What Is a System of Systems?

Author: | Category: Systems Engineering | Published on: August 12, 2025

In systems engineering, the term System of Systems (SoS for short) comes up time and again, often leading to discussions about its exact definition. In fact, it is not always clear when you're dealing with a single monolithic system and when you are confronted with a System of Systems. This blog article is intended to provide more clarity.

System of Systems Definitions

Let's get one thing out of the way, there are, of course, already numerous definitions of the term System of Systems. One definition can be found in an international standard:

set of systems or system elements that interact to provide a unique capability that none of the constituent systems can accomplish on its own
International Organization for Standardization (ISO®). ISO/IEC/IEEE 21839:2019 Systems and software engineering — System of systems (SoS) considerations in life cycle stages of a system. Last accessed on: 2025-08-11.

Another definition comes from the International Council on Systems Engineering (INCOSE), which has had a working group on this topic since 2012. In their so-called SoS Primer, which serves as an effective introduction to the SoS area, the working group defines System of Systems as follows:

A System of Systems (SoS) is a collection of independent systems, integrated into a larger system that delivers unique capabilities. The independent constituent systems collaborate to produce global behavior that they cannot produce alone.

If we compare these SoS definitions with the general definition for systems from the discipline of systems engineering, there is hardly any difference at first glance. At INCOSE, for instance, a system is defined as follows:

A system is an arrangement of parts or elements that together exhibit behaviour or meaning that the individual constituents do not.

The difference becomes clear when you take a closer look at what constitutes a System compared to a System of Systems. While the general definition still refers quite abstractly to parts or elements that make up a system, the SoS definitions already refer to systems as their constituents.

Characteristic Properties of a System of Systems

But why, then, do we not refer to a car, a satellite, or even a physically large system such as an Airbus A380 aircraft as a System of Systems? After all, these systems also appear to consist of systems, such as the jet engine in the case of the latter aircraft.

Five key characteristics are helpful in distinguishing SoS from other systems that may be very large and complex, but are nevertheless monolithic:

  1. Operational independence of the constituents: The constituents that make up a system of systems are themselves completely self-sufficient systems, i.e., these systems can also be operated and used independently of each other. They can therefore also leave a system of systems compound to join another system of systems. This means that the coupling between these systems is very loose: Usually they mostly exchange only data via IT interfaces.
  2. Independent management of the constituents: The constituent systems of a SoS can not only operate independently of each other, they are also usually developed, procured, and managed independently. Their life cycle is decoupled from the life cycle model of the entire system of systems. It is therefore not unusual for these systems to have already existed before the SoS to which they were later added.
  3. Evolutionary development: Compared to other products or systems, there is no such thing as a full-fledged and finalized System of Systems. Its development and existence is evolutionary, i.e., functions—and thus also constituent systems—are added, removed, modernized, or changed throughout the entire life cycle based on experience, new requirements or changing framework conditions.
  4. Geographical extent: The geographical extent and distribution of the constituent systems of a System of Systems is often relatively large. This is certainly a vague and fuzzy characteristic, but in relation to a SoS, it often means that, due to their spatial distance from each other, its constituent systems can essentially only exchange information (data) and not significant amounts of mass, materials, or energy (see 1.).
  5. Emergent properties: Although this property can also be observed in monolithic systems, it is often much more distinct in SoS. Emergence (from the Latin emerge, meaning “to appear,” “to come out,” or “to rise”) is the spontaneous development of new properties or behaviors in a system as a result of the interaction of its elements. Emergent properties can be desirable, but they can also occur unexpectedly and cause problems.

Coming back to the previous example of the Airbus A 380 and why it is not a System of Systems: Although its jet engine is obviously a (sub-)system of this aircraft, it cannot easily be disconneced from the whole and serve as a self-sufficient entity with a purpose. The parts of an aircraft are far too closely coupled—they are mechanically connected and exchange considerable amounts of energy and material (liquids, gases, etc.)—and such cannot be considered as constituent systems of a SoS.

System of Systems Examples

Examples of SoS can be found in various domains like defense, space, healthcare, energy economy, finance and transportation. Here is a small selection:

The Art of System of Systems Engineering

Even though complexity is nowadays an overused term that can be applied to almost all kind of systems, Systems of Systems can have a vast high degree of complexity at various levels. Therefore, the planning and development of an SoS, known as System of Systems Engineering (SoSE), is a demanding task. It requires very good skills of Systems Thinking. Here is the top-3 of biggest challenges that SoS engineers will face:

  1. Interoperability Management: Achieving seamless interaction between the constituent systems is difficult because each system may use different technologies, standards, communication protocols and data formats. The challenge is to create stable interfaces and common protocols without forcing all constituent systems into a uniform interface design.
  2. Emergent Behavior Prediction and Control: As already mentioned in the key characteristics of SoS above, unexpected behaviors can arise in a System-of-Systems-Compound from the interactions between its constituent systems, even if each system individually works as specified. Predicting and managing these emergent effects requires advanced modeling, simulation, and adaptive control strategies.
  3. Stakeholder Alignment and Governance: A lot of different stakeholders are involved. Each constituent system may be owned and operated by different organizations with distinct objectives, timelines, and constraints. Establishing governance structures and shared goals without undermining each system’s autonomy is a persistent challenge.

Many systems engineers work on and develop systems that are intended for integration into a SoS compound, even if they are sometimes unaware of this. Certain methods and tools have become established for System of Systems Engineering. One of these tools is known as so-called enterprise architecture frameworks. One example of this is the Unified Architecture Framework (UAF®) standardized by the OMG. I will blog about these tools later.

References