6-2] BASIC PRINCIPLES 293 



Unfortunately, there is a tremendous variety of possible choices. In 

 terms of generally recognized system types and subtypes, there are pulse^ 

 continuous-wave {CW), pulsed doppler, monopulse, correlation ^high-resolution, 

 and moving target indication {MTI) radars. Some of these types represent 

 genuinely different approaches; some of them represent merely alternative 

 means for performing the same job; and some of them are derivatives of 

 particular system types. In each case, however, the selection of one of these 

 types commits the radar system designer to a problem approach that is 

 confined within uncomfortably narrow limits. The radar system designer 

 must therefore have a good general knowledge of the basic system types and 

 the general laws that govern their performance characteristics. Toward 

 this end, this chapter will attempt to accomplish two things. 



(1) It will summarize basic radar laws in a rule-of-thumb fashion to 

 provide a means for understanding the operation of any radar 

 system. 



(2) It will describe the performance characteristics and limitations of 

 generally recognized radar system types and will indicate their 

 general areas of application. 



6-2 BASIC PRINCIPLES 



The operation of almost any radar system may be visualized and under- 

 stood by asking and answering the following basic questions: 



(1) Is the system active, semiactive, or passive (see Paragraph 1-4) ? 



(2) What information is contained in the signal return from the 

 assumed target complex? 



(3) What are the system sampling frequencies? 



(4) How are the radar data detected and processed in the receiving 

 system ? 



(5) Where does the processed information go? 



Each of these questions may now be considered in greater detail. 



Type of Radar System. The most basic division of radar system 

 types is a classification based on the origin of the target signal information. 

 An active system generates the signals which are ultimately scattered back 

 to the point of signal origin. A passive system is simply a receiving system 

 which utilizes target-generated radiations as its signal source. A semiactive 

 system employs separate transmission and receiving systems which may be 

 at some distance from each other. Depending upon the degree of coupling 

 between the transmitted and received signals, a semiactive system may 

 resemble either an active or a passive system insofar as its basic operation 

 is concerned. 



