SITING 
scale of about 1 inch to 16 miles and 1,000-ft 
contours are suitable for checking distant echoes. 
From the maps, profiles are prepared for various 
azimuths about the radar station. The first mile or so 
should be plotted accurately; at greater distances, 
the critical points, such as hills and breaks, should 
1eceive the most attention. On each profile is drawn 
the tangent line from the center of the antenna to 
the point on the profile which determines the shield- 
ing, as in Figure 9. The angular elevation a of this 
line of sight is marked on the diagram. If a is nega- 
tive, the profile should be checked out to the radar 
horizon to obtain the correct shielding angle. On a 
plane earth diagram, the line of sight is actually 
curved, but for distances up to 10 miles it may be 
taken as straight with small error. The height 
difference, with a in radians, is then equal to 
he — ky = dtana. (18) 
When the distance is larger so that earth curvature 
has to be taken into account, to the above expression 
for the height difference he — h; must then be added 
the amount by which the earth is sloping away over 
the distance d. This amount is d?/2ka, and the 
complete expression for hy — hi become: 
ta liv CTA sere (19) 
Wor easier handling of this equation, a set of 
curves may be drawn where hz — hy is plotted against 
d for various constant values of a. These curves 
may then be used to determine the height of the 
shielded region at any range. Thus all other moun- 
BS 
479 
tains along the profile in Figure 9 might be checked 
for visibility by comparing the height of the moun- 
tain with the value of h, — h; read from the curve 
a =0.5°. Any desired allowance for diffraction 
may be made by using a different curve such as 
a = 0. When the shield consists of several ridges 
close together, an equivalent 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- 
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, but where the terrain 
is questionable, such as in a region of low hills, the 
profiles should be taken at 5-degree intervals. 
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 (broken lines in Figure 10). These lines repre- 
sent the limits of the heights of the shielded regions. 
Targets or mountains below and beyond the cover- 
age contours will not be visible except by diffraction. 
These contours may be drawn for several heights. 
Where they are close together, the shielding is good 
but the coverage is poor. Where the lines are widely 
separated, as toward the sea, there is little or no 
shielding except that due to earth curvature. With 
the coverage contour diagram superimposed on a 
, 
a 
ay, 
4 
r_} 
-— 
rn 
Ficure 10. Theoretical permanent echo diagram 
