330 



GENERIC TYPES OF RADAR SYSTEMS AND TECHNIQUES 



A = Transmitted Pulses 

 I = Received Pulses 



True Range = ^- 



_ 2 X Desired Max. Range 



max- ^ 



/ \\ Al A! Al Al A! 



-f2^ 



JA. 



lA 



lA 



lA 



kA,^ 



Fig. 



6-28 Two-PRF Range Measure- 

 ment. 



A second means is to employ- 

 various modulations of the pulse 

 repetition frequency. As an ex- 

 ample, step switching of the PRF 

 between several values can provide a 

 ranging capability. The basic prin- 

 ciple is shown in Fig. 6-28. Trans- 

 mission occurs on two PRF's which 

 are multiples of relatively prime 

 numbers (in the example in the 

 figure the numbers are 5 and 4). 

 Because of ambiguities, a target at a 

 true range corresponding to ttr will 

 appear as a target return with time 

 delays /i and /2 relative to the near- 

 est transmitted pulse on each PRF, 

 respectively. Thus we may write 



, ^1 



/2 + 



(6-42) 



where «i, «2 = number of unambiguous range intervals in each PRF (in 

 the figure, «i = 2 and «2 = !)• 



In the example shown there are two possible relationships between «i 

 and «2: 



Wl = ^2 



«i — 1 = «2 



Substituting these relationships in Equation 6-42 we obtain, as expres- 

 sions for the time delay corresponding to true range, 





h/rl-tJr2-{-l 



(6-43) 



frl-/r 



If the first expression is negative, the second must be employed. Thus, 

 the use of two PRF's can provide unambiguous range over a maximum 

 desired ranging interval corresponding to the time delay 



/rl-/r 



(6-44) 



