Build a Modular Open Systems Approach (MOSA)

Modular Open Systems Approach (MOSA) is a systems engineering approach that involves dividing large entities into smaller modules with standard interfaces. It builds on principles such as “high cohesion, low coupling”  to produce systems that are more robust, maintainable, and easier to update over time.

The term “modular design” refers to building a large system out of discrete pieces, usually built to standard dimensions and attributes. This often allows for rapid assembly of modules into a system, as well as easy customization, depending on which modules are incorporated. Lego blocks are an example of modular design.

Open Systems use standards that are independent of a particular vendor or designer, increasing the diversity of potential suppliers. These standards are particularly important at interfaces. Again, a Lego block’s standardized interfaces allow for connections between any two blocks.

Program management is responsible for evaluating and implementing a MOSA to the maximum extent feasible and cost effective. This approach integrates technical requirements with contracting mechanisms and legal considerations to support a more rapid evolution of capabilities and technologies throughout the product life cycle through the use of architecture modularity, open systems standards, and appropriate business practices. The Acquisition Strategy for the system should identify where, why, and how a MOSA will or will not be used in the program. (Source: DODI 5000.02 Enclosure 2)


Program Managers, with support from the Lead Systems Engineer, are responsible for applying modular approaches in product designs where feasible and cost-effective. They are also responsible for acquiring data and IP that are both appropriate and essential to achieving the expected benefits. Modular designs coupled with an appropriately open business model provide a valuable mechanism for continuing competition and incremental upgrades, and to facilitate reuse across the joint force. (Source: DODI 5000.02 Enclosure 3)


Per law, 10 USC 2446, A major defense acquisition program that receives Milestone A or Milestone B approval after January 1, 2019, shall be designed and developed, to the maximum extent practicable, with a modular open system approach to enable incremental development and enhance competition, innovation, and interoperability. The term “modular open system approach” means, with respect to a major defense acquisition program, an integrated business and technical strategy that

  • employs a modular design that uses major system interfaces between a major system platform and a major system component, between major system components, or between major system platforms;
  • is subjected to verification to ensure major system interfaces comply with, if available and suitable, widely supported and consensus-based standards;
  • uses a system architecture that allows severable major system components at the appropriate level to be incrementally added, removed, or replaced throughout the life cycle of a major system platform to afford opportunities for enhanced competition and innovation while yielding—
    • significant cost savings or avoidance;
    • schedule reduction;
    • opportunities for technical upgrades;
    • increased interoperability, including system of systems interoperability and mission integration; or
    • other benefits during the sustainment phase of a major weapon system; and
    • complies with the technical data rights set forth in section 2320 of this title.


Benefits of MOSA (Source: DASD/SE)

  1. Enhance competition – open architecture with severable modules, allowing elements to be openly competed.
  2. Facilitate technology refresh – delivery of new capabilities or replacement technology without requiring change to all elements in the entire system.
  3. Incorporate innovation – operational flexibility to configure and reconfigure available assets to meet rapidly changing operational requirements.
  4. Enable cost savings/cost avoidance – reuse of technology, modules, and/or elements from any supplier across the acquisition life cycle.
  5. Improve interoperability – severable software and hardware modules that can be changed independently.
Actions You Can Take
  • Review the MOSA Guidebook at the link below.
  • Incorporate open standards into the system requirements and the Systems Engineering Plan.
  • Perform an engineering review to identify opportunities for using modular design approach.
  • Use the MOSA Program Assessment and Rating Tool (PART) to assess your program
  • Contact a SME for additional assistance.
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