6-5] 



FM/CW RADAR SYSTEMS 



313 



The information matrix of this system may be visualized in Fig. 6-16. 

 The maximum possible range is limited by scanning speed; that is to say, 

 beyond a given range, targets cannot be seen because the dwell time on the 



N, 



Total Doppler Acceptance Band 



'N.xNgxN.xNA 



Individual Doppler Filter Bandwidth 

 = Total Doppler Acceptance Band 



Fig. 6-16 CW Radar Information Matrix. 



target is less than the time required for a round trip of the radar energy. 

 Since a simple CW system has no range-measuring capability, the maximum 

 range is also the size of the minimum range resolution element. 



Range Measurement in CW Systems. To measure target range, 

 frequency modulation (FM) of the transmitted energy is generally used in 

 CW systems. The maximum deviation of the transmitted signal determines 

 the range resolution obtainable, while the frequency of the FM-ing deter- 

 mines the maximum unambiguous range.* 



Consider typically a simple sine wave FM as expressed by the trans- 

 mitted FM /CW waveform of Equation 6-21 : 



et = Et sin ( co«/ + y- sin oo™/ j- (6-21) 



The received doppler signal er as detected by a coherent FM/CW radar is 



c-r = Er cos cod^ H J— sin ir/mT cos (comt + Tr/mr) |- (6-22) 



/m 



where Er is the peak received voltage, cod is the doppler frequency, AF is the 

 peak transmitted deviation, /^ is the deviation rate, and r is the round trip 



^For a rough comparison with pulse radar the reader may consider the reciprocal of the 

 bandwidth of the FM transmission to be analogous to an "effective" pulse length, while the 

 FM rate is analogous to the PRF of a pulse radar. 



