A digital twin, like a virtual prototype, is a dynamic digital representation of a physical system. However, unlike a virtual prototype, a digital twin is a virtual instance of a physical system that is continually updated with the latter's performance, maintenance, and health status data throughout the physical system's lifecycle. 

Digital twin of AOCS equipment can be integrated in the AOCS development cycle, combined with model based subsystem engineering, to simulate the physical AOCS hardware at the right level of abstraction, in consistency with the AOCS test bench used and corresponding Verification and Validation objectives along the AOCS lifecycle. 

The objective of the study ''Digital Twin for AOCS Hardware Unit Modelling: Definition of TRL Scale'' was to define the maturity (TRL) scale, and the corresponding products and milestones in the lifecycle of the AOCS hardware Digital Twins used in AOCS model based development, inspiring from the TRL definition for the equivalent physical hardware models. 

Provide a readiness scale for interchangeability between physical hardware and simulator/virtual models in the AOCS subsystem engineering V&V cycle.