4-2] REFLECTION OF RADAR WAVES 175 



differential radial movement of a half-wavelength of the target or a portion 

 of it is sufficient in many cases to produce a profound variation of echo 

 amplitude, even such targets as trees, towers, and buildings, normally 

 considered stationary, frequently give fluctuating echoes. For a given 

 target, the rate of fluctuation usually will be proportional to radar fre- 

 quency. 



The current distribution set up in the target depends on the distribution 

 of the incident field. In many common situations, the incident field is 

 rather uniformly distributed over the target aperture, so that the target 

 may be considered to be illuminated by a uniform plane wave. Then the 

 scattering characteristics of the target may be analyzed independently of 

 the propagation factors. This is permissible in the case of most airborne 

 or elevated targets. More generally, however, the incident field may be 

 distributed nonuniformly over the target, because of the nature of the 

 propagation phenomena obtaining between the transmitter and various 

 portions of the target. A ship is an example of a target in which the incident 

 field varies over the target aperture because of the interference between 

 direct and surface-reflected rays, which gives a resultant amplitude that 

 varies with height. In such cases the scattering properties of the target 

 cannot be separated from the propagation factors, so that a specification 

 of the target properties becomes more complicated and involves the 

 propagation factors. This same type of complication is also involved in 

 sea and ground return. 



The principal propagation factors which affect airborne radar are the 

 following. 



1. Reflection from the ground 



2. Attenuation by liquid water drops in the air 



3. Absorption by atmospheric gases 



4. Refraction in the atmosphere 



This chapter will be devoted to a discussion of these factors and to a 

 description of the characteristics of the principal radar targets of interest 

 in airborne applications; viz., aircraft, sea return, ground return, and rain. 



4-2 REFLECTION OF RADAR WAVES 



The radar equation for free space is derived in Chapter 3 (Equation 3-9). 

 It may be modified to account for the effect of obstacles such as the earth's 

 surface or an inhomogeneous atmosphere by introduction of a quantity 

 called the propagation factor., which is the factor by which the free-space 

 field is to be multiplied to obtain the actual field. This factor, which we 

 denote by F, is a complex quantity, or phasor, representing the modification 

 in amplitude and phase of the free-space field by the actual propagation 



