222 REFLECTION AND TRANSMISSION OF RADIO WAVES 



the ground is wet the curve for 1.25 cm falls much below the other two. 

 Grant and Yaplee state that this behavior was always found on 1.25 cm 

 when the ground was wet, and suggest that this anomaly may be associated 

 with the water vapor absorption peak near this wavelength (see Paragraph 

 4-16). Aside from the large rise near vertical incidence, the remainder of 

 the curve lies approximately 5 db higher when the ground is wet than when 

 it is dry. 



The large increase of o-" near vertical incidence when the ground is wet 

 can be explained as caused by patches of water which are viewed broadside, 

 as in the case of the facets which have been proposed as the scattering 

 elements for sea clutter. This emphasizes the importance of plane surfaces 

 whose dimensions are comparable to or large compared with the wavelength 

 when they are viewed broadside. Hence, in attempting to generalize on the 

 basis of the rather meager experimental results which have been reported 

 in the literature, this characteristic should be kept in mind. 



An important example of this is the case of cultural areas, especially 

 cities. Here, in addition to the presence of large flat surfaces, such as 

 building walls, windows, roofs, and streets, there are many possibilities for 

 corner reflectors. Since corner reflectors have a large radar area over a wide 

 range of angles, they have a very large effect on the radar return. For 

 example, observations of ground painting by airborne radar^^ show that the 

 signals from man-made structures are often too strong to be fully explained 

 in terms of their size, and that a certain amount of corner-reflector action 

 ("retrodirectivity") in the targets must be present. This action is present 

 principally at long ranges (small depression angles) and is responsible for 

 sharp contrast in the return from arrays of buildings at long ranges. At 

 short ranges, where the depression angle is outside the range of corner- 

 reflector action, this contrast tends to fade. These principles have to be 

 kept in mind, for example, in estimating the effect of STAE from a city on 

 the performance of the radar in an AI, an AEW, or a target-seeking missile 

 application. 



4-13 ALTITUDE RETURN 



In Paragraphs 4-10 to 4-12, we have discussed the back-scattering 

 properties of the sea and ground in terms of the scattering parameter o-". 

 This has been done in order that the properties could be applied to radars 

 with a wide range of parameters. In order to determine the response for a 

 particular radar, one needs to consider the radar parameters in connection 

 with the scattering characteristics of the surface. One case which is of some 

 importance is that of the altitude return in pulse radar. This is the signal 

 received from the ground directly beneath the aircraft. On a PPI display 



32L. E. Ridenour (Ed.), Radar System Engineering, Vol. 1, pp. 100-101, McGraw-Hill Book 

 Co., Inc., New York, 1947. 



I 



