4-5] 



REFLECTION OF PLANE WAVES FROM THE GROUND 



181 



In ordinary (non-doppler) radar, this shift in frequency due to the average 

 approach velocity of the target is not noticed in the case of a point target. 

 For extended targets, such as rain clouds and the ground or sea, for which 

 various portions of the target area fill all or an appreciable part of the radar 

 beam, the approach velocity varies over the beam, so that the composite 

 echo has a spectrum of frequencies. In a doppler radar this spectrum will 

 be properly discernible as frequency shifts relative to the radar frequency. 

 In a non-doppler radar, beats between the various frequencies will be pro- 

 duced in the final detector, so that an echo spectrum will also be obtained. 

 In the case of an aircraft, a turn, pitch, or yaw will also introduce doppler 

 beats which are discernible in a non-doppler radar. For example, consider 

 the effect of a turn, which imparts an angular velocity c3 of the target about 

 its center of gravity. Two fixed points on the target a distance D apart will 

 then have a relative radial velocity toward the radar of 



A/^ = coD cos (4-22) 



as can be seen from Fig. 4-3. Hence 

 by Equation 4-21 the difference in 

 doppler frequency between these 

 two reflection points is 



Af=?^=?5°^. (4-23) 



A A 



Thus the doppler frequency will be 

 proportional to radar frequency, to 



= SD cos d 



Fig. 4-3 Differential Doppler Effect 

 Due to Turning of Target. 



the angular velocity of the target, 

 and to the gross aspect of the 

 target. 



These and other effects which result in fluctuations of the target echo will 

 be discussed further in later sections. 



4-5 REFLECTION OF PLANE WAVES FROM THE GROUND 



The reflection of radar waves from the ground or sea surface is an 

 important factor in a number of phenomena associated with airborne radar. 

 Among these may be cited the lobe structure which is encountered in 

 tracking low-altitude targets, height-finding errors for such targets, and the 

 dependence of sea and ground clutter upon polarization and depression 

 angle. In all these cases, an understanding of the basic phenomena can be 

 obtained from a consideration of the reflection of plane waves from a plane 

 homogeneous surface. 



The reflection of a plane wave from flat ground depends on the frequency, 

 polarization, and angle of incidence of the wave, and on the electrical 

 properties of the ground (dielectric constant and conductivity). A wave of 



