1-4] FUNCTIONAL CHARACTERISTICS OF RADAR SYSTEMS 13 



nature of microwave propagation (see Chapter 4), it has a number of 

 potential advantages: (1) high directivity, increasing the security of the 

 communications link; (2) dual utilization of the same antenna and carrier 

 power source; and (3) relative predictability of the transmission character- 

 istics. 



Radiation Detection. The radiation detection or passive listening 

 function that may be performed by a radar system has already been men- 

 tioned in the preceding discussions of IFF, ECM, beacon, and com- 

 munications systems. 



A passive radar system consists of only a receiving channel or channels 

 designed to detect and — in some applications — to track microwave 

 energy that is emitted or scattered by a separate source. Passive radars 

 cannot measure range without auxiliary devices. 



There is a variety of means for obtaining range measurements from a 

 passive system — e.g., triangulation using several passive tracking systems 

 at different locations; but all these methods are complicated and inaccurate 

 when compared with the convenience of range measurement in an active 

 radar system. 



Several important functions may be performed by passive radar systems 

 in addition to those already discussed. In the Ferret application, radar 

 receivers tuned to cover a wide band of frequencies are used to detect 

 enemy radiations, thereby providing intelligence data on the characteristics 

 and capabilities of enemy radar systems. Such information is of great 

 value in determining the tactics and countermeasures to be employed in 

 subsequent operations. 



A variation of the above application is one in which the enemy radiation 

 is used as a source upon which a guided missile homes — a system known 

 colloquially as a "radar buster." Despite their simplicity of concept, such 

 systems may present formidable systems engineering and design problems. 

 The multiplicity of enemy signal sources, the intermittency of trans- 

 mission from a scanning source, and the importance of having a stand-by 

 mode of operation in the event that the enemy ceases to radiate for exten- 

 sive periods of time, all contribute to the difficulties. 



A special case of the "radar buster" passive radar homing system is the 

 "home-on-jam" system. This system might be used as an alternative 

 mode of operation for an active radar system. When the active radar is 

 jammed, the jamming source could be detected and tracked by the passive 

 system. 



A passive radar system also forms a vital part of a semiactive guidance 

 system. This application is discussed later in this chapter. 



Illumination. A common form of radar system is the semiactive 

 system. The functional operation of such a system is shown in Fig. 1-8 



