1-5] 



THE MODULATION OF RADAR SIGNALS 



21 



sidebands to an extent depending upon the rate of target reflection char- 

 acteristic fluctuations. 



The target reflection entails phase changes with reference to the trans- 

 mitted signal incident to the finite time required for propagation of micro- 

 wave energy to and from the target. These phase changes occur in all 

 the frequencies of the transmitted wave. The phase changes are linear 

 with frequency and have a proportionality constant which depends upon 

 the distance to the target. The phase modulation that occurs in the 

 portion of the transmitted signal that is reflected back from the target 

 provides the basic means for measuring range to the target. Pulse radars, 

 for example, measure the phase (or time) difference between transmitted 

 and received pulse trains. 



Phase modulations of a somewhat different sort may result from the 

 motion of the target in conjunction with the space-modulation character- 

 istic of the radar. As an example of this process, consider a radar which 

 scans a directional beam through an angle of 360° once each second. If 

 there is a stationary target at an angle of Qt with respect to the reference 

 axis, a return from the target will be obtained as the radar beam sweeps 

 past this point. The amplitude of the return signal will have the general 

 shape of the radar beam resulting in a return signal having the envelope 

 shown in Fig. 1-15. Thus, the scanning process gives rise to an angle in- 





A. 



A 



[- — 1 sec — H 



27r 47r 



ANGLE (rad) 



^Lll sec-^ 

 A , A 



2 



TIME (sec) 



, J\ Stationary 

 67r Target 



I A Moving 

 3 Target 



Fig. 1-15 Effect of Target Motion. 



formation subcarrier which has a fundamental frequency of 1 cps. The 

 angle information is carried on the phase angle of this subcarrier funda- 

 mental. 



Now let us assume that the target flies in a circle around the radar station 

 in the same direction the beam is revolving, at a speed of 1 revolution 



