Within defence, digital twins are rapidly gaining ground as indispensable tools for enhancing capabilities in platform and concept development, experimentation, test and evaluation, and operational activities within today's intricate, multi-domain environment.  With a growing need to compress training to skills deployment timescales, incorporating digital twins adds a new dimension to simulation systems by introducing a dynamic coupling between digital and physical assets.  Using a digital data thread from physical military assets, the digital model and physical counterpart can be synchronised at a speed of relevance between simulation and physical domains, enabling timely analysis, insight and actions by military decision-makers using accurate, adaptive and responsive simulation environments.

This paper builds on research conducted for the NATO ACT (Allied Command Transformation) M&S (Modelling and Simulation) Future Technology Watch programme which has identified digital twins, along with quantum computing, big data and AI as key technologies and disruptors of the M&S domain and solutions.  The research investigates, proposes, designs and demonstrates approaches to digital twin integration into simulation systems within the context of aggregate level constructive simulation.  By reducing the gap between simulated and real-world situations, digital twins make it possible to model battlefield scenarios, logistics, command structures and predict outcomes more precisely, evaluate various tactical and strategic options and enhance the effectiveness, realism and responsiveness of constructive simulations, allowing for right-time, data-driven insights. 

This paper reports on research conducted into the state of digital twins, concepts, frameworks and standards. It considers key challenges, technologies, best practises, and approaches to digital twin integration with simulation systems and next steps in their application to constructive simulation and enhanced mission effectiveness.  This analysis is consolidated into a proof of concept system design and implemented as a modular, interoperable, standards-based systems architecture that demonstrates component encapsulation and seamless integration of a digital twin and simulation system.