SITING AND COVERAGE OF GROUND RADARS 97 
TasLe 3. Prediction of permanent echoes.* 
Relative intensities from Figure 40. 
Distance 
inmiles 1.18 1.00 0.75 0.50 0.25 0.10 0.03 
1 MSB MSB MSB MSB MSB MSB MSB 
10 MSB MSB MSB MSB MSB MSB M 
20 MSB MSB MSB MSB MSB M M 
50 M M M M M M 
100 M M M M M 
200 M M 
*This table will apply only fox the conditions of Example 8. 
area. Contours are drawn for the first few thousand 
feet, and prominent peaks are indicated. From topo- 
graphical sheets of a 20-ft interval and a scale of 
2 in. to the mile the profiles of Figures 43 and 44 
are obtained. From the center of the antenna to the 
“effective” shielding, the line of sight has been drawn 
and the angle of the line of sight noted. In some cases, 
as at 20 degrees (Figure 43) a near sharp ridge is not 
considered an effective shield because of the large 
diffraction around such obstacles. The map is 
inspected between the azimuths used and the hori- 
zontal limit of shielding of a ridge noted. Thus the 
shielding ridge on 120 degrees (Figure 44) is found 
PLL eT 
iTS 
500 
ANS 320 
Lee 
DISTANCE IN FEET 
6 
RANGE IN MILES 
Fieure 43. Profiles for Example 8. 
to drop off at 138 degrees. From the curves of 
Figures 38 and 39 are read the ranges for hz — hy = 
1,000, 5,000, 10,000, and 15,000 ft for the line-of- 
sight angles at various azimuths. These points are 
plotted on Figure 42; they are connected by heavy 
dashed lines and are the coverage contcurs. 
In Figure 45 are plotted the predicted echoes. It 
will be noted that the shielding to the east is very 
good and most of the mountains are not visible. To 
the north the numerous mountains are unshielded 
and give rise to many echoes which extend into the 
search sector to the west. The islands are inherently 
bad and cannot be shielded without drastic loss of 
coverage. In some cases, as along azimuth 345°, 
ridges which cause large echoes shield more distant 
ridges. The broken terrain in this region is taken to 
ol Ae 
HE eR 
ge | i 
i a a | i 
DISTANCE IN FEET 
in 
(esc 
r 
rast a 
JnaecEeeen 2 
oo 
a 
4 
RANGE IN MILES 
Ficure 44. Profiles for Example 8. 
give one large echo rather than a number of small 
echoes. In most cases the simple rules for plotting 
echoes may be applied directly. 
Where diffraction is involved the procedure should 
be more detailed. In Figure 43 azimuth 20° will be 
examined to determine the visibility of the hill at 
4.65 miles. The following data are obtained from 
the profile. 
h, = 387 ft; d’; — d’. = 1.45 miles 
hy = 550 ft; d’, = 3.20 miles 
H = 425 ft; dy = 4.65 miles 
From equation (8): 
hie 4.65 X 550 — 3.20. 387 i 4.65 X 3.20 
a 4.65 — 3.20 2 
= 917.2.ft . 
From equation (10): 
425 — 917.2 
fa = 5980 K 4.65 
X 57.3 = —1.15°. 
From Figure 40 the intensity is found to be 15 
per cent. At the very short range of this hill a strong 
echo would be expected at this intensity, and all 
lobes would be plotted. 
At 138.5° azimuth and 160 miles is a 10,000-ft 
mountain (not shown in any figure). The data for 
this case are: 
hy = 387 ft; d’; — d’, = 0.27 miles 
he = 380 ft; d', = 160 miles approximately 
H = 10,000 ft; d’, = 160 miles approximately 
j — 160. x 380 — 160 x 387 , 160 x 160 
ay 0.27 2 
= 16,950 ft. 
jy c= SU ea < 57.3 = —0.472°. 
5,280 X 160 
From Figure 40 the relative intensity is 43 per 
cent. A main lobe echo is plotted on the second sweep 
