TARGETS FOR MICROWAVE RADAR NAVIGATION 853 



A eff = efifective area of target.^ 

 X = wavelength. 



d = distance between radar and target. 

 The above formula applies to the case where free-space propagation prevails; 

 that is, where multiple path or anomalous transmission effects are absent. 



It is apparent from the formula that, at a given wavelength and range, the 

 received echo power can be increased by increasing the transmitted power, 

 the size of the antenna, or the effective area of the target. The present paper 

 will consider only the latter. 



In some cases the effective area of a target can be calculated from simple 

 geometrical optics. For the more complicated structures it is always pos- 

 sible to measure the effective area by comparing the signal reflected by the 

 object in question to the signal reflected from a simple target of known 

 effective area. 



Flat Plates 



The simplest target for which the effective area can be calculated is a flat 

 metal plate oriented so as to be perpendicular to the incident radiation. It 

 can be demonstrated that a flat plate with all linear dimensions large in pro- 

 portion to the wavelength of the incident radiation has an effective area 

 which is substantially equal to its geometrical area. Diffraction effects at 

 the edges of such a plate are small in comparison with the energy reflected 

 from the central portion of the plate. 



Flat plates, however, have the serious disadvantage that, in order to create 

 strong echoes, they must be maintained accurately perpendicular to the 

 incident rays. At other angles of incidence the echoes fall off rapidly. For 

 this reason flat plates are of limited value as targets for use in navigation. 



DrsEDRAL Corner Reflectors 



A dihedral corner reflector consists of two perpendicular, plane conducting 

 surfaces which are usually arranged so that they intersect along a common 

 line. Figure 1 shows a typical dihedral reflector. The dihedral reflector 

 has the important property that a ray which enters the corner will experience 

 a reflection from each of the surfaces and will return in the direction from 

 which it came, provided of course that the entering ray lies in a plane which 

 is perpendicular to the line of intersection of the planes which form the 



2 The term "effective area" as used in this paper refers to the equivalent flat plate area 

 of a target. The echoing effectiveness of a target may alternatively be expressed in terms 

 of the cross section of an equivalent isotropic reflector as described by Schneider, "Radar," 

 Proc. I.R.E., Vol. 34, p. 529, August 1946. The alternative unit is called the "scattering 

 cross section" and is frequently denoted by the symbol <x, although Schneider uses S. The 

 two quantities are related by the equation a = iir A^eff/X-. Both units are useful. For 

 most of the targets considered in the present paper, Aeff does not vary with \ and is there- 

 fore preferable. 



