The SysAOCS project: from ESA AOCS/GNC Documents to Digital Models
Authors:   SENER Aeroespacial, Tres Cantos, Madrid , Spain, Martinez Alvaro , Orte Sofia , Tarabini Castellani Lorenzo , Wijnands Quirien 
Published: January 2024

SysAOCS was a 12-month European Space Agency project developed by SENER Aeroespacial in the frame of the Open Space Innovation Platform (OSIP) campaign “Model-Based System Engineering: from documents to models”. Executed during 2022, its main objective was the digitalisation of the AOCS/GNC design documents using the SysML language. The project tasks included the definition of the digitalisation approach and its application to ESA AOCS/GNC missions, to validate and improve the modelling process.
Systems Model Language (SysML) is a specification language which makes use of universal graphical elements organised in diagrams for the description of systems. The use of SysML language is defined by a methodology and requires a modelling tool. The methodology contains the system decomposition strategy, to flow-down the information from mission specification to system physical design. Among the different methodologies, the ESA SysML Solution was selected for the SysAOCS project. ESA SysML Solution organizes the system description in viewpoints or layers and makes a clear distinction between elements belonging to the system definition and decomposition (solution space) and those corresponding to the system context and environment (problem space).
In the frame of the project, the ESA SysML Solution was tailored to describe the specifics of the design of AOCS/GNC subsystems. SENER included the “Mission Modes Cycle”, defining the spacecraft operational modes, specific diagrams on Fault Detection, Isolation, and Recovery (FDIR) elements, models on trade-offs and analyses, together with design documents and budgets. The tailored Solution was used to describe the AOCS/GNC subsystem of the Space Rider and Euclid missions.
For both missions, the Design Definition File (DDF) was modelled to capture the static AOCS/GNC design, while the Design Justification File (DJF) was employed to account for the design iterations, including trade-offs and analyses. The information from several design documents was combined to generate the digital versions of the DDF and DJF, strictly following the ECSS standards. Thanks to this digitalisation process, it was possible to combine the Euclid AOCS modes in a single digital document, recovering the information from several sources.
The information contained in both the Space Rider and Euclid models was structured in different packages. A package is a SysML component designed to group modelling elements to be maintained and reused. The model packages in the SysAOCS implementation are: the methodology package, containing the tailored ESA SysML Solution, the common database package, with the complete models of the AOCS/GNC, and dedicated packages for each of the documents to be modelled. The document packages present the information according to the ECSS Document Requirement Description (DRD). Information duplicity was avoided by linking the contents of each digitalised document to the applicable elements in the common database, where the definition of all design data is performed. Additionally, the models contain extensive navigability hyperlinks between the different packages and diagrams, allowing model elements direct access from the diagrams.
The digital representation of the DDF of both study cases was organised in different chapters, containing information about the mission and its context, the AOCS/GNC mode architecture, the functional and physical system decomposition and the AOCS/GNC budgets. The functional and physical description layers contain the detailed definition of the interfaces among the different AOCS/GNC functions, sensors and actuators, and external systems. Since the information within the model is unique, the risk of inconsistency in the definition of the different interfaces is minimised, when compared to approaches based on documents. The DDF is flexible enough to account for the different types of AOCS/GNC architectures and interface definitions. On the one hand, Space Rider GNC is divided by Guidance, Navigation and Control functions and its mode architecture is sequential. On the other hand, Euclid AOCS follows a classical AOCS recursive mode scheme. This implies that Euