Systems of Systems (SoS)

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System of systems engineering (SoSE) is not a new discipline; however, this is an opportunity for the systems engineering community to define the complex systems of the twenty-first century (Jamshidi 2009). While systems engineering is a fairly established field, SoSE represents a challenge for the present systems engineers on a global level. In general, SoSE requires considerations beyond those usually associated with engineering to include socio-technical and sometimes socio-economic phenomena.


Each part of the SEBoK is divided into knowledge areas (KAs), which are groupings of information with a related theme. The KAs in turn are divided into topics. This KA contains the following topics:

Definition and Characteristics of Systems of Systems

There are several definitions of system(s) of systems (SoS), some of which are dependent on the particularity of an application area. Maier (1998) postulated five key characteristics (not criteria) of SoS: operational independence of component systems, managerial independence of component systems, geographical distribution, emergent behavior, and evolutionary development processes, and identified operational independence and managerial independence as the two principal distinguishing characteristics for applying the term 'systems-of-systems.' A system that does not exhibit these two characteristics is not considered a system-of-systems regardless of the complexity or geographic distribution of its components.

In the Maier characterization, emergence is noted as a common characteristic of SoS particularly in SoS composed of multiple large existing systems, based on the challenge (in time and resources) of subjecting all possible logical threads across the myriad functions, capabilities, and data of the systems in an SoS. As introduced in the article Emergence, there are risks associated with unexpected or unintended behavior resulting from combining systems that have individually complex behavior. These become serious in cases which safety, for example, is threatened through unintended interactions among the functions provided by multiple constituent systems in a SoS.

ISO/IEC/IEEE 15288 Annex G (ISO, 2015) provides a definition of SoS:

System of Systems (SoS)A system of systems (SoS) brings together a set of systems for a task that none of the systems can accomplish on its own. Each constituent system keeps its own management, goals, and resources while coordinating within the SoS and adapting to meet SoS goals.

  It should be noted that according to this definition, formation of a SoS is not necessarily a permanent phenomenon, but rather a matter of necessity for integrating and networking systems in a coordinated way for specific goals such as robustness, cost, efficiency, etc.

ISO/IEC/IEEE 15288 Annex G also describes the impact of these characteristics on the implementation of systems engineering processes.  Because of the independence of the constituent systems, these processes are in most cases implemented for engineering both the systems and the system of systems, and need to be tailored to support the characteristics of SoS. These processes are shown in the table below highlighting the fact that these processes are implemented at both the system and SoS levels, with SoSE often constrained by the systems.

SE Process Implementation as Applied to SoS
Agreement processes Because there is often no top level SoS authority, effective agreements among the systems in the SoS are key to successful SoSE.
Organizational project enabling processes SoSE develops and maintains those processes which are critical for the SoS within the constraints of the system level processes.
Technical management processes SoSE implements technical management processes applied to the particular considerations of SoS engineering - planning, analyzing, organizing, and integrating the capabilities of a mix of existing and new systems into a system-of-systems capability while systems continue to be responsible for technical management of their systems.
Technical processes SoSE technical processes define the cross-cutting SoS capability, through SoS level business/mission analysis and stakeholder needs and requirements definition. SoS architecture and design frame the planning, organization and integration of the constituent systems, constrained by system architectures. Development, integration, verification, transition and validation are implemented by the systems. with SoSE monitoring and review. SoSE integration, verification, transition and validation applies when constituent systems are integrated into the SoS and performance is verified and validated.

Finally, based on work done by the INCOSE Systems of Systems Work Group (Dahmann, 2014), the major challenges facing SoSE have been catalogued in terms of seven pain points.  These challenges are presented in the SoSE section of the INCOSE SE Handbook. (INCOSE 2015). These challenges include:

