4-12] GROUND RETURN 221 



where Rg = range of ground reflector 

 Rt = target range 

 Rprf = range corresponding to an interpulse period. 



Echoes from objects in the range interval Rt + Rprf {n = 1) are known as 

 second-time-around echoes (STAE). It is not unusual for STAE to be 

 comparable to or stronger than the desired target echo. The range of angles 

 for which STAE may be troublesome depends upon the geometry and radar 

 parameters of the particular system under consideration. Obviously, a 

 knowledge of the characteristics of ground return is of importance in this 

 and in other applications. 



Some measurements of ground return at wavelengths of 0.86, 1.25, and 

 3.2 cm are given in a paper by Grant and YapleCj^^ who used vertical 

 polarization. Fig. 4-42 shows their results for a tree-covered terrain with 

 the trees in full foliage. It will be noted that a^ is very roughly independent 

 of the angle of incidence. o-° also increases with the frequency, but even at 

 X = 8.6 mm does not exceed — 13 db at any angle. Thus this type of terrain 

 absorbs most of the incident energy. 



20° 40° 60° 80° 

 ANGLE OF DEPRESSION 



Fig. 4-43 Comparison of a^ for Green 

 Fig. 4-42 o^ for a Tree-Covered Terrain. Grass and Dry Grass. 



Fig. 4-43 shows the results for ground covered with tall weeds and grass 

 in the spring when the grass was green and the ground wet and marshy, and 

 in the fall when the grass and ground were dry. Two effects are clearly 

 evident from this figure. (1) There is a very large and rapid rise in o-° near 

 vertical incidence, amounting to 15-20 db, when the ground is wet. (2) 

 Although a" increases steadily with frequency under dry conditions, when 



