14 



ELEMENTS OF AIRBORNE RADAR SYSTEMS DESIGN PROBLEM 



Fig. 1-8 Semiactive Guidance System. 



and is described elsewhere in greater detail.^ In this system, the target is 

 illuminated by a source of microwave energy. A portion of this energy 

 is scattered by the target and may be detected and tracked by a passive 

 receiver located at some distance from the transmitting source. 



Semiactive systems find their greatest use in guided missile systems, 

 where it is often desirable to retain the basic advantages of an active 

 system without incurring the weight penalty and transmitting antenna 

 size restrictions that would result from placement of the transmitter in the 

 guided missile. 



It is possible to obtain a crude measurement of range in a semiactive 

 system if the missile is illuminated by the same energy transmission as the 

 target. The accuracy of this range measurement is greatest when the 

 illuminator, missile, and target are in line as shown in Fig. 1-9. In this 



&^ 



r) 



Rf^ct, 



L_ U ' 



Fig. 1-9 Rane;e Measurement in a Semiactive system. 



case, the target receives energy from the interceptor-borne radar /o Msec 

 following transmission. The illuminating energy is also received directly 



lA. S. Locke, Guidance (Principles of Guided Missile Design Series), D. Van Nostrand Co., 

 Princeton, N. J., 1955. 



