6-6] 



PULSED-DOPPLER RADAR SYSTEMS 



329 



interval would be much higher; thus, there would be many more "holes" 

 than shown in this example. For purposes of calculating the cumulative 

 probability of detection, it is often convenient to approximate the notched 

 blip-scan curve with a "smoothed" curve. 



If the pulsed-doppler system is operated with a fixed PRF, there will be 

 certain closing velocities which could result in the target's appearing in a 

 detection notch on each successive scan. For example, the interpulse period 

 of the numerical example was 0.74 n.mi. If the first detection of the target 

 occurred at a range corresponding to a "hole," and if the target moved a 

 multiple of 0.74 n.mi. between scans, then detection would never occur. 

 Eventualities such as this may be largely eliminated by a slow variation of 

 the PRF which would have the effect of producing a smoothed — but 

 nevertheless, degraded — blip-scan curve. 



Pieces of Information 



Range Measurements in 

 Pulsed-Doppler Systems. As pre- 

 viously mentioned, the range gate 

 position measurement produces an 

 ambiguous range indication because 

 of the high repetition frequency that 

 must be used in a practical pulsed- 

 doppler system. The high repetition 

 frequency reduces the total number 

 of separate unambiguous range in- 

 tervals (Nr = 1 /rfr) and gives the 

 pulsed-doppler radar an information 

 matrix such as is shown in Fig. 6-27. 

 In almost all practical cases, it is 

 desired to operate the radar against 

 targets at ranges far exceeding the 

 unambiguous range interval. Thus 

 a means must be employed to 

 circumvent the range ambiguity 

 problem in a range-measuring pulsed-doppler system. 



There are several means for measuring true range: all are inconvenient 

 and all degrade radar performance in terms of information rate and /or 

 signal-to-noise ratio. One means for accomplishing range measurement is 

 to employ the FM method used for the CW radar. The operating charac- 

 teristics of this method are essentially the same as for an FM/CW radar; 

 particularly, if the duty cycle of the pulsed-doppler system is relatively 

 high. The range accuracy of this method is relatively poor if a narrow 

 detection and tracking bandwidth is maintained. The range resolution is 

 also poor because the pulse shape information is never utilized. 



Fig. 6-27 



NrxN.xNcxNy 



Nr = VTfr 

 N = fr/b 



B = Doppler Filter 

 Detection Bandwith 



Pulsed-Doppler Information 

 Matrix. 



