6-5] FM/CW RADAR SYSTEMS 311 



system has been illustrated here, and numerous ramifications become 

 evident. 



The ultimate goal in search radar using correlation and storage tech- 

 niques will be achieved when the range accuracy and resolution are limited 

 only by the transmitted bandwidth, and the range-rate accuracy and 

 resolution are limited only by the total observation time during which the 

 target doppler remains coherent. 



6-5 FM/CW RADAR SYSTEMS 



Previous discussions have dealt primarily with pulsed radar systems — 

 i.e., systems where transmission and reception occur at different times. 

 Another important class of radar systems is composed of systems that 

 employ continuous transmission (CW systems). In these systems, the 

 transmitting and receiving systems operate simtultaneously rather than on 

 a time-shared basis. 



For certain applications — such as semiactive missile guidance systems 

 — continuous-wave (CW) systems can offer important advantages, 

 particularly with respect to high clutter rejection for moving targets, 

 and relative simplicity. 



Basic Principles of Operation. In a CW system, the transmitted 

 and received signals are separated on a frequency basis rather than on a 

 time basis as for a pulsed system. This is accomplished by maintaining 

 phase coherence between transmission and reception — a process which 

 permits the measurement of the doppler shift caused by the continual rate 

 of change of phase in the radar reflection from a relatively moving object. 



The principles of operation of a simple CW system are illustrated in Fig. 

 6-14. A signal at frequency/^ is transmitted. The return echo from a target 

 moving with relative velocity Vr is shifted by the doppler effect to a new 

 frequency /< +/d, where 



Ja = 1ft Vrlc. (1-20) 



Closing geometries shift the received frequency higher than the trans- 

 mitted; opening geometries lower the frequency. 



The transmitted and received signal frequencies are mixed to recover the 

 doppler frequency, which is then passed through a filter whose bandpass is 

 designed to accept the doppler frequency signals from moving targets and 

 suppress the return signals from fixed targets such as ground clutter. (For 

 the example in the diagram, the clutter is shown at zero frequency: a 

 condition that would exist for a ground-based radar.) For a moving 

 airborne radar, the clutter would also possess a doppler shift relative to 

 the transmitted frequency. This complicates the design of the filter; 

 however, the basic principle remains the same. 



