180 



VARIATIONS IN RADAR COVERAGE 



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. 



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 

 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- 

 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 

 is a 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 not 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 elevation 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. 



As a 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. 



RADAR 

 STATION 



DUCT 



Figure 4. Wave paths illustrated as rays in ground- 

 based duct. 



162 GUIDED PROPAGATION 



It is obvious that excessive 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 



