22 ELEMENTS OF AIRBORNE RADAR SYSTEMS DESIGN PROBLEM 



every 10 seconds (i.e., one-tenth of the scanning velocity). The return 

 signals now have the form shown in the lower diagram. The effect of the 

 target motion has been to shift the fundamental frequency of the angle 

 information subcarrier by 10 per cent to 0.9 cps for a target moving in 

 the same direction as the scan. This process might also be viewed as in- 

 troducing a time-varying phase shift in the 1-cps subcarrier (phase modula- 

 tion). How the target modulates the information subcarriers is one of the 

 most important problems of radar design. The choice of frequency and 

 bandwidth for the subcarrier frequency information channels is largely 

 governed by these characteristics. 



One other target modulation — the doppler frequency shift mentioned 

 in preceding paragraphs — is of fundamental importance. 



Motion of the target along the direction of propagation (see Fig. 1-16) 



^=> 



OAAAAAAAAA. 



•« 



Fig. 1-16 The Doppler F.ffect. 



causes each frequency component of the transmitted wave that strikes 

 the target to be shifted by an amount 



/d = {VtIc)/ (1-19) 



where Vt = the velocity of the target 



c = the velocity of light 



/ = the radio frequency. 



When this signal is reflected or reradiated back to the radar, the total 

 frequency shift of each component is 



fn = {2FtIc)/. (1-20) 



The frequency modulation caused by target motion is important; an 

 entire family of radars known as doppler radars has been developed to 

 exploit this characteristic. However, whether use is made of this char- 

 acteristic or not, the doppler shift occurs in all signals reflected from 

 objects that possess relative radial motion. 



Thus, it can be seen that the target generates a large number of am- 

 plitude, phase, and frequency modulations of the transmitted signal. 



