38 ELEMENTS OF AIRBORNE RADAR SYSTEMS DESIGN PROBLEM 



Iconic Models. An iconic model is the most literal. It "looks like" 

 the system it represents. Iconic models can quickly portray the role that 

 each subsystem plays in the operation of the overall system. It is therefore 

 particularly well adapted to illustrating the qualitative aspects of system 

 performance, such as information flow and functional characteristics of 

 various portions of the system. 



The iconic model is not well adapted to the representation of dynamic 

 characteristics of the system because it does not reveal the quantitative 

 relationships between various elements of the system. For the same reason, 

 it is not very useful for studying the efi^ects of changes in the system. 

 Because of its pictorial value, most system analyses usually begin with the 

 construction of an iconic model (block diagram) in order to establish the 

 characteristics of the system and to provide the investigator with a realistic 

 frame of reference for subsequent studies. This process will be illustrated 

 by examples later in this book. 



Analogue Models. Analogue models are made by transforming 

 certain properties of a system into analogous properties, the object being 

 to transform a complicated phenomenon into a similar form that is more 

 easily analyzed and manipulated to reveal at any early time the initial 

 elements of system performance. For example, fluid flow through pipes 

 can be replaced by the flow of electrical current through wires. A slightly 

 more abstract example would derive from the problem of calculating the 

 probability of a mid-air collision in a situation where only the laws of 

 chance were operative — i.e., where no special equipment or techniques 

 were used to prevent collisions. The imaginative investigator might per- 

 ceive that this problem bears a striking similarity to the problem of calculat- 

 ing the mean free path of a gas molecule. Having established the validity 

 of this insight, we would then be free to make appropriate transformations 

 between the two problems and apply the kinetic theory of gases to his 

 problem. 



Unlike the iconic model, the analogue model is very effective in repre- 

 senting dynamic situations. In addition, it is usually a relatively simple 

 matter to investigate the efi^ects of changes in the system with an analogue 

 model. For these reasons, analogue models form very powerful tools for 

 the solution of complex system problems — particularly problems involving 

 many nonlinearities. 



The great utility of analogue models is evidenced by the large-scale 

 analogue computer installations that form a part of almost every major 

 weapons system engineering organization. 



Symbolic Models. The symbolic model represents the components of 

 a system and their interrelationships by mathematical or logical symbols. 



