Chapter 9 

 TARGETS 



9.1.1 



SCATTERING PARAMETERS 

 Radar Cross Section 



IX DETERMINING the Coverage to be expected of 

 radar systems, it is important to know what 

 fraction of the power incident upon a target will be 

 returned to the receiver. A parameter involving the 

 dimensions and orientation of the target, and usually 

 also the wavelength, and which measures the propor- 

 tion of power returned, is called a scattering para- 

 meter. 



The most generally used of these parameters is the 

 radar cross section introduced in Section 2.4.1. It 

 is denoted bj^ a- and is defined Ijy 



= 47ra" — , 



(1) 



where W^ is the scattered power per unit area at the 

 receiver and Wj is the incident power per unit area at 

 the target. In terms of a, the radar gain is 



- = G1G2 A/. 



1 iTCl- \8Td/ 



(2) 



This equation may also be written in the form 



Po 



Pi 



9X- 



where 



^^ it) ''-■'-'' 



is the gain factor introduced earlier (see Section 5.1), 

 Aq is the free-space gain factor and Ap is the path- 

 gain factor. 



9.1.2 



Target Gain 



Another scattering parameter is 0^, the target 

 gain, discussed in Section 2.4.2. It is the gain of the 

 target in the direction of the receiver relative to a 

 shorted (dummy) douljlet antenna. The target gain 

 is connected with a by the relation 



V47r(r 



dj. 



3X 



(3) 



The corresponding radai- gain is 



— = -ifnOifiR-AK 

 P, 



(4) 



The factor 4 is due to the calculation of Gn relative to 

 a shorted doublet rather than to a matched load 

 doublet. If the calculation of G^ were made relative 

 to the matched load doublet, the factor 4 would be 

 replaced by 1. 



'^ Echo Constant 



The echo constant, denoted bv K, is defined bj^ 



and is related to a by 



A' = 



(6) 



47rX= 

 The corresponding power ratio is 



P 



P, 



Except for the factor Mx, A' is just a measured 

 in square wavelengths. 



^.^.,ft(|)'.. 



(7) 



' '"' Equivalent Plate Area 



A plate of area S placed normal to the direction of 

 propagation has a radar cross section gi^•en by 



S2 



(8) 



provided the linear dimensions of the plate are large 

 compared with X. An\' target may be supposed to 

 scatter (in the direction of the radar) an amount of 

 energj' equal to the amount that a plate of area S 

 would scatter in this direction. This area S is called 

 the equivalent plate area of the target. The corre- 

 sponding I'adar gain is 



P2 

 Pi 



\ ' \ axv 



(9) 



182 



