TARGETS FOR MICROWAVE RADAR NAVIGATION 



857 



reflector is being viewed, as well as the geometrical configuration of the 

 reflector. For some of the simpler configurations the effective area can be 

 readily determined by the following procedure. 



Project the aperture of the reflector through the apex O to form the image 

 (A' B' C of Fig. 4); then project the aperture and its image upon a plane 

 perpendicular to the incident rays. The area common to the projections of 

 the aperture and its image is equal to the effective area. The effective area 

 of the triangular reflector of Fig. 4 is, therefore, represented by the hexagon 

 a b c d e f . Only those rays, perpendicular to the plane of the paper, which 



-r — 1 ST IMAGE 



2 ND IMAGE 



-3 RD IMAGE 



Fig. 5 — Determination of effective area of trihedral comer reflector. 



fall inside the hexagon will be returned. Exactly the same procedure is used 

 in determining the effective area for other aspect angles. 



The above rule must, however, be applied with caution. Situations arise 

 in which rays falling upon the area determined by this method do not return 

 to the source. Figure 5 shows a reflector in which this difficulty is encoun- 

 tered. This reflector differs from the previous reflector in that it has a notch 

 cut in one of the reflecting surfaces. The projection of the aperture upon the 

 plane of the paper is indicated by the solid line; that of its image by the 

 dotted line. According to the rule of the preceding paragraph, one would 

 expect the effective area to be defined by the total shaded area of the figure. 