  • SoS Authorities.  In a SoS each constituent system has its own local ‘owner’ with its stakeholders, users, business processes and development approach. As a result, the type of organizational structure assumed for most traditional systems engineering under a single authority responsible for the entire system is absent from most SoS.   In a SoS, SE relies on cross-cutting analysis and on composition and integration of constituent systems which, in turn, depend on an agreed common purpose and motivation for these systems to work together towards collective objectives which may or may not coincide with those of the individual constituent systems.
  • Leadership.  Recognizing that the lack of common authorities and funding pose challenges for SoS, a related issue is the challenge of leadership in the multiple organizational environment of a SoS.  This question of leadership is experienced where a lack of structured control normally present in SE of systems requires alternatives to provide coherence and direction, such as influence and incentives.   
  • Constituent Systems’ Perspectives. Systems of systems are typically comprised, at least in part, of in-service systems, which were often developed for other purposes and are now being leveraged to meet a new or different application with new objectives.  This is the basis for a major issue facing SoS SE; that is, how to technically address issues which arise from the fact that the systems identified for the SoS may be limited in the degree to which they can support the SoS.  These limitations may affect the initial efforts at incorporating a system into a SoS, and systems ‘commitments to other users may mean that they may not be compatible with the SoS over time.  Further, because the systems were developed and operate in different situations, there is a risk that there could be a mismatch in understanding the services or data provided by one system to the SoS if the particular system’s context differs from that of the SoS.
  • Capabilities and Requirements. Traditionally (and ideally) the SE process begins with a clear, complete set of user requirements and provides a disciplined approach to develop a system to meet these requirements. Typically, SoS are comprised of multiple independent systems with their own requirements, working towards broader capability objectives.  In the best case the SoS capability needs are met by the constituent systems as they meet their own local requirements. However, in many cases the SoS needs may not be consistent with the requirements for the constituent systems.  In these cases, the SoS SE needs to identify alternative approaches to meeting those needs through changes to the constituent systems or additions of other systems to the SoS.  In effect this is asking the systems to take on new requirements with the SoS acting as the ‘user’.
  • Autonomy, Interdependencies and Emergence. The independence of constituent systems in a SoS is the source of a number of technical issues facing SE of SoS.  The fact that a constituent system may continue to change independently of the SoS, along with interdependencies between that constituent system and other constituent systems, add to the complexity of the SoS and further challenges SE at the SoS level.  In particular, these dynamics can lead to unanticipated effects at the SoS level leading to unexpected or unpredictable behavior in a SoS even if the behavior of constituent systems is well understood.
  • Testing, Validation, and Learning. The fact that SoS are typically composed of constituent systems which are independent of the SoS poses challenges in conducting end-to-end SoS testing as is typically done with systems.  Firstly, unless there is a clear understanding of the SoS-level expectations and measures of these expectations, it can be very difficult to assess level of performance as the basis for determining areas which need attention, or to assure users of the capabilities and limitations of the SoS.  Even when there is a clear understanding of SoS objectives and metrics, testing in a traditional sense can be difficult.  Depending on the SoS context, there may not be funding or authority for SoS testing.  Often the development cycles of the constituent systems are tied to the needs of their owners and original ongoing user base.  With multiple constituent systems subject to asynchronous development cycles, finding ways to conduct traditional end-to-end testing across the SoS can be difficult if not impossible.  In addition, many SoS are large and diverse making traditional full end-to-end testing with every change in a constituent system prohibitively costly.  Often the only way to get a good measure of SoS performance is from data collected from actual operations or through estimates based on modeling, simulation and analysis. Nonetheless the SoS SE team needs to enable continuity of operation and performance of the SoS despite these challenges.
  • SoS Principles.  SoS is a relatively new area, with the result that there has been limited attention given to ways to extend systems thinking to the issues particular to SoS.  Work is needed to identify and articulate the cross cutting principles that apply to SoS in general, and to developing working examples of the application of these principles.  There is a major learning curve for the average systems engineer moving to a SoS environment, and a problem with SoS knowledge transfer within or across organizations.

Types of SoS

In today’s interconnected world, SoS occur in a broad range of circumstances. In those situations where the SoS is recognized and treated as a system in its right, an SoS can be described as one of four types (Maier 1998; Dahmann and Baldwin 2008):

  • Directed - The SoS is created and managed to fulfill specific purposes and the constituent systems are subordinated to the SoS. The component systems maintain an ability to operate independently; however, their normal operational mode is subordinated to the central managed purpose;
  • Acknowledged - The SoS has recognized objectives, a designated manager, and resources for the SoS; however, the constituent systems retain their independent ownership, objectives, funding, and development and sustainment approaches. Changes in the systems are based on cooperative agreements between the SoS and the system;
  • Collaborative - The component systems interact more or less voluntarily to fulfill agreed upon central purposes. The central players collectively decide how to provide or deny service, thereby providing some means of enforcing and maintaining standards; and
  • Virtual - The SoS lacks a central management authority and a centrally agreed upon purpose for the SoS. Large-scale behavior emerges—and may be desirable—but this type of SoS must rely on relatively invisible mechanisms to maintain it.

