5-11] 



DETERMINATION OF SIGNAL'S TIME OF ARRIVAL 



287 



Random 

 Noise 



_L_ 



1st 

 Detector 



Scanning 

 Antenna 



IF Amp. 

 Matclned 

 To Pulse 



Square Law 



2d 



Detector 



Pulse 

 Gate 



Video Filter 

 Matched To 



Scan 

 Modulation 



Output 



Giving Minimum 



Angular Error 



Fig. 5-20 Block Diagram of Receiver of Scanning Radar. 



with a uniform power density is introduced at the input to this amplifier. 

 A square-law second detector is used to generate the video envelope of the 

 signal plus noise. The video signal-plus-noise pulses are gated into a video 

 filter which is matched to the scan modulation. That is, this filter is matched 

 to the fundamental component of the gated video signal plus noise. All 

 signals, information, and noise at the repetition frequency and its higher 

 harmonics are filtered out. We assume that the number of pulses per 

 beamwidth is sufficiently large that the signal spectrum about the first 

 harmonic of the repetition frequency does not overlap the fundamental 

 component of the signal spectrum. We also assume that the video noise is 

 sufficiently uniform over the bandwidth of the scan modulation signal for 

 the assumption of a constant noise spectrum under which we derived 

 Equation 5-145 to be valid. Other system configurations are possible. A 

 more practical design might stretch the gated signal-plus-noise pulses before 

 smoothing by the scan modulation filter. Such a system, however, would 

 give slightly greater angular errors than the one we have chosen to study. 



The following special notation is adopted for this example. 



a = voltage amplitude of received signal 



T = repetition period 



d = duty ratio 



■^ = antenna angle 



y^ = antenna angular rate 



9 = antenna beamwidth (half-power points — one-way) 



5 = pulse width 



n = Q l\j/T = number of pulses per beamwidth 

 The signal received from the target will have the following form: 



Received signal = a (scan modulation) (pulse modulation) cos coc/- 



(5-146) 



