Modular open systems approach (MOSA)
MOSA is a US defence business and technical strategy that builds systems from modular parts with defined, standards-based interfaces so components can be replaced and competed.
The modular open systems approach, usually shortened to MOSA, is a US Department of Defense strategy for designing, buying and sustaining systems. It combines a technical method, in which a system is divided into modules joined by defined interfaces, with a business method, in which those modules can be competed and sourced from different suppliers over the life of the system. The aim is to avoid a situation in which one vendor controls every upgrade because the architecture is closed.
What it is
The US Department of Defense describes MOSA as an integrated business and technical strategy for competitive, affordable acquisition and sustainment across a system’s life cycle. It names five aims: more competition, easier technology refresh, more innovation, cost savings through reuse, and better interoperability between software and hardware modules that can change independently.
The statute, 10 U.S.C. 4401, gives the concept legal form. It defines the approach as using modular designs with defined interfaces, verifying those interfaces either through widely supported consensus standards or through delivery requirements for machine-readable, software-defined interfaces, and using an architecture that lets components be added, removed or replaced. It also requires compliance with the technical data rights provisions of Title 10. Under the section, the largest acquisition programmes, if they received Milestone A or B approval after 1 January 2019, must use the approach where practicable, and other programmes are covered by the same expectation.
How it works in practice
A MOSA design identifies the points where change is most likely, such as sensors, processors or mission software, and places stable interfaces there. Interface specifications are documented and, where possible, drawn from open standards so that more than one supplier can build to them. Programme offices then specify interface conformance in contracts and test for it. Data rights matter as much as the engineering: an interface that exists on paper but cannot be shared with competitors does not deliver the competitive benefit.
Origins and users
The concept grew out of earlier open-architecture thinking in defence acquisition and was later consolidated in policy and then in statute. Its main users are US programme offices and the contractors that bid to them. Other nations and NATO pursue similar goals through different instruments, including standardisation agreements such as STANAG documents that fix interfaces for shared use.
Why it matters
For defence technology, MOSA is the architectural counterpart to software-defined defence. Frequent software updates and the insertion of new sensors or processors are only practical if the system can accept them without redesign. MOSA also affects market structure: smaller firms can compete for a module without bidding for a whole platform, and a customer can replace a supplier without rebuilding the system.
What to watch
Key questions are how consistently programmes apply the statute, whether standards for implementation are published and used, how disputes over technical data rights are resolved, and whether interface conformance is verified through testing rather than asserted. The practicability exception also leaves room for programmes to explain why the approach was not used.
Sources
Related: Authority to operate (ATO) · Software-defined defence · STANAG