332 GENERIC TYPES OF RADAR SYSTEMS AND TECHNIQUES 



filters — is a vital design consideration. A fixed range gating, fixed filter 

 bank pulsed-doppler system may have hundreds or even thousands of these 

 narrow band filters; thus the trade-ofF between filter performance and size 

 and weight is a vital consideration. 



Angle tracking poses certain special problems in a pulsed-doppler radar. 

 The doppler frequency as well as the range must be tracked prior to angle 

 lock-on. The bandwidth of the velocity loop corresponds to the width of 

 the doppler filter. If conical scanning is employed, this filter must be wide 

 enough to transmit the scan modulation sidebands. Actually, the doppler 

 filter width should be about three times the scan rate in order to minimize 

 phase and amplitude variations of the error signal. For example, a 40-cps 

 scanning frequency would require a doppler filter band width of at least 

 120 cps. 



The use of monopulse angle tracking (see Paragraph 6-3) poses a most 

 difficult problem in a pulsed-doppler system. The sum and the difference 

 signals must be handled in completely separate receiver channels — each 

 with its own mixer, amplifiers, range gates, and doppler filters. In addition 

 to the obvious disadvantages of weight and size, the problem of maintaining 

 the proper alignment of these channels relative to each other represents a 

 prodigious design problem. 



Pulsed-Doppler Systems Applications. As previously mentioned, 

 pulsed-doppler systems are best employed in systems requiring substantial 

 ground clutter rejection, a common transmitting and receiving antenna, 

 and accurate range and /or velocity measurement. 



One other characteristic of a doppler system — either CW or pulsed- 

 doppler — also has great tactical utility. This is the automaticity potential 

 of such systems. Detection in such systems is inherently automatic since 

 the signal is detected by the comparison of a filter output with a preset bias. 

 While the same thing can be done in a pulse radar system, the problem of 

 setting a bias level is enormously more difficult because of false alarms 

 caused by clutter. This necessitates the use of bias levels considerably 

 higher than would be dictated by thermal noise considerations. Thus the 

 detection performance of an automatic pulse radar system is appreciably 

 poorer than can be obtained when a human being is used as the detection 

 element, since the human operator can discriminate between true targets 

 and random clutter peaks so long as the clutter does not completely obscure 

 the target. However, a doppler radar separates closing targets from clutter; 

 thus the bias level may be set on thermal noise considerations alone. For 

 this reason, as well as the others mentioned, pulsed-doppler systems are 

 particularly suited for application as AI radars and guided missile active 

 seekers which must find and lock on to a target buried in clutter in a high 

 closing-rate tactical situation. 



