322 GENERIC TYPES OF RADAR SYSTEMS AND TECHNIQUES 



and pulse repetition frequencies and to provide coherent references for the 

 receiver mixing processes. The transmitted signal thus consists of an RF 

 pulse train as shown in Fig. 6-21 (a) which has the frequency spectrum 

 shown in Fig. 6-21 (b). The width of the frequency spectrum is a function 

 of pulse length; the separation between adjacent spectral lines is equal to 

 the pulse repetition frequency /r. 



The operation of a pulsed-doppler system can best be visualized by 

 examining the character of the return spectra from targets and clutter at 

 various points in the receiving system. The target and clutter spectra for a 

 single spectral-line transmission from a moving platform is shown in Fig. 

 6-22. Because of sidelobes, the frequencies of the fixed clutter returns can 



Relative Target Velocity 

 Velocity of Radar Aircraft 



Sidelobes 



MC^ 



Fig. 6-22 Target and Clutter Spectra for a Single Spectral Line Transmission. 



vary ±2/^f/X from the transmitted frequency. The clutter possesses a high 

 peak resulting from clutter return in the main beam. The position of this 

 clutter peak depends upon the angle between the antenna pointing axis and 

 the aircraft velocity vector. Quite obviously, if the antenna is scanning, 

 the frequencies of the clutter returns will change as functions of time; in no 

 case, however, can the returns from fixed clutter be doppler shifted by more 

 than 1VfI\. For closing targets {Fr > Fp), the target returns will be 

 shifted by IV^jX and will therefore appear in a clutter-free portion of the 

 frequency spectrum. 



The effects of scanning and target-induced modulations (see Paragraph 

 4-8) cause a broadening of the target spectrum. Generally, the latter effects 

 are small compared with the modulation induced by scanning, so that the 

 width of the target spectrum may be expressed 



^ftarort = ^ (6-30) 



where td = dwell time of the main beam on the target. 



By analogy, then, the frequency spectrum of the signal and fixed clutter 

 returns for a ^«/j-^^ transmission consisting of an assembly of spectral lines 



