SITING AND COVERAGE OF GROUND RADARS 75 
Azimuths of the Sun, H. O. 71, published by the 
U. S. Navy, Hydrographic Office. The equation of 
time may be obtained from a current copy of The 
American Nautical Almanac, United States Naval 
Observatory, Washington, D. C. 
This method will be illustrated by the data from 
Example 1. The LAT is obtained as before. Between 
September 23 and March 21 the sun is in south 
declination and since the latitude in this case is 
north, the second part of the book labeled ‘‘Declina- 
tion Contrary Name to Latitude” is used. For 
latitude 40° an interpolation is made between 12:40 
and 12:50 obtaining 164°. The table is marked “the 
angular departure of the sun west of north” for 
readings in the afternoon, and the tabular value is 
therefore subtracted from 360°, giving 196° as the 
azimuth of the sun. It is usually more convenient 
to plot a curve of azimuth against time for the hours 
during which it is expected that the observation will 
be made. Such a curve may be used for several days 
without much error. 
A method that is léss convenient but requires no 
calculation is the equal altitude method. This con- 
sists in measuring the horizontal angles between the 
sun and a mark, when the sun is at the same altitude 
on both sides of the meridian of the observer. The 
bisector of the horizontal angle between the two 
equal altitude positions of the sun during the obser- 
vations is very close to true south, and the azimuth 
of the mark may be determined. 
A horizontal radiation pattern should be obtained 
to determine whether the electrical and mechanical 
axes of the antenna coincide and to discover any 
abnormalities in the main or secondary lobes. Defec- 
tive patterns should be corrected by appropriate 
maintenance. 
Visibility Problems 
It is frequently necessary to estimate the effect on 
rays of intervening obstacles or the curvature of the 
earth, or to compute the distance to the horizon, or 
the amount a ray would have to be diffracted to clear 
K 
un intervening hill. The methods described here 
enable one to solve such problems quickly and 
simply. 
DISTANCE TO THE Horizon 
The distance d of the horizon on a spherical earth 
as seen by an observer at elevation h is given by the 
well-known formula: 
Heme nes 
a= <5 or h=3@, (2) 
with d in statute miles and h in feet. This expression 
makes no allowance for refraction and is commonly 
used in visual work. 
In radio propagation work the refraction of the 
standard atmosphere is sufficient to increase the 
distance of the “radio horizon” to 
d=V2h or h= 
where d is expressed in statute miles and h. in feet. 
This corresponds to the use of an effective radius of 
the earth equal to ka where k is % and a is 3,960 
miles. This value of k will be used throughout this 
report. If it is desired to use other values of k, 
equation (8) may be written as 
Bkh 2d? 
d= ay OE A are 
1 4 
a (3) 
Points at heights h; and he which are separated by 
the sea or smooth earth are visible fronr each other 
if the distance between them is less than’ 
Drie anp RiIsz 
Over land, visibility is determined by the profile 
of the path involved. Elevations obtained from map 
contours may be plotted on a profile so as to take 
the effective earth curvature into account, and visi- 
bility can then be determined by graphical means. 
However, construction of such profiles on a curved 
datum line is tedious, and it is easier to compute 
Ficurs 2. Relations between various heights on earth’s surface. Dip and rise. 
