318 



GENERIC TYPES OF RADAR SYSTEMS AND TECHNIQUES 



because of excessive feedihrough of the transmitter energy directly into the 

 receiver. By physically spacing the transmitting from the receiving antenna 

 on a common radar vehicle (spaced-aclive operation), the isolation problem 

 becomes resolvable at the cost, however, of degradation in the vehicle's 

 aerodynamic profile. Semiactive operation involves transmission and 

 reception on separate radar platforms, which also minimizes the feed- 

 through problem. The main advantages of semiactive operation in homing 

 missilery are that the transmitting hardware is deleted as an internal missile 

 requirement and a greater on-target illumination power density is practical 

 from the large parent radar. Both a spaced-active and a semiactive system 

 are illustrated in Fig. 6-19. 



Missile Semiactive 

 Radar 



Fig. 6-19 FM/CW Airborne Radar Systems. 



(3) The ranging accuracy obtainable with an FM /CW set designed for 

 high ground clutter rejection may be relatively coarse — perhaps of the 

 order of 1—2 n. mi. This is not usually a serious drawback for guided missile 

 applications, although it is an important limitation for fire-control systems 

 employing unguided weapons. 



FM/CW Radar Performance. The detectability of the target is 

 determined, for a given false-alarm rate, by the signal-to-noise ratio after 

 final detection. Neglecting the effects of clutter and transmitter modu- 

 lations, the signal-to-noise ratio may be calculated by approximate modifi- 

 cation of the previously derived radar range formula (see Equation 3-9): 



S/N ^ 



{AttYR'FKTB 



(6-28) 



