128 TECHNICAL SURVEY 
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ee eo STRAIGHT LINE 
Figure 2. Modified presentation of the information shown in Figure 1. 
lower atmosphere are usually bent downwards. 
As can be seen from the illustration of the actual 
pattern (Figure 1), the bending of the waves, or rays, 
by the atmosphere permits one to see farther than 
he would otherwise. In the figure the vertical dimen- 
3ions have been strongly exaggerated so that the 
earth’s gurvature becomes clearly visible. Under 
average weather conditions the horizon distance is 
increased by about 15 per cent, but at an elevation 
near the first lobe the increase in range is much less 
than this amount. This is the case of standard 
refraction, or standard propagation. 
It, is rather inconvenient to draw curved rays in 
radar coverage and calibration diagrams. This can 
be avoided by assuming that the earth’s radius is 
4, the actual radius. Then in the diagrams the rays 
appear as straight lines when the propagation is of 
the standard type. This method often is adopted in 
radar calibration practice, with coverage diagrams 
drawn or printed to the % value of the earth’s radius 
(see Figure 2). This corrects for the effect of normal 
bending in the atmosphere. The radar operator 
merely plots the position of his target on such a 
diagram and assumes that the radiation travels along 
a straight line between the radar and the target. In 
this way he takes into acgount the effects of standard 
refraction while doing his work. 
Wave propagation deviating from standard occurs 
under special weather conditions. The most import- 
ant type is called “guided propagation,” “trapping,”’ 
or “superrefraction’”’—formerly referred to as 
anomalous propagation. The main feature of this 
type of propagation is an excessive bending of the 
rays due to refraction. This bending occurs prin- 
cipally in the lower layers of the atmosphere and 
mainly in the lowest few hundred feet. In certain 
regions, notably in warmer climates, excessive bend- 
ing is observed as high as 5,000 ft. The amount of 
bending in regions above this height is almost always 
that of the standard atmosphere. 
As a consequence of the excessive bending in the 
lower layers the coverage pattern of a radar set is 
deformed, as illustrated in Figure 3. The fact that 
ee Ree: 
Figure 3. Radar lobe pattern in nonstandard atmos- 
phere. A duct has been formed on the surface of the 
ocean and a ship is detected. Lobe No. 1 is,bent down- 
ward more than normal, but the other lobes remain sub- 
stantially unchanged by the duct. 
atmospheric influences are effective only in the lower 
layers does not imply that the echo strength from a 
target will be affected only as it lies in these layers, 
though the effects will be strongest there. It merely 
means that excessive bending is suffered by the rays 
only while passing through the lower layers. How- 
