9-11] 



AUTOMATIC RANGE TRACKING 



501 



purposes are shown in Fig. 9-15. In the double signal type, the original 

 video signal is transformed into a bipolar signal by adding to the video 

 signal its reflection from a shorted delay line. Since the delay line is shorted, 

 the reflection is the negative of the original video. By making the delay 

 equal to one-half the pulse width, the total delay of this negative signal 

 equals the pulse width. A schematic representation of this process is shown 

 in Fig. 9-16. 



The time modulator generates two 

 gates: one is the range gate used to 

 time-select the video information 

 which goes to the angle tracking sys- 

 tem; the other is a delayed range 

 gate used to integrate the energy in 

 the bipolar pulse to produce a range 

 error signal. The typical output of 

 the time discriminator as a function 

 of gate position with respect to the 

 echo center is shown in Fig. 9-17. A 

 second means for tracking is the so- 

 called split gate system of Fig. 9- 

 15b. In this scheme- two adjacent 

 gates are employed. The pulse 

 energy falling in one gate is sub- 

 tracted from that in the other to 

 form an error signal. The time dis- 

 criminator characteristic produced 

 thereby is the same as that of the 

 other system. 



A variation of these methods is the so-called jittered range gate method. 

 With this technique, the position of a range gate is modulated at some low 

 frequency — say 30 cps. The product of the jittered range gate and the 

 received pulse will possess a modulation component at the jitter frequency 

 whose amplitude and phase signify the misalignment between the pulse and 

 the average gate position. This a-c error can be demodulated to provide an 

 error voltage for controlling the gate. One disadvantage of this particular 

 scheme is that there is an extra modulation frequency upon which extra- 

 neous noise and jamming can enter the system. 



Time Discriminator Charac- 

 teristic. 



Transmission Shaping Unit. This is a filter whose characteristics 

 determine the bandpass and lag errors of the range tracking system. 

 Subsequent discussion in this chapter will deal with the methods for 

 selecting the parameters of this unit. 



