320 GENERIC TYPES OF RADAR SYSTEMS AND TECHNIQUES 



methods can maintain the target signal in the velocity gate after lock-on. 

 By allowing adequate time for the energy to build up in the gate,^ the 

 maximum allowable sweep rate is proportional to the bandwidth squared. 

 It is this "squared" sweep constraint that makes sweeping very narrow band 

 filters very time consuming. As the filter bandwidth approaches the 

 scanning frequency (or the reciprocal of the time on target), fixed filters 

 become mandatory (see Fig. 6-15). 



From a performance standpoint, the behavior of the system in the 

 presence of multiple targets is most important. Multiple radar targets 

 include terrain and weather returns as well as reflections from man-made 

 objects; for an airborne radar, each radar target will have a finite radial 

 velocity with respect to the radar platform. Because of their physical size, 

 man-made objects are usually point source targets, whereas clutter is most 

 always angularly expansive. 



In the presence of a hypothetical hemisphere of homogeneously reflective 

 clutter, the clutter doppler amplitude versus frequency spectrum would be 

 similar to the integrated transmit-receive antenna radiation pattern versus 

 angle as viewed at the receiver. A typical CW radar doppler clutter 

 spectrum is shown in Fig. 6-18. Note the complete absence of interference 

 at all frequencies above own-speed dopplers; only nose-aspect closing 

 targets appear in this uncontaminated spectrum. In practice the clutter 

 spectrum may be smeared somewhat by noise modulation on the trans- 

 mitted energy. Reasonable prediction of specific clutter is possible, but 

 there are a myriad of possible situations. Clutter can assume staggering 

 proportions; a terrain return 100 db above the minimum detectable signal 

 power is not inconceivable. Ultrahigh linearity in a receiving system is 

 obviously required to avoid generating the additional interference of 

 distortion products. Most radar systems are advisedly employed in a 

 manner avoiding the most adverse clutter conditions. 



With some performance compromises, a practical degree of automaticity 

 can be achieved in sorting multiple-target data in an FM/CW airborne 

 radar. Doppler filtering can provide the necessary resolution to distinguish 

 numbers of targets, but the decision as to which target is wanted may 

 require the application of considerable intelligence. With experience a 

 human being can reduce visual or aural doppler data satisfactorily for some 

 applications. 



6-6 PULSED-DOPPLER RADAR SYSTEMS 



Pulsed-doppler radar systems represent an eflFort to combine the clutter 

 rejection capabilities of doppler sensing radars with the range measurement 



^Commonly, values of from 2 to 17 time constants of the filter are used in practical appli- 

 cations. 



