VARIATIONS IN RADAR COVERAGE 129 
ever, the deformation of the coverage pattern itself 
will in general extend to a greater height. 
Two factors are operative in producing a rapid 
change of refractive index with height: variation of 
moisture with height and variation of temperature 
with height. Excessive refraction occurs when there 
isa rapid decrease of moisture with height (“moisture 
lapse”) and, to:a lesser degree, when there is a rapid 
increase of temperature with height (“temperature 
inversion”). The most pronounced cases of excessive 
refraction occur when both these conditions prevail 
at the same time. These conditions will be discussed 
later from the meteorological viewpoint. 
Since the atmosphere is a very tenuous substance, 
the amount of refraction, that is, the amount of 
angular deflection of the rays, is very small and in 
no case exceeds a fraction of a degree. How then can 
these small effects influence radar operations? The 
answer is that they do nof influence operations unless 
the angle between the ray itself and the horizontal 
is very small. If radar is used for fire control, search- 
light control, or fighter intercept control, the targets 
are usually at medium or short ranges, and the angle 
between the line of sight and the horizontal is usually 
larger than one to two degrees. Refraction has 
practically no effect on such an application of radar. 
However, the same equipment may be used for 
long-range search and then the story is different. 
With early warning radar the target may be an 
airplane 50 or 100 miles away, and it may fly at an 
elevation of only a few thousand feet. In this case 
the angle of élevation of the target above the hori- 
zontal, as seen from the radar, is only a fraction of 
a degree. This applies still more to seaborne targets. 
The atmospheric effects then become operationally 
important. It should always be kept in mind that 
only low-angle search is affected By meteorological 
conditions. 
Asa rule, the operational characteristics of a radar 
for angles of elevation of the target exceeding 1 
degree may be calculated on the assumption of a 
standard atmosphere, with confidence that all non- 
standard meteorological effects are negligible. 
GUIDED PROPAGATION 
It is obvious that excéssive bending of the rays in 
the lower layers of the atmosphere must distort 
radar coverage patterns. One case of special import- 
ance is illustrated in Figure 4. Four rays, out of 
many, are shown which leave the transmitter at 
different angles with the horizontal. 
Ray 1 is bent so much that after some distance it 
returns to the ground; there it is reflected and then 
the same course is repeated again. In this way the 
ray may be reflected a number of times in succession, 
remaining always in the lowest layer. This super- 
refraction “traps” the rays in a ‘“‘duct’’ and results 
in guided propagation of the radar waves. Trapping 
does not occur under standard atmospheric condi- 
RADAR 
STATION 
‘THE GRITICAL ANGLE IS ALWAYS LESS THAN +e 
Fiaure 4. Wave paths illustrated as rays in ground- 
based duct. 
tions. A ray, under standard conditions, may be re- 
flected by the earth’s surface only once before it 
‘escapes into space. 
Ray 2 is also bent in the lowest layer but not 
enough to keep it from escaping into the upper 
atmosphere whence it does not return to earth. 
Ray 3 is similar to 2 except that it undergoes one 
reflection by the ground before it escapes into the 
upper atmosphere. 
Ray 4 separates the two types of rays illustrated 
by rays 1 and 2. This ray becomes horizontal when 
it reaches the top of the trapping layer or duct and 
from there on travels along at the same height. All 
rays are divided into two groups: those that leave 
the trahsmitter at an angle with the horizontal less 
than the critical angle and are trapped, and those 
that leave the transmitter at a larger angle and 
proceed into the upper atmosphere. 
The critical angle is aways small, practically never 
larger than Y% degree. Its magnitude may be taken 
as a measure of the intensity of guided propagation, 
that is, of the amount of radiant energy trapped 
within the duct. Rays that leave the transmitter at 
a somewhat larger angle up to about twice the critical 
angle are sufficiently deflected while passing through 
the lowest layers to distort that part of the radar 
coverage pattern lying just above the duct. Rays 
leaving the transmitter at a still larger angle are not 
appreciably affected. 
The ground-based duct or trapping layer guides 
the wave along the earth’s surface in much the same 
way that hollow metal tubes guide microwaves. 
Within the duct there is less decrease of signal 
strength with distance than there is above the duct. 
Radar ranges on surface craft and low-flying aircraft 
located within a duct, similar to the one illustrated 
in Figure 5, are increased—sometimes to two, three, 
or four times the normal ranges. Ground echoes 
would be increased at the same time and might, in 
some cases, obscure partly, or even entirely, the 
echoes from incoming aircraft. 
When the radar is located within the duct, ranges 
on aircraft flying above the duct will be decreased 
only slightly, if at all. Often there may be a slight 
