MOSA: Modular Open System Approach

By: PeopleTec Senior Strategic Advisor, MG William Gayler (U.S. Army Retired), and PeopleTec Technical Fellow, Casey Carter

What is MOSA?

The term “MOSA” has certainly been in the news lately, especially in and around Department of Defense (DoD) programs. And for years, it has been floating through technical conference circuits. But what is MOSA and why is it a good thing?

MOSA (Modular Open System Approach) is a design philosophy that meets an organization’s or enterprise’s business and technical objectives. These objectives can range from the ability to easily maintain and upgrade, to enable third party development of components, and to reducing supply chain risk and schedule pressures.

In years past, many thought the “A” in MOSA stood for “architecture.” However, that term was often confused with avionics—especially in Aviation—and, as a result, industry focused their MOSA efforts on a system’s “computational resources.” This limited application of MOSA left many elements of a system without the inherent benefits of a modular design and/or the use of open interface standards. The change from “architecture” to “approach” clarified the DoD’s intent to apply MOSA across an entire system.

The concept of MOSA has been applied by the U.S. Army for decades in its combat rifle solutions, making them modular and easy to maintain with easily replaced parts (versus replacing the entire rifle). The Army’s Next Generation Weapon System (NGWS) embraced MOSA even further with their Sig Sauer design that provided modularity for sustainment and interchangeability as well as mechanical standard interfaces to easily adapt and add accessories for any given mission. The NGWS also uses a common ammunition across multiple systems, where previous units had to carry at least two types of ammunition.

More recently, several supporting tools and capabilities have surfaced that enable MOSA across entire systems and even broader enterprises. These tools include Model-Based Systems Engineering (MBSE) and the widely applied Digital Engineering. In general, Digital Engineering encapsulates the tools, models, simulations, and collaboration processes to expedite system development. MBSE is an extension of the modern application of traditional systems engineering (SE) that uses system models instead of traditional documentation to enhance requirements and design techniques and arrive at a system solution quicker than traditional SE approaches. For more on MBSE, read our blog poster here.

What is the difference between MOSA, MBSE, and Digital Engineering?

MOSA is a design philosophy that is applied during the system engineering lifecycle to better achieve an affordable and sustainable system. Digital Engineering and MBSE solutions streamline the acquisition process. You can implement MOSA without using MBSE or Digital Engineering; however, by combining these tools and approaches, we form a more comprehensive development approach that enables rapid deployment of new technologies and systems to meet emerging threats.


PeopleTec’s MG Bill Gayler (U.S. Army Retired) has firsthand experience on why the DoD community is so focused on MOSA and why industry should continue to invest in incorporating MOSA into their standard practices:

“I’ve been living the reality of incremental modernization for a long time. I spent 34 years in the Army and most of it has been flying and commanding aviation units during multiple tours in several challenging theaters of operation. I have always thought the best way to care for our soldiers is to train them so well that there is nothing they see in combat more difficult than what they saw in training.”

The need for MOSA in Aviation, with noted experiences from MG Gayler (Army, Ret.)

At an enterprise level, the U.S. Government wants to ensure the best equipment is available and the best capability possible before entering a hazardous situation. Ideally, units have adequate time to ‘staff, ‘equip’, and ‘train’ for employment into combat, but the process never seems to be clean and easy. Since staffing is an entirely separate challenge in and of itself, we focus on equipping and training throughout. Surprisingly, these two approaches occasionally compete with the desire to care for our most precious resource: our people.

MG Gayler provides a real-world example of this challenge. Looking back on when he took command of a battalion a few months prior to deploying to combat, he says,