This taxonomy is based on the degree of independence of constituents and it offers a framework for understanding SoS based on the origin of the SoS objectives and the relationships among the stakeholders for both the SoS and its constituent systems. In most actual cases, an SoS will reflect a combination of SoS types. Other taxonomies may focus on nature/type of components, their heterogeneity, etc.

A set of systems may interact in an SoS manner, but may not be recognized as an SoS. Kemp (2013) has described such ad hoc SoS as ‘accidental’. These are not engineered as SoS although the individual systems will be engineered and engineering may need to be applied to resource particular issues among systems. Further, in most actual cases, an SoS will reflect a combination of SoS types.


Emergence is key to SoS, since in effect multiple, independent systems are brought together in an SoS explicitly to create new capability based on the interaction among the constituent systems.

Emergent behavior can be viewed as a consequence of the interactions and relationships between system elements rather than the behavior of individual elements. It emerges from a combination of the behavior and properties of the system elements and the systems structure or allowable interactions between the elements, and may be triggered or influenced by a stimulus from the systems environment. One of the consequences of emergence of significant concern to all applications of SE is emergent behavior which is unexpected or cannot be predicted by knowledge of the system’s constituent parts. These are often referred to as emergent property . See the Emergence article for more details.

As discussed in the US Department of Defense Systems Engineering Guide for Systems of Systems (DoD 2008) “for the purposes of a SoS, unexpected means unintentional, not purposely or consciously designed-in, not known in advance, or surprising to the developers and users of the SoS. In a SoS context, not predictable by knowledge of its constituent parts means the impossibility or impracticability (in time and resources) of subjecting all possible logical threads across the myriad functions, capabilities, and data of the systems to a comprehensive SE process."

Application Domains and the Difference between System of Systems Engineering and Systems Engineering

Application of SoSE is broad and is expanding into almost all walks of life. Originally addressed in the military environment, SoSE application is now much broader and still expanding. The early work in the defense sector has provided the initial basis for SoSE, including its intellectual foundation, technical approaches, and practical experience. Now, SoSE concepts and principles apply across other governmental, civil and commercial domains. Some examples include:

  • Transportation - air traffic management, the European rail network, integrated ground transportation, cargo transport, highway management, and space systems,
  • Energy - smart grid, smart houses, and integrated production/consumption,
  • Health Care - regional facilities management, emergency services, and personal health management,
  • Natural Resource Management - global environment, regional water resources, forestry, and recreational resources,
  • Disaster Response - responses to disaster events including forest fires, floods, and terrorist attacks,
  • Consumer Products - integrated entertainment and household product integration,
  • Business- banking and finance, and

  • Media - film, radio, and television.

Observations regarding differences between individual or constituent systems and SoS are listed in Table 1. These differences are not as black and white as the table might suggest. In each case, the degree of difference varies in practice and with complexity of current systems and system development environments - many of the SoS characterizations may apply to systems in certain circumstances.

Table 1. Differences Between Systems and Systems of Systems as They Apply to Systems Engineering. (SEBoK Original), adapted from Dahmann and Baldwin (2008) and Neaga et al. (2009)
Systems Engineering Systems of Systems Engineering
Management and Oversight
System Physical engineering Socio-technical management and engineering
Stakeholder Involvement Clear set of stakeholders Multiple levels of stakeholders with mixed and possibly competing interests
Governance Aligned management and funding Added levels of complexity due to management and funding for both SoS and systems; SoS does not have control over all constituent systems
Operational Focus (Goals)
Operational Focus Designed and developed to meet common objectives Called upon to meet new SoS objectives using systems whose objectives may or may not align with the SoS objectives
Acquisition/Development Aligned to established acquisition and development processes Cross multiple system lifecycles across asynchronous acquisition and development efforts, involving legacy systems, developmental systems, and technology insertion
Process Well-established Learning and Adaptation
Test and Evaluation Test and evaluation of the system is possible Testing is more challenging due to systems' asynchronous life cycles and given the complexity of all the parts
Engineering and Design Considerations
Boundaries and Interfaces Focuses on boundaries and interfaces Focus on identifying systems contributing to SoS objectives and enabling flow of data, control and functionality across the SoS while balancing needs of the systems OR focus on interactions between systems. Difficult to define system-of-interest
Performance and Behavior Performance of the system to meet performance objectives Performance across the SoS that satisfies SoS use capability needs while balancing needs of the systems
Metrics Well defined (e.g., INCOSE handbook) Difficult to define, agree, and quantify


Works Cited

Dahmann, J., and K. Baldwin. 2008. "Understanding the Current State of US Defense Systems of Systems and the Implications for Systems Engineering." Presented at IEEE Systems Conference, April 7-10, 2008, Montreal, Canada.

