Introduction 94

Model-Based Systems Engineering (MBSE) is defined as “the formalized application of

modeling to support system requirements, design, analysis, verification and validation activities beginning in the conceptual design phase and continuing throughout development and later life- cycle phases.” (See INCOSE 2007.)

Systems engineers have used models of various types to help understand, describe, and analyze different aspects of a system. Indeed, we all are using models of a system: within our minds; in drawings, budgets, or equations on paper; or in information that we access or process with computers. One difference between traditional document-centric methods and model-centric methods is that in model-based systems engineering, models are expressed, developed, and matured in a machine-usable form external to the engineer.

Traditional practices tend to rely on multiple, stand-alone models, resulting in disconnected system representations. These are often discipline-specific models—systems, mechanical, electrical, thermal, etc.—that may be connected by awareness of the engineer, but are disconnected from each other in the sense that they can only be made mutually consistent through acts of human labor. Much of the communication among different engineering teams takes place orally or visually in a discipline-centric viewpoint using a variety of documents that includes human-readable text, diagrams, and spreadsheets. In this approach, the systems engineer can be challenged to ensure consistency among all the disparate models, especially as the models are changed over time by their custodians. With disconnected system representations, it can be difficult to get an accurate system-level understanding of the technical baseline.

MBSE includes a paradigm shift from disconnected system representations to systems

descriptions in the form of integrated system models. MBSE uses formal system models as the preferred way to represent systems, systems engineering activities, and their resulting artifacts, and manage the process of engineering. Because formal models can be subjected to formal tests of completeness, accuracy, and consistency, the integrated system models of MBSE offer an improved way to analyze the system architecture, providing the ability to detect problems earlier in the project life cycle. Formal systems models offer these advantages because they introduce additional rigor and flexibility, because they are both human and computer understandable, and because they are logically verifiable. Additionally, when the system models are integrated by machines, it becomes possible to keep engineering information consistent rapidly.

Integrated systems models help systems engineers manage the many kinds of interrelated

information in systems of increasing size and complexity. Systems engineers have always had to capture, in one form or another, information about a system’s structures, behaviors, constraints,

and requirements. With the increased existence of standard modeling languages for systems engineering such as OMG SysML, systems engineers can specify and maintain semantically rich relationships among model elements, such as how one component is part of another, how one function depends on another, which requirements specify a component’s interfaces, what work package has delivery responsibility for a subsystem, what analysis shows that a performance requirement can be satisfied, etc.

MBSE shifts the locus of authority of the systems descriptions from documents to models. This does not mean eliminating required documents or other traditional systems engineering

deliverables. Instead, these artifacts can be increasingly produced automatically from

information in the models—the “one source of truth”—ensuring consistency among the artifacts. The key assumption made by the MBSE approach is that the integrated system model and its representations, or views, describing the system are more capable of describing systems than are documents. There can be a wider variety of views of the system, tailored to the stakeholder interests when MBSE is applied, instead of a standard set of limited documents. There may still be a mix of models and documents generated from the models; some things might be better conveyed in document form, while some others might be better conveyed in models. A benefit of MBSE is the possibility of generating document-formatted reports consistently from the

information in various models, as depicted in Figure 8.2-1.

Figure 8.2-1 Automated Generation of Engineering Artifacts

The documents and other artifacts such as reports, power point presentations, etc., are produced from the system model using automated procedures that transform the system model into models of the artifacts.

One of the main benefits of MBSE emerges from incorporating all the information about the system into an integrated collection of interrelated models that represents the system from different perspectives (e.g., compositional, functional, operational, cost) with increased ability to correlate and retrieve any desired information. It ensures that data needed by programs and projects (e.g., for milestones, reviews, mission operations, and anomalies or investigations,

decisions, and outcomes) are identified and managed to provide traceability of the data used in decision-making. Interrelations defined between the model elements enhance the ability to maintain overall system representation consistency and enable efficient propagation of changes.

“Models have been used as part of document-based systems engineering approach for many years, and include functional flow diagrams, behavior diagrams, schematic block diagrams, N2 _{charts, performance simulations, and reliability models, to name a few.} However, the use of models has generally been limited in scope to support specific types of analysis or selected aspects of system design. The individual models have not been integrated into a coherent model of the overall system.” (Source: Friedenthal 2008)