270 



TECHNIQUES FOR SIGNAL AND NOISE ANALYSIS 



shall consider is a noncoherent delay-line cancellation system. In such a 

 system, both ground clutter and target reflections are received simul- 

 taneously. The RF carrier frequency of the ground clutter is denoted 

 by Wc> while that from a target moving relative to the ground will possess 

 a doppler shift ud and is denoted by coc + cod. When the sum signal is 

 detected, a beat is produced at the doppler frequency wd- If there is no 

 target present, there is no doppler beat, and the spectrum of the detected 

 video is concentrated at d-c and in the neighborhoods of harmonics of the 

 pulse repetition frequency. The doppler signal can be separated from the 

 clutter background by means of a delay-line cancellation unit. This unit 

 provides the difference of successive returns as an output, that is, returns 

 separated by the repetition period T. Fig. 5-14a shows a block diagram of 

 such a cancellation unit, while Figure 5-14b illustrates its operation. This 



Input 



Time 

 Delay 



p-iujT 



Output 



Fig. 5-14a Delay-Line Cancellation Unit. 



Input 



Signal HI- 

 Delayed 

 Signal 



i~x,.npv^^xi ^ 





-\ 



'^t 



/^Tx „ /IN ,, /I , 



Cancelled 

 Signal il-^ ^^\ly '^^\iy 



Fig. 5-14b Cancellation of Clutter Echc 



sort of unit will attenuate the d-c component and all harmonics of the 

 repetition frequency and in this manner cancel most of the clutter. When 

 the doppler frequency lies between these harmonics, it will be transmitted 

 through the cancellation unit. If by chance the doppler frequency coincides 

 with one of the repetition rate harmonics, it will be canceled along with the 

 clutter and produce a blind region or range of doppler frequencies to which 

 the system is insensitive. Blind regions represent one of the most serious 

 limitations of this type of system. 



Proceeding with the analysis, the clutter echo at a given range is repre- 

 sented before detection as a narrow band noise process: 



Clutter echo = xU) cos uj + v(/) sin cor/. (5-97) 



The modulating functions .v andjy are independent Gaussian noise processes 

 with identical spectra. The clutter spectrum is determined by the motion 



