6-6] 



PULSED-DOPPLER RADAR SYSTEMS 



323 



Target Frequencies 

 n = 0,1,2 y 



1 li 



^^0^^. 



Main Beam and 

 Sidelobe Clutter 



Closing Target 



Frequency 



Fig. 6-23 Received Signal Frequency Spectrum for an Airborne Pulsed-Doppler 

 System, Showing Clutter Spectrum and Return from a Closing Target. 



can be represented as shown in Fig. 6-23. This diagram illustrates one of 

 the basic limitations of a pulsed-doppler radar system : in order to maintain 

 a clutter-free region for all closing targets of tactical interest, the spacing 

 between spectral lines must be 



where Vr, 



,fr> 2{Vf^ ^..,nax)/X 



maximum closing velocity dictated by tactics. 



(6-31) 



If this spacing is not maintained, some of the closing targets will be buried 

 in the clutter from adjacent spectral lines. This consideration leads to the 

 use of very high PRF's in pulsed-doppler systems. For example, an X-band 

 (3.2-cm) system designed for operation in a 2000-fps aircraft against 

 2000-fps targets will require a minimum PRF of 112 kc — a value that is 

 several orders of magnitude larger than the PWF's commonly used inpulsed 

 radars. 



When the return is mixed with the coherent reference signal as shown in 

 Fig. 6-20, the output spectrum shown in Fig. 6-24 is produced. This 

 heterodyning operation causes an effect known ?is folding; i.e. each of the 



II 



-Doppler Filter 

 Bandpass 



f 



(b) 



Fig. 6-24 Received Signal Spectrum After Heterodyning to Zero Frequency, 

 Showing Effects of Spectrum Folding: (a) Video Spectrum, (b) Filtered Spectrum. 



