142 ; TECHNICAL SURVEY 
STANDARD ATMOSPHERE NONSTANDARD ATMOSPHERE 
CURVED EARTH h 
RADIUS a 
HEIGHT ~ 
h CURVED EARTH h 
RADIUS ka 
(HOMOGENEOUS 
ATMOSPHERE) 
INVERSION 
PLANE EARTH 
(MODIFIED INDEX 
CURVES) 
M M 
Figure 15. Types of index curves. 
where d,, a, and hare all expressed in the same units. 
For the particular value of k=%, 
= V17hi + V1The*, (14) 
where d, is measured in kilometers and h is in meters; 
and 
dy = V/2hi + V2h2 , (15) 
where d, is given in statute miles and A in feet. 
INTERFERENCE 
q nREGION TT 2S 
ag —S— 3 DIFFRACTION 
ny eam arin W Ty Grier * REGION 
RA, 
Us a 
hp IN FEET 
jefleer sates 
coh 
-200 -180 “160 Lo “120 -100 -80 
0B 
Ficure 17. Diffraction and interference fields at height 
h,. Field strength at 50 statute miles over sea water in 
db relative to field at 1 m from transmitter. Horizontal 
polarization. Transmitter height 30 feet. 
The field strength at different elevations hz (Figure 
16) for a given range varies in the manner illustrated 
in Figure 17. The field is given in decibels; relative to 
the intensity at 1 m from the transmitter, for a range 
of 50 miles over sea. water for frequencies of 100, 200, 
500, and 3,000 me. The horizon elevation for this 
point is 888 ft. Above point P in Figure 16, is the 
interference region where, with increasing height, the 
field strength first increases rapidly and then oscil- 
lates between maxima and minima determined by 
the lobe patterns of the coverage diagrams. 
Below point P, the field strength declines rapidly 
with decreasing height to a minimum at ground 
level; the rate of decrease is larger for the higher fre- 
quencies. Neither the direct nor the reflected rays 
can penetrate into this region, which therefore, re- 
ceives radiation entirely by diffraction of the energy 
around the earth’s curvature. 
‘Radar targets are rarely visible when they are in 
the diffraction region. This is certainly true for air- 
plane targets. Very large targets, such as warships or 
islands, are occasionally visible in this region; but 
more often the detection of targets is caused by 
superrefraction. For communication work, on the 
other hand, the diffraction region is of importance, 
especially at the longer wavelengths. 
ATMOSPHERIC STRATIFICATION 
AND REFRACTION 
Origin of Refractive Index Variations 
The variation with height of the index of refraction 
n controls the curvature of rays in the atmosphere. 
The value of n exceeds unity by only a few hundred 
parts in a million and may be computed from the 
following formula: 
79p _ lle i 3.8 X 10%e 
T T 1? 
(n — 1) 108 = - (16) 
in which n = index of refraction at height h above 
ground; 
p = barometric pressure of the atmosphere 
in millibars at height h. (1 mm Hg 
pressure = 1.334 mb); 
e = partial pressure of the water vapor in 
millibars (order of 1 per cent of p); 
T = absolute temperature ((C + 273) at 
height h. 
In equation (16) the term 1le/T is very small in 
comparison with the other terms and may, without 
serious error, be neglected. This simplification has 
been used in obtaining the values in ‘the last two 
columns of Table 1 and in designing the nomogram, 
Figure 19. 
Workers in the field may prefer to use mixing ratio 
(practically equal to specific humidity) in place of 
