336 GENERIC TYPES OF RADAR SYSTEMS AND TECHNIQUES 



First of all, the generation of a short, high-power pulse is a difficult 

 problem in itself. The design of the transmitting tube, the modulator, and 

 the TR switching all are complicated by the short-pulse operation. 



Short-pulse operation also limits radar performance. The required 

 receiver bandwidth is inversely proportional to pulse length. Thus, the 

 S /N rsLt'io for a given value of peak transmitted power is directly propor- 

 tional to the pulse length: 



S/N^Pt/B = Ptt. (6-51) 



Usually, peak power cannot be increased to compensate for this effect 

 because of voltage breakdown limitations in the transmitter, antenna, and 

 waveguide. Thus, for a given state of the art in RF components, the S jN 

 performance will decrease with decreasing pulse width. Actually, because 

 of the previously mentioned transmitter design problems, this decrease 

 proceeds at greater than a linear rate. For these reasons, short-pulse 

 systems are limited to relatively short-range operation (such as fuzes) or 

 operation against targets of large cross section (ground mapping) where it is 

 feasible to sacrifice *S'/A^ ratio for improved resolution. Short pulse lengths 

 can also complicate certain other problems. For example, if delay line 

 AMTI is employed, the tolerances on the pulse repetition frequency control 

 and the delay line calibration must be held within proportionally closer 

 limits. In addition, the bandwidth requirements of the delay-line elements 

 are increased proportionately. 



As a result of limitations such as these, there are certain tactical applica- 

 tions where no physically realizable noncoherent pulse radar system can 

 provide the requisite resolution and range capabilities. To fill this gap, a 

 family of radar systems has grown up during recent years which — for 

 lack of any more suitable name — are called "wide bandwidth coherent 

 systems." 



Wide Bandwidth Coherent Systems. From an information theory 

 standpoint, the fine range resolution capability of a short-pulse system 

 derives from the wide bandwidth of such a system. In fact the range 

 resolution capability is a direct function of the bandwidth of the trans- 

 mitted spectrum. This suggests that 

 any system which employs a wide 

 bandwidth has the inherent capabil- 

 ity for fine range resolution. Several 

 other observations — useful for in- 

 venting new radar systems — may 

 also be made from an examination of 

 the transmitted spectrum of a pulse 

 Fig. 6-31 Pulse Radar Spectrum. radar as shown in Fig. 6-31. 



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-Frequency 



