Systems Engineering and Architecture

Modular Open Systems Approach

The MOSA ecosystem depends on active involvement by multiple stakeholders across the Department of Defense.

A Modular Open Systems Approach (MOSA) is an integrated business and technical strategy to achieve competitive and affordable acquisition and sustainment over the system life cycle. In the development of Department of Defense (DoD) systems, MOSA is an acquisition and design strategy, consisting of technical architectures, that adopts open standards and supports a modular, loosely coupled, and highly cohesive system structure.

MOSA implies the use of a modular design, including system interfaces designed according to accepted standards with which conformance can be verified. MOSA can be employed in both defense and non-defense sectors by using a modular architecture and an open business model to add, modify, replace, and remove system components across the acquisition life cycle.

DoD can use MOSA to design systems with highly cohesive, loosely coupled, and severable modules that can be competed separately and acquired from independent vendors. DoD is actively pursuing MOSA in the life cycle activities of its Major Defense Acquisition Programs and Major Automated Information Systems, keeping pace with the rapid evolution in technology and threats that require faster cycle time for fielding and modifying warfighting capabilities.

A key enabler for MOSA is the adoption of an open business model, which requires doing business in a transparent manner and leveraging the collaborative innovation of numerous participants across the DoD enterprise. The open business model permits sharing risk, maximizing reuse of assets, and reducing ownership cost. MOSA allows the Department to incrementally acquire warfighting capabilities, including systems, subsystems, software components, and services, with more flexibility, competition, and innovation.

MOSA Benefits

There is no single way to implement MOSA. The user must determine the desired outcomes up front to fully realize its benefits. DoD seeks five primary benefits of MOSA:

Enhance competition – open architecture with severable modules, allowing components to be openly competed.
Facilitate technology refresh – delivery of new capabilities or replacement technology without changing all components in the entire system.
Incorporate innovation – operational flexibility to configure and reconfigure available assets to meet rapidly changing operational requirements.
Enable cost savings/cost avoidance – reuse of technology, modules, and/or components from any supplier across the acquisition life cycle.
Improve interoperability – allow severable software and hardware modules to be changed independently.

The benefits to MOSA implementations complement one another, and it is likely that a program will seek to configure the benefits to achieve the necessary outcome.

As a comprehensive strategy, MOSA influences numerous aspects of SE&A and DoW acquisition processes. As illustrated in the graphic, MOSA is not a single, isolated step but is integrated throughout the system lifecycle. This ensures that systems are developed with modularity, interoperability, and adaptability in mind.

MOSA is involved in many areas in engineering across the system life cycle.

Resources

- 10 U.S.C. 144, Modular Open Systems Approach

- 10 U.S.C. 4401: Requirements for modular open system approach in major defense acquisition programs; definitions

- Air Force Materiel Command (AFMC) Guidebook for Implementing Modular Open Systems Approaches In Weapon Systems, Version 2.0

- Naval Modular Open System Approach Guidebook

- Dept of the Air Force Guidance Memorandum to DAFI 20-101/63-101, Integrated Life Cycle Management

- Dept of the Army Pamphlet 70-3, Army Acquisition Procedures, August 15, 2025

- SecNav Instruction 5000.26, Department of the Navy Implementation of the Defense Acquisition System and the Adaptive Acquisition Framework

Training and Education Resources

CLE 019 (DAU): This course describes MOSA principles from a business and technical perspective. It provides program examples of successful MOSA implementation, as well as sources that can assist an organization in implementing MOSA. This course is designed to familiarize students with MOSA best practices throughout the life cycle.

Definitions

The Tri-Service Tiger Team reviewed and updated the following definitions, which can be found in the DAU Glossary.

Modular Open Systems Approach

Modular Open Systems Approach: an integrated business and technical strategy that:

(A) 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;

(B) Is subjected to verification to ensure major system interfaces comply with, if available and suitable, widely supported and consensus-based standards;

(C) 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- 

(i) Significant cost savings or avoidance;

(ii) schedule reduction;

(iii) opportunities for technical upgrades;

(iv) increased interoperability, including system of systems interoperability and mission integration; or

(v) other benefits during the sustainment phase of a major system; and

(D) Complies with the technical data rights set forth in Sec 2320, title 10.  [10 U.S.C. 2446a. (b) Sec. 805 (FY 2017 National Defense Authorization Act Modular Open Systems Approach (MOSA) definition for DoD Acquisition, Dec 2016)]

 Open Systems Approach

 Open Systems Approach: 

(A) Employs a modular design and uses widely supported and consensus-based standards for key interfaces;

(B) Is subjected to successful validation and verification tests to ensure key interfaces comply with widely-supported and consensus-based standards; and

(C) Uses a system architecture that allows components to be added, modified, replaced, removed, or supported by different vendors throughout the life cycle of the system platform to afford opportunities for enhances competition and innovation, while yielding-  

     (i) Significant cost savings or avoidance; and (ii) increased interoperability.

 [P.L. 113-291 Sec. 801 (FY15 NDAA Open Systems Approach (OSA) definition, Dec. 2014)] (DAU and Tri-Service Tiger Team)

Modular Design

Modular Design: a design (organization) where functionality is partitioned into discrete, cohesive, and self-contained units with well-defined, open and published interfaces that permit substitution of such units with similar components or products from alternate sources with minimum impact on existing units. (A Modular Open Systems Approach (MOSA) to Acquisition, Open Systems Joint Task Force)

Open Architecture

Open Standards

Open System

Open System Architecture

Open System Acquisition of Weapons Systems

Open System Acquisition of Weapons Systems: an integrated and business strategy that defines key interfaces for a system (or a piece of equipment under development) in accordance with those adopted by formal consensus bodies (recognized industry standards' bodies) as specifications and standards, or commonly accepted (de facto) standards (both company proprietary and non-proprietary) if they facilitate utilization of multiple suppliers. (DAU Glossary)

Open Interface

Open Interface: a public standard for connecting hardware to hardware and software to software. With regard to hardware, it implies that there is more than one brand of product that can be hooked up to the device with the open interface. In the case of software, it implies that more than one program exists to interface with the application that has the open interface or that a program can be readily written to communicate with it. (OSA Contract Guidebook for Program Managers, v1.1, June 2013)

Closed Interface