BUNDING 



179 



t 



CURVED* 



EARTH -ACTUAL RADIUS 



Figure 1. Actual pattern showing radar coverage for standard propagation. 





STRAIGHT LINE 



EARTH -4/3 ACTUAL. RADIUS 



Figure 2. Modified presentation of the information shown in Figure 1. 



change with height are temperature, pressure, and 

 moisture content. These changes from one level to 

 another are very small compared with that from 

 water to air, and the resulting refraction itself is 

 small. Nevertheless this refraction is of great import- 

 ance in radar operations and radio communications 

 above 30 mc. 



If the atmosphere were composed of a number of 

 successive layers each having a different index of 

 refraction, a wave passing across the successive boun- 

 daries of the layers would be abruptly deflected at 

 each surface. The atmosphere does not consist of such 

 distinct layers. Instead, the change in its physical 

 properties and its index of refraction is gradual, 

 continuous. There is, then, no sudden change in 

 direction of the waves; the change in direction 

 becomes gradual and continuous. In other words, a 

 bending of the waves occurs as they pass through 

 the atmosphere. Radio waves passing through the 

 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- 

 sions have been strongly exaggerated so that the 

 earth's curvature 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 

 % 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 account the effects of standard 

 refraction while doing his work. 



