(y-S] 



FM/CW RADAR SYSTEMS 



317 



200- 



150- 



50- 



C^Speed of Electromagnetic Propagation 

 Vf = Speed of Radar Platform 

 V^ = Radial Target Speed 



Clutter 



'/"' 



\ 0? y^ Receiver 

 ^^ hE / Noise 



^ 



Density 



Ike lOkc 



2Vc 



AM Noise 

 Density 



lOOkc IMc lOMc lO^Mc lO^Mc lO^Mc 



f Frequency — 



c ^ 



Fig. 6-18 Typical Signal Levels in an Airborne FM/CW Radar. 



A principal advantage of CW doppler systems is their simplicity, when 

 compared with other means for obtaining high clutter rejection such as 

 pulsed doppler and coherent AMTI systems. Provided that some of the 

 limitations to be discussed below do not seriously limit tactical utility, an 

 FM/CW system offers a lightweight and potentially reliable answer to 

 many airborne radar system problems. 



The use of doppler techniques places several constraints upon the tactical 

 usage of an airborne weapons system. (1) There are approach aspects where 

 the target doppler frequency can be zero, or where the target doppler 

 frequency falls within the clutter spectrum. These conditions lead to "blind 

 regions" and regions of poor signal-to-noise ratio which must receive careful 

 consideration and analysis in the overall system design. For example, 

 approaches in the rear hemisphere of a target can be degraded by these 

 considerations. Fortunately, the most interesting approach region from 

 many tactical standpoints — forward hemisphere or head-on — is a region 

 where doppler sensing devices are most effective in detecting and tracking 

 targets in heavy ground clutter. 



(2) Buplexed-active operation (transmission and reception through a 

 common antenna) is generally impractical in FM/CW doppler equipment 



