326 GENERIC TYPES OF RADAR SYSTEMS AND TECHNIQUES 



A generic range-gated pulsed-doppler system is shown in Fig. 6-25. In 

 this system, the return signal is converted to IF and passed through an 

 amplifier with a bandwidth approximately equal to the reciprocal of the 

 pulse length. The IF amplifier output then is "gated" before the final 

 mixing and doppler detection takes place. The width of the gate usually is 

 made approximately equal to the pulse length. 



The operation of the gate is best understood by considering first a fixed 

 gate which opens up the receiver at a time ti following transmission and 

 closes the receiver tj seconds later at /i + xg. This action accomplishes the 

 following: 



1. The only returns going into the final detection stage are those from 

 ranges falling between the values 



Rn = f/2(/i + n/fr) and 



•^ (6-37) 



Rn + ^R = r/2(/i + n/fr + r) n = 0,1,2,3,-. 



Clutter originates — in the main — from area extensive targets, 

 whereas the desired signal originates from point targets. Thus the 

 gating will improve the signal-to-clutter ratio of a target in the gate 

 by a factor which is, on the average, equal to the duty cycle of the 

 gate dg^ where 



dg = rjr. (6-38) 



2. Noise enters the receiver only during the gating interval. Thus the 

 average noise power is reduced by the duty cycle of the gate. 



3. Since the position of the gate is known with respect to the trans- 

 mitted pulse, any target doppler detected must come from a target 

 in one of the range intervals indicated by Equation 6-35. 



The improvement in signal-to-clutter ratio represents an improvement 

 over and above what can be done with a CW radar system. Thus a range- 

 gated pulsed-doppler system can provide greater clutter rejection than any 

 other generic radar system type. The reduced receiver noise incident to 

 gating tends to restore the ^ /A" ratio to the same value as would exist for a 

 CW system of the same average power and bandwidth. In fact, if the gate 

 width equals the pulse length, a target in the middle of the gate would 

 possess the same SIN as the comparable CW system. 



The range measurement made by a gated pulsed-doppler system is not 

 exactly the same as a range measurement of a pulse radar. The high PRF 

 that must be employed in a pulsed-doppler system causes the unambiguous 

 range interval to be relatively short compared with the maximum detection 

 and tracking ranges. Since the maximum unambiguous range is 



D ^^^^^ ■ if. ^Q\ 



