9-11] AUTOMATIC RANGE TRACKING 499 



2. To provide means for discriminating against returns which 

 originate at ranges different from those of the desired target 



3. To provide means for effecting the transition from range search 

 to automatic range tracking. 



The first function involves measuring the round-trip transit time of the 

 radar energy; the second involves positioning a range gate such that only- 

 signals in time coincidence with the target pulse are passed through the 

 receiving system; the third involves the dynamic problems of stopping a 

 sweeping range gate. 



Range gating — as illustrated in Fig. 5-13 and described in Paragraph 5-8 

 — is necessary to the satisfactory operation of the angle tracking system. 

 In fact the target pulse train must be time selected (i.e. range lock-on must 

 occur) before angle tracking can be started. ^^ This sequence of operations 

 ensures that returns from other targets on the ground or clouds — at ranges 

 other than the range of the desired target ^ do not compete with the 

 desired target signal. 



Range measurement generally is employed to provide inputs to the threat- 

 evaluation and fire-control systems. Accuracy requirements vary widely; 

 errors of no more than a few yards may be allowed for unguided weapons, 

 while guided missile systems can often tolerate rather large range errors, 

 as was shown in the example of Chapter 2 (Table 2-4). 



Typical means for providing automatic range tracking and gating are 

 shown in Fig. 9-14. Also shown (in dotted lines) are the parts of the radar 



To Angle Tracking 



Radar I I 2d Detector i 



IF 1— H and i- 



Amplifier \ j^Video Amp. J 



Fig. 9-14 Automatic Range Tracking System. 



i^In doppler radars which do not employ range gating, a similar function is served by 

 the velocity gate — in this case frequency selection replaces time selection. In some pulsed 

 doppler systems both time and frequency selection are utilized preparatory to angle lock-on 

 (see Paragraph 6-6). 



