6-6] PULSED-DOPPLER RADAR SYSTEMS 331 



Three or more PRF's can be used to extend the unambiguous ranging 

 interval further. In these cases, the data processing becomes more compli- 

 cated for two PRF's; however, methods similar to those used for Equations 

 6-42 to 44 may be used to derive the required relationships. The multiple 

 FRF system of ranging is severely limited if more than one target return 

 at the same doppler frequency is present. In a two-PRF system, two 

 targets would yield four possible range values : two correct ranges and two 

 "ghosts." Eclipsing also can cause difficulty, since it is quite likely that the 

 target return for one of the PRF's will be eclipsed. The accuracy of this 

 method of ranging is comparable to that of a pulse radar employing the 

 same pulse length. 



If the "looks" at the target are taken by sequentially switching the PRF 

 from one value to another, the required dwell time on the target for the 

 same system bandwidth is increased by the number of PRF's employed. 

 Alternative procedures such as simultaneous transmission of the PRF's or 

 wider bandwidth reception could be used to keep dwell time constant. 

 However, these methods will decrease the available *S'/A^ ratio for a given 

 amount of total average transmitter power. In addition, simultaneous 

 transmission greatly increases systems complexity in both the transmitter 

 and receiver and gives rise to serious eclipsing problems because of the 

 higher effective duty ratio. 



Pulsed-Doppler System Design Problems. Pulsed-doppler systems 

 have the same basic problems of transmitter stability as CW systems. 

 These problems are, in fact, common to any coherent system. 



Because of its high duty cycle, the duplexing problem is particularly 

 difficult in a pulsed-doppler system. To cut eclipsing losses to a minimum, 

 the transition from transmit to receive must be made as quickly as possible. 

 Ordinary transmit-receive (TR) tubes are not satisfactory for this appli- 

 cation; however, ferrite circulators (see Paragraph 10-16) and ferrite 

 switches have found considerable application because of their low insertion 

 losses (0.5 db) and their very rapid recovery time. 



In the receiving system, particular care must be taken to provide suffi- 

 cient dynamic range to accommodate the maximum clutter amplitudes .^^ 

 Linearity must be maintained over this range to avoid intermodulation 

 products which spread the signal spectrum and cause loss of signal-to-noise 

 ratio at the doppler filters. 



The design of the doppler filtering system — particularly, the bandpass 

 characteristic and the maintenance of proper frequency spacing between 



iiln many designs, the clutter from the main beam and the altitude line is eliminated prior 

 to amplification and doppler filtering. This greatly reduces the dynamic range requirements 

 of subsequent stages of the receiver; however, it also makes the system completely "blind" 

 at these frequencies. 



