298 



GENERIC TYPES OF RADAR SYSTEMS AND TECHNIQUES 



RF 



Signal 



Bandpass 



»^ — ► Amplifier 



(Af=Af, 



Detector 



Integration I Output 

 Filter Signal* 



Noncoherent 

 Reference 



NONCOHERENT INTEGRATION 



RF 



Signal 



1 



Coherent 

 Reference 



COHERENT INTEGRATION 



Fig. 6-2 Noncoherent and Coherent Integration Processes. 



Thus, noncoherent detection does not make optimum use of system infor- 

 mation redundancy; in fact, in the region of low S /N ra.tio, the S /N ratio 

 improves approximately as the square root of the number of samples 

 integrated. 



In a coherent detection process, the phase relations between the trans- 

 mitted and received signals are maintained. This makes it possible to add 

 successive samples before detection to obtain a direct enhancement of S jN 

 ratio (predetection or coherent integration). In this case improvement in 

 6" /A^ ratio may be expressed 



M^SIN)= n. (6-14) 



Thus, in a coherent system, the S jN ratio can increase linearly with 

 information redundancy. This means that all other things being equal 

 (average power, frequency, antenna aperture, etc.) a coherent detection 

 system can obtain longer detection ranges than a noncoherent system, the 

 difference between the two being particularly noticeable for high informa- 

 tion redundancy. In addition, as will be shown in Paragraph 6-4, a coherent 

 system can employ a more efficient detection law than a noncoherent 

 system, thereby enhancing the relative detection capability of coherent 

 systems even for short observation times. These characteristics coupled 

 with the doppler frequency measurement ability of coherent systems (see 

 Paragraphs 6-4, 6-5, and G-G) has resulted in a significant shift of develop- 

 ment emphasis to coherent systems in recent years. 



The number of channels required in a radar system depends upon the 

 detection bandwidth and the scanning time. The basic relationships may 

 be ascertained by considering a radar which is designed to measure range, 



