6-2] BASIC PRINCIPLES 297 



The transmission periodicity Jr places an upper limit on the scanning 

 frequency. If, for example, A >/r, then radar returns from the far end of 

 the unambiguous range interval will not be received because the radar 

 antenna will have moved to a new angular position by the time the signal 

 has returned. For this case 



i?n.ax = 1^^ fs>fr. (6-10) 



Since the angular scanning frequency defines the minimum signal band- 

 width, it also defines the minimum possible velocity resolution element. 

 Thus 



Ar>-^. (6-11) 



where A/^ = velocity resolution element, and the total number of separate 

 unambiguous velocity elements is, from Equations 6-8 and 6-11, 



N^^^^n^<-fi. (6_i2) 



For passive radar and IR systems, the scanning frequency is the basic 

 system sampling frequency. The interrogation rate of such systems is 

 roughly equal to the scanning bandwidth. 



Detection and Data Processing. The detection and data-processing 

 system of any radar may be characterized by several basic properties: 

 (1) type of detection process, i.e., coherent or noncoherent; (2) number of 

 channels; (3) filtering techniques; (4) signal-to-noise ratio as a function of 

 target size, range, etc. A knowledge of these properties can provide a ready 

 means for estimating the performance potential of any system. Each of 

 these properties will be discussed briefly; subsequent sections of this chapter 

 will provide illustrations of the various possibilities for the generic systems. 



In a noncoherent detection process, no attempt is made to correlate the 

 phases of the transmitted and received signals. Thus the signal returns 

 from each target must be detected separately and added together after 

 detection (postdetection integration) as shown in Fig. 6-2. As was shown 

 in Paragraph Z-'}), this process improves the S jN ratio of the target return 

 by a factor which may be expressed as 



A(^/yV) = n^ (6-13) 



where n = number of samples integrated; n ^ frifs if all the samples in 

 one scan period are integrated 



7^ = 0.5 - 1.0 (0.5 for low S jN and 1.0 for high S jN). 



