Early Validation of the Data Handling Unit of a Spacecraft Using MBSE

Model-Based Systems Engineering (MBSE) represents a move away from the traditional approach of Document-Based Systems Engineering (DBSE), and is used to promote consistency, communication, clarity and maintainability within systems engineering projects. MBSE offers approaches that can address issues associated with cost, complexity and safety. Focus groups with Airbus spacecraft functional avionics engineers have highlighted that one way this can be achieved is by performing early functional validation of the high-level spacecraft functional avionics system. This paper defines an approach to the application of MBSE to perform early functional validation of a spacecraft. This approach acts as an extension to a methodology in development by Airbus. It focusses on the definition of the Concept of Operations during Phase B. Information traditionally contained in a Mission Operations Concept Document is presented through a Mission Operations Concept Model. The aim is to improve the clarity, consistency and quality of the information being communicated by providing a model template to contain the relevant system information and enable the high-level simulation of the design. This high-level simulation is enabled by the execution of static systems engineering diagrams. The mission phases are defined and a mission profile, determined by the orbit characteristics, specifies the duration of each phase. The system mode diagram details the response of the system to a change in the mission phase, and activity diagrams describe the functions that must be performed by the system in each mode. Executing this information allows the response of the system to be analysed and validated against high-level mission needs. Traditionally, this level of analysis would not be available at this early stage. The approach replaces ad hoc calculations with a formal representation of the system that can be executed, interrogated and quantified. The structure of the spacecraft system is represented by block definition and internal block diagrams, and the functionality by executable state machine and activity diagrams. Textual requirements are presented and maintained within the model and are formally linked to the physical and functional architectures. These requirements are refined by mathematical constraints that can be satisfied by calculations performed during the simulation. The use case discussed in this paper focusses on the data handling unit onboard an Earth-observation spacecraft. The system response in terms of the onboard memory usage throughout the mission is calculated. The onboard memory consists of three directories - one for housekeeping telemetry and two for science data. The simulation shows that for the chosen orbit, the total onboard memory allocation is adequate and provides a solution for the optimum memory allocation between the three directories. Ultimately, a flexible Mission Operations Concept Model template will be derived for use on future projects, which will enable them to realise the benefits demonstrated in the use case: improved control, communication and early validation of the functional avionics system design.