6-7] HIGH RESOLUTION RADAR SYSTEMS 335 



platform moving with a horizontal velocity Vp. The beam illuminates two 

 closely spaced fixed targets, 1 and 2; however, because of the angular 

 relation, and the velocity of the radar platform, the returns from these 

 targets differ slightly in frequency. Thus, narrow band filtering may be 

 employed in the receiver to distinguish between the two targets. 



If a single narrow band filter of width A/d centered about the carrier 

 frequency is used in the receiver, the effective beamwidth may be expressed 



e,// = ^ radians. (6-46) 



However, the filter bandwidth is limited by dwell time requirements to a 

 value 



A/, = \/t,= VF/hQeff cps. (6-47) 



Substituting, and solving for the minimum value of Qeff, we obtain 



Qefs ^ VV2A radians. (6-48) 



If multiple receiver channels are used in conjunction with appropriate 

 signal storage and correlation techniques, the return from each target can 

 be integrated over the entire dwell time of the actual beamwidth. In this 

 case, the minimum possible effective beamwidth becomes 



Qeff = X/2^e = d/lh radians. (6-49) 



and the number of channels required is 



n, = IKKld'' channels. (6-50) 



Thus, in theory at least, the resolution performance of a very long antenna 

 (possibly much longer than the aircraft itself) can be obtained by coherently 

 combining signals transmitted and received from various positions along 

 the flight path. Quite obviously, this principle could have application to 

 ground mapping radar systems. It is of some interest to note that the 

 effective angular resolution of the multiple channel correlation system 

 actually improves as the actual antenna beamwidth becomes larger. The 

 reader should also note, however, that achievement of effective beamwidths 

 approaching the minimum possible requires a radar system of enormous 

 complexity. For example, a 3.2-cm system with a 4-ft antenna operating at 

 10,000 ft altitude has a theoretical angular resolution limit of 0.1 mil. 

 However, such a system would require the equivalent of 267 separate 

 coherent receiver channels to realize this potential. 



Short-Pulse Systems. The most obvious means for obtaining high 

 range resolution is to employ a short pulse length in a conventional pulse 

 radar system. However, such a system has a number of important design 

 problems which severely limit the usefulness of this approach. 



