144 



SITING AND COVERAGE OF GROUND RADARS 



Figure 45. Predicted permanent echoes. (Example 8.) 



there is trapping, and the size of this echo indicates 

 the unusual weather conditions at the time the data 

 were taken. This echo is due to a mountainous island 

 260 miles away and about 5,000 ft high. There is 

 no shielding except from the curvature of the earth. 

 The relative intensity compared to the free space 

 intensity computed from the formulas for the dif- 

 fractive region (not given here) is 0.5 per cent. This 

 echo would not ordinarily be plotted in spite of the 

 large area of the mountain side. 



In correlating the predicted and actual fixed echo 

 diagrams, Figures 45 and 35 respectively, it will be 



noted that the degree of success achieved depends 

 on the effort expended. Numerous small echoes were 

 not predicted, but these are unimportant from an 

 operating standpoint. "Permanent" echoes vary over 

 wide limits with changes in weather conditions and 

 efficiency of the equipment so that only a fair 

 agreement should be expected in their predictions. 



15.5.7 



Microwave Permanent Echoes 



With microwave equipment a simple analysis of 

 the terrain is generally sufficient. The beam may 



