6-6] PULSED-DOPPLER RADAR SYSTEMS 327 



A PRF of 112 kc, as derived in the previous example, would yield an un- 

 ambiguous range interval of only 0.74 n.mi. Values of this order of magni- 

 tude are typical for airborne pulsed-doppler systems which are constrained 

 by antenna considerations to operate in the general range of S to X band 

 (10 cm to 3 cm). As a result, additional techniques — to be described 

 below — must be employed to measure true range in a gated pulsed- 

 doppler system. 



Range gating also levies a cost on the system; a price must be paid in 

 terms of system complexity and /or information rate. The previous dis- 

 cussion considered a single fixed gate. To cover the complete interpulse 

 period, this gate would have to be swept. A sweeping range gate will 

 increase the total required dwell time on the target tdf by the reciprocal of 

 the gating duty factor dg-. 



tdt = tdf/dg. (6-40) 



where tdf = buildup time for the doppler filter. 



An alternative solution is to employ contiguous fixed range gates covering 

 the entire interpulse period (see Fig. 6-11). This "brute force" solution 

 requires a separate doppler filtering system for each range gate interval; 

 however, in combination with fixed contiguous doppler filters it does permit 

 the maximum information rate to be extracted from the system because the 

 separate doppler components of each range interval are examined simul- 

 taneously. Paragraph 6-4, Correlation and Storage Radar Techniques, 

 suggested still another means for processing pulsed-doppler radar infor- 

 mation. 



Range Performance. The idealized range of a pulsed-doppler system 

 may be calculated by the following modification of the basic radar range 

 equation (3-1): 



" ~ [{4Tr)'FkTBdg\ 



(6-41) 



where ds = signal duty cycle 



dg = gating duty cycle [equal to (l-ds) for an ungated system] 



B = doppler detection filter bandwidth^''. 



When "folding" occurs in the detection process, an additional factor of 2 is 

 required in the denominator of the one-fourth power expression. 



^"In some cases, postdetection filtering will be employed to improve the final signal-to-noise 

 ratio without increasing the number of doppler filters required. In such cases the effective 

 detection bandwidth is Bbff = V725 • Bpd as derived in Paragraph 3-5 (Equation 3-62). 

 In these cases, the dwell time should be matched to the bandwidth of the postdetection filter. 



