136 



SITING AND COVERAGE OF GROUND RADARS 



MffO 



LAND 



Figure 35. Permanent echo diagram. 



or the roughness of the surface becomes excessive. 

 3. Frequency variations will affect the echo from 

 complex reflectors such as rugged terrain. Peaks 

 which are separated in distance such that the returns 

 from a single pulse overlap are said to be frequency 

 sensitive. In the overlap portion the echo strength 

 will depend upon the relative phase of the two 

 returns. Thus if the pulse width is 10 ^sec the wave 

 train will be about 2 miles long. If there are peaks 

 at 10 miles and 10M miles their echoes will appear 

 as follows on an A scope : 



10 to 103^2 miles near peak echo only 



10^ to 12 miles 

 12 to 12)^ miles 



combined echo of both peaks 

 far peak echo only 



The combined portion of the echo may have a height 

 from zero to twice that of the individual echoes and 

 usually fluctuates rapidly as the frequency drifts. A 

 change of a half wavelength in the separation of the 

 peaks will change the combined echo from maximum 

 to minimum. This means that a frequency stability 

 of the order of one part in a million is required for 

 a steady combined echo. 



Permanent echoes used for testing should therefore 

 be (1) nearby but distinct from ground clutter and 



