■V1RCK VKT TAHGKTS 



H3 



210 



190 ISO 



10.2 



AIRCRAFT TARGETS 



Diagrams showing the dependence of a on the 

 orientation of the aircraft indicate very large and 

 irregular fluctuations. The radar cross section can 

 change by 100 to 1 with a change of aspect of only 

 a few degrees. These varying values of the radar 

 cross section are dependent on wavelength, polari- 

 zation, details of plane design, etc. Reflection pat- 

 terns such as shown in Figure 1 have been measured 

 in the laboratory for a few simplified models. Actually 

 an observer would see only the time average of the 

 radar cross section of a plane, and it is only this 

 average value which is of operational importance. 



Table 2 gives measured values of a for various 

 aircraft. These are the values to be used in equation 

 ( 6) . As far as is known, these empirical cross sections 



Figure 1. Aspect diagram of a B-17E at 5 degrees 

 above horizon. 



are independent of wavelength. This result may be 

 interpreted to mean that a plane in motion behaves 

 more or less like a collection of good reflecting 

 surfaces oriented at random. It is worth noting in 

 this connection that the radar cross section of a 

 circular plate of radius a, whose normal is at an 

 angle 6 with the direction of incidence, is 



a = to, 2 



cot 6 X Ji 



(Aira 



sin 6J , 



(8) 



Table 2. Airplane radar cross sections. 



