to PRF to maintain a constant scope 

 brightness. The change-over frequency is 

 not necessarily the threshold of background 

 uniformity; this too depends on bias. With 

 dim scopes, visibility improves with PRF 

 even after the scope is uniform because more 



300 450 600 750 



PULSE REPETITION FREQUENCY 



900 



1050 



Fig. 9. Visibility and pulse repetition frequency 

 The effect of PRF depends both on bias and rotation rate. The lower curves are for a bright ,_^ nearly 

 saturated screen; the upper, for a dim screen. Conditions: 7BP7 tube; no video noise; 2 fis x 2° pip. 



No one has yet worked out in detail all of 

 the interrelations among the parameters al- 

 though Payne-Scott's theory is the best ap- 

 proximation. Suffice here to illustrate a few 

 relations. 



PRF and Rotation Rate. Data of 

 Williams (52) show that visibility may im- 

 prove with increasing PRF up to a point 

 and then be essentially constant, as Payne- 



pulses per second can still add a favorable 

 brightening to the screen. With very high 

 PRF and very low bias, rotation speed can 

 be removed as a factor in pip visibility. 

 Otherwise it is inversely related to visibility. 

 (See Fig. 9.) Equipment limitations have 

 thus far prevented exploration of the very 

 high PRF's and rotation speeds. 

 PRF and Bias. Part of the improvement 



