How Is A System Defined?
In an earlier post, I described how systems engineering differs from other types of engineering. In this post, I will provide a definition of a system.
Though a system can be a challenge to define, Benjamin S. Blanchard and Wolter J. Fabrycky have developed a definition that is useful. They describe a system as, “An assemblage or combination of functionally related elements or parts forming a unity whole … that include physical elements and have useful purposes .” In other words, a system is composed of components (also called parts or pieces) with properties and functional relationships between each other. It is important to remember that not every set of components is a system. A group of random pieces, for instance, is not a system. This definition of a system is approximately illustrated in Figure 1. Note how there are components numbered from Component 1 to Component j. Each component has its own set of attributes. For instance, Component 1 has Attribute1_1 to Attribute1_n and Component j has Attributej_1 to Attributej_n. Also notice that there are relationships between the various components. Blanchard and Fabrycky make the distinction that a system has interrelated components that function “together toward some common objective(s) or purpose(s) .” The relationship characteristic of a system is important since it illustrates how the components have connections which are necessary for the function of the system. Together these traits of a system function together to meet a common goal or purpose that is known from a customer need.
Figure 1. Approximate graphical representation of a system as defined by Blanchard and Fabrycky .
Example: Why an AESA Radar Is Rightly Considered A System?
Using this definition from Blanchard and Fabrycky, an Active Electronically Scanned Array (AESA) radar is a system for at least four reasons. First, it has multiple components. An AESA has radiating antenna elements, transmit/receive (T/R) modules, circulator assemblies, beam forming network, digital receiver, digital exciter, power conditioning circuitry, and many other components. Second, each of the components has its own attributes. For instance, some of the attributes of the antenna element are that it launches electromagnetic waves into free space, has physical dimensions and materials, and has the constraint that it must not couple energy too strongly to other features of the array. Third, each of the components has relationships with other components that are engineered as part of the function of the AESA. For example, T/R modules are designed to interface properly to the circulator assemblies. The T/R modules are also designed to accept signals from the beam forming network and the power conditioning circuits. In fact, every component in the AESA has a relationship with at least one other component, but most components have multiple relationships. Fourth, the relationships are engineered so that the AESA provides the required functions desired by the user. For these reasons, an AESA is considered a system.
In a different post, I answer the question: What Is Systems Engineering?
 B. S. Blanchard, W.J. Fabrycky, Systems Engineering and Analysis, Fifth Edition (Upper Saddle River, NJ: Prentice Hall, 2011), p. 3.
 Blanchard, Fabrycky, Systems Engineering and Analysis, p. 3.