DeLaurentis, D., and W. Crossley. "A Taxonomy-Based Perspective for System of Systems Design Methods." Paper 925, presented at IEEE Conference on Systems, Man, and Cybernetics, October 10-12, 2005, Waikoba, HI, USA.

DoD. 2008. Systems Engineering Guide for Systems of Systems. Arlington, VA: US Department of Defense, Director, Systems and Software Engineering, Deputy Under Secretary of Defense (Acquisition and Technology), Office of the Under Secretary of Defense (Acquisition, Technology and Logistics). Accessed November 12, 2013. Available:

Kemp, D., et. al.. 2013. Steampunk System of Systems Engineering: A case study of successful System of Systems engineering in 19th century Britain." Presented at INCOSE International Symposium, June 24–27, 2013, Philadelphia, PA.

Neaga, E.I., M.J.d. Henshaw, and Y. Yue. 2009. "The influence of the concept of capability-based management on the development of the systems engineering discipline." Proceedings of the 7th Annual Conference on Systems Engineering Research, April 20-23, 2009, Loughborough University, Loughborough, England, UK.

Maier, M.W. 1998. "Architecting Principles for Systems-of-Systems." Systems Engineering. 1 (4): 267-284.

Primary References

Dahmann, J., and K. Baldwin. 2008. "Understanding the Current State of US Defense Systems of Systems and the Implications for Systems Engineering." Presented at IEEE Systems Conference, April 7-10, 2008, Montreal, Canada.

Jamshidi, M. (ed). 2009a. Systems of Systems Engineering – Innovations for the 21st Century. Hoboken, NJ, USA: Wiley.

Jamshidi, M. (ed). 2009b. Systems of Systems Engineering - Principles and Applications. Boca Raton, FL, USA: CRC Press.

Maier, M.W. 1998. "Architecting Principles for Systems-of-Systems." Systems Engineering. 1 (4): 267-284.

DoD. 2008. Systems Engineering Guide for Systems of Systems, version 1.0. Washington, DC, USA: US Department of Defense (DoD). Available:

Additional References

Barot, V., S. Henson, M. Henshaw, C. Siemieniuch, M. Sinclair, S.L. Lim, M. Jamshidi, and D. DeLaurentis. 2012. Trans-Atlantic Research and Education Agenda in Systems of Systems (T-AREA-SoS) SOA Report. Longborough, England, UK: Longborough University. Ref. TAREA-RE-WP2-R-LU-7.

Boardman, J., and B. Sauser. 2006. "System of Systems - the Meaning of Of." IEEE Conference on Systems of Systems Engineering, April 24-26, 2006, Los Angeles, CA.

Carlock, P., and J.A. Lane. 2006. System of Systems Enterprise Systems Engineering, the Enterprise Architecture Management Framework, and System of Systems Cost Estimation. Los Angeles, CA, USA: Center for Systems and Software Engineering (CSSE), University of Southern California (USC). USC-CSE-2006-618.

Checkland, P.B. 1999. Systems Thinking, Systems Practice. Chichester, UK: John Wiley & Sons Ltd.

Dahmann, J., Rebovich, G., Lane, J., Lowry, R. & Baldwin, K. 2011. "An Implementer's View of Systems Engineering for Systems of Systems." IEEE Systems Conference, April 4-7, 2011, Montreal, Canada. p. 212-217.

Keating C.B., J.J. Padilla, and K. Adams. 2008. "System of systems engineering requirements: Challenges and guidelines". EMJ - Engineering Management Journal. 20 (4): 24-31.

Luzeaux, D., and J.R. Ruault. 2010. Systems of Systems. London, UK: ISTE.

Poza, A.S., S. Kovacic, and C. Keating. 2008. "System of Systems Engineering: An Emerging Multidiscipline". International Journal of System of Systems Engineering. 1 (1/2).

Rebovich Jr., G. 2009. "Chapter 6: Enterprise System of Systems," in Systems of Systems Engineering - Principles and Applications. Boca Raton, FL, USA: CRC Press.

Ring J. 2002. "Toward an ontology of systems engineering." INSIGHT. 5 (1): 19-22.

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