4-7] 



RADAR CROSS SECTIONS OF AIRCRAFT 



193 



Fig. 4-15 shows the cumulative amplitude distribution of a 2-second 

 sample of echoes from the B-36, plotted on so-called Rayleigh coordinates. 



30 

 25 

 20 

 15 

 o 10 



-10 



30 



25 ~h 



20 # 



15 2 



10 I 



■a 

 5 







-5 



-10 



0.010.11 5 10 30 50 70 80 90 95 98 99 99.5 99.9 



PERCENTAGE OF THE TIME THE VALUES EXCEED THE ORDINATE 

 Fig. 4-15 Cumulative Amplitude Distribution of B-36 Echo, Approach Aspect. 



The straight lines through the points represent the Rayleigh distribution. ^^ 

 Even with such a short sample (in this case, of only 240 pulses), the fit to a 

 Rayleigh distribution is quite good. From the data obtained, it was 

 concluded that for a 2-second sample the echo amplitude (and thus the 

 radar area) is Rayleigh distributed for most aspects, except at broadside 

 aspect. At broadside the amplitudes were compressed into a rather narrow 

 range. 



The Rayleigh distribution signifies that the target consists of a large 

 number of elements whose relative phases are independent and vary 

 randomly during the time of the observation. The number of independent 

 elements which constitute a "large" number, however, need be only about 

 four or five if their amplitudes are comparable. Thus the conclusion to be 

 drawn from the B-36 amplitudes distribution is that, except at broadside, 

 the target consists of just such a "large" number of independent scatterers, 

 and that in 2 seconds their relative phases pass through substantially all 

 possible combinations.^^ At broadside, however, the echo from the flat 



13J. L. Lawson and G. E. Uhlenbeck, "Threshold Signals" Mass. Inst. TechnoL, Laboratory 

 Series 24, 53, McGraw-Hill Book Co., Inc., New York, 1950. 



i^Practically, "all possible combinations" probably is satisfied if the phases vary over one 

 or two times 360°. 