“I had been in and out of this unit for several years and knew it well. We were in the middle of a gunnery exercise when I took command. I thought this training was appropriate as gunnery skills were paramount to our success and immediately went out to the gunnery range to observe training. While awaiting my iteration on the range, what I saw was not inspiring. Crews were having a difficult time hitting targets along with less than impressive flight techniques. The Master Gunner, Standardization Instructor Pilot, and I put an immediate pause on the gunnery; we asked for additional ammunition to be sent to the range, and we spent several days teaching crews how to better shoot. We then wove in flight techniques that would maximize survivability. We shot all the ammo we could in the remaining time on the range. We didn’t score for record, but rather scored for confidence and competence. With training complete, we assessed how the unit’s proficiency could have slipped so much. What we noticed was the unit had been retrofitting the fleet of aircraft with the latest version of software and hardware components (commonly referred to as A-Kits) in preparation for a new type of equipment to be installed on the aircraft. This resulted in the aircraft being unavailable for extended periods of time resulting in less training time and a loss of proficiency in the tasks critical to success in combat. In the final few weeks prior to deployment, the army’s plan was to begin the installation of the ‘B-Kits’ (the actual final component). This meant we would have had even fewer aircraft to train with during the last critical weeks before deploying. I argued, unsuccessfully, that there is no ‘thing’ you can put on an aircraft that is more important than our crew’s training as much as possible in that critical month prior to deploying to combat. This experience reaffirmed my belief that we must find a way to rapidly introduce capability to prevent the loss of readiness created by our legacy approach to modernization.”

Prior to a MOSA, the Army had to take an inefficient approach to modernize enduring Army Aviation platforms given the operational tempo with these forces in high demand around the world. Capabilities were introduced as federated solutions and not integrated into existing architectures of the airframe. Improvements such as Communication, Command, and Control capabilities, redundant navigation systems, survivability equipment, and battle command systems were necessary but added after a system was fielded, which resulted in size, weight, and power trade-offs. Though necessary, this ad hoc method to enhance capabilities required multiple installation kits that were not fully integrated or optimized for the platform. This problematic method of integration was costly, and, as seen in MG Gayler’s example, it adversely affected unit readiness and operational availability and increased the logistical burden on our forces.

MG Gayler recalls similar situations throughout his career where more effective integration of new capabilities would improve readiness and availability.

“Fast forward several years, I had the privilege of commanding the Aviation Branch responsible for modernizing our fleets, among other equally important aspects of the branch. Working with other aviation senior leaders, we were determined to find a better way to introduce capability utilizing a modular and open system approach.”

In this case, the old approach of developing a capability in a stove pipe, solely controlled by a single supplier who designed an aircraft-specific solution, had to be modernized. This status quo approach was not focused on enterprise-wide needs and created size, weight, and power challenges for each aircraft. Innovation was also limited since outside suppliers could not offer more optimal solutions.

In addition, units were continually burdened by lengthy periods of unavailable aircraft, which restricted training time and required units to carry multiple, unique spare parts for each new capability, creating a logistical nightmare. The result was costly, burdensome, slow, and adversely affected readiness to gain a limited increase in capability.

For Army Aviation units to expedite the introduction of new capability and enhance sustainability at an affordable cost while minimizing the impact to unit readiness, the Army had to pursue something that existed in other areas, including in the commercial aviation industry, for decades… MOSA.

Can MOSA help a unit quickly install new equipment (technical objective) while maintain readiness (business objective)?

MOSA establishes a standards-based severable architecture for supported systems to easily integrate and optimally operate within the Weapon System. Fleet-wide commonality reduces costs and decreases supply chain risk and logistical footprint. With the ability to increase readiness, modular technology lowers impact on power and weight and improves overall platform and equipment sustainment. Addressing the design and development of the architecture early in the program schedule with a clear definition of requirements and interfaces, a modular approach, and use of open standards, affords commonality and enables reusability across a family of systems within an enterprise.

To enable lower size, weight, power, and cost solutions that support long-term sustainment of the Army’s Aviation Fleet, PeopleTec is using model-based development and a MOSA to analyze the available architecture products and frameworks for the U.S. Army’s Program Executive Office, Aviation. Our analysis considers the unique aviation implications of cybersecurity and airworthiness that fully exploit the interoperability of the fleet. By combining common functions and defining open software interfaces using the Future Airborne Capability Environment (FACE™) standard, PeopleTec’s approach supports MOSA objectives of reduced supply chain and schedule pressures, increased readiness, and the introduction of affordable capabilities providing for future adaptations and modifications while enabling configurable flexibility to meet Multi-Domain Operations.

Using our systems engineering, modeling and simulation, cybersecurity, and DevSecOps (Development, Security, Operations) experience, PeopleTec supports the U.S. Army’s development of a comprehensive Digital Engineering ecosystem. We also support various MOSA and enterprise commonality efforts, including the Architecture Collaboration Working Group (ACWG) and the FACE™ consortium.