1-4] FUNCTIONAL CHARACTERISTICS OF RADAR SYSTEMS 5 



2. There is a target within the searched volume, but for one reason 

 or another its presence is not detected by the radar. 



3. There is, in fact, no target within the searched volume and none 

 is indicated by the radar. 



4. There is no target within the searched volume; however, the 

 presence of a target is indicated by the radar. 



In cases (1) and (3) the radar provides the proper answer to the question. 

 In case (2) the radar /^z7j to provide the proper answer by failing to provide 

 any information whatsoever. In case (4) the radar provides the wrong 

 answer by providing spurious information. 



The manner in which the identification function is performed varies 

 widely according to the type of radar and the tactical use to which it is 

 put. In some cases, the detection and identification functions may be 

 combined by a logical nonmechanical process which uses a suitable choice 

 of a detection criterion and a prior knowledge of the probable target 

 characteristics. For example, in the search and detection system, Fig. 

 1-1, one might specify that the appearance of a target indication on 

 each of three successive scan cycles constitutes a detection — the assump- 

 tion being that it is not likely that a spurious indication would be repeated 

 on three successive scan cycles. One might further stipulate that any 

 target thus detected shall be considered an enemy target if it is approaching 

 at predetermined altitudes, speeds, or courses. 



The identification function is sometimes performed by a completely 

 separate radar system designed specifically to accomplish some part of 

 the identification problem. Many forms of identification-friend-or-foe 

 (IFF) systems fall into this category: e.g., in Fig. 1-2 a presumably friendly 



Fig. 1-2 IFF System. 



aircraft is equipped with a passive receiver that detects the search radar 

 signals. These signals are used to initiate transmission of a coded signal 

 back to the search radar location. This coded signal is correlated with 

 the search radar target return signal to establish and define the presence 

 of a "friendly" aircraft. 



