6-7] HIGH RESOLUTION RADAR SYSTEMS 333 



Another application of pulsed-doppler systems — doppler navigation — ■ 

 is covered in Chapter 14. In this application, precise velocity measurement 

 coupled with freedom from CW radar duplexing problems make the pulsed- 

 doppler system most attractive. 



6-7 HIGH RESOLUTION RADAR SYSTEMS 



Certain radar applications such as fuzing and ground mapping often 

 require very fine resolution; i.e. effective radar pulse lengths of from 0.002 

 to 0.2 Msec (which correspond to range resolution elements of from 1 to 100 

 feet, respectively) and /or angular resolutions of the order of 0.1-10 mils. 

 High resolution is also tactically useful for counting the number of separate 

 targets in a given space volume. The AEW radar example of Chapter 2 

 discussed this basic problem. In this case, high resolution in one dimension 

 — for example range — can provide the requisite capability. Finally, high 

 resolution provides a means for improving signal-to-clutter ratio when the 

 clutter originates from area extensive targets. This is shown by Equation 

 4-60, where the instantaneous illuminated area of ground is a direct function 

 of pulse length and antenna beamwidth. 



There are a number of means for obtaining high resolution in a radar 

 system. Basically, all of them are variations of the following approaches to 

 the problem: 



1. Angular Resolution 2. Range Resolution 



(a) Large antenna aperture (a) Short pulse length 



(b) High frequency (b) Wide bandwidth 



(c) Beam sharpening 



(d) Doppler sensing 



Angular Resolution. This resolution problem has already been 

 discussed in some detail in Paragraph 3-6. There it was shown that the 

 angular resolution element of a radar system was approximately equal to a 

 beamwidth where the antenna beamwidth can be expressed 



- -J radians 12 (6_45) 



a 



where X and d are the wavelength and aperture size respectively in consist- 

 ent units. 



Increases in the antenna aperture d or the operating frequency (/ == 1 /X) 

 will directly increase the angular resolution capability. One limitation on 

 the benefits of increasing antenna aperture size is worthy of mention at this 

 point. For purposes of resolution, the pattern of an antenna has the shape 



i^For a practical antenna, a value of = \.2\/d radians generally is a closer approximation 

 when the effects of nonuniform illumination of the aperture are considered. 



