194 



SITING 



shielded region at any range. Thus all other moun- 

 tains along the profile in Figiu-e 9 might be checked 

 for visibility by comparing the height of the moun- 

 tain with the value of /(2 — fh read from the ciu've 

 a = 0.5°. Any desired allowance for diffraction 

 may be made by using a different cur\'e such as 

 a = 0. When the shield consists of several ridges 



lined. From an examination of the map, the azi- 

 muths at which profiles should be prepared are 

 determined. This will normally be about every 

 10 degrees. Where the shielding is obviously good, 

 the interval may be 20 degrees, IduI where the terrain 

 is (juestionable, such as in a region of low hills, the 

 |)rofiles should be taken at 5-degree inter\als. 



Figure 10. Theoretifal permanent echo diagram. 



close together, an ecjuivalent shield should be used. 

 This is derived by enclosing the ridges in a triangle, 

 whose apex is taken as the shield (see Figure 6). 



The general procedure to be followed in preparing 

 a prediction of permanent echoes will now be out- 



An overlay of the region is then prepared, showing 

 important geographical features and a polar-grid 

 system. On this chart is drawn the coverage contour 

 lines (Ijroken lines in Figure 10). These lines repre- 

 sent the limits of the heights of the shielded regions. 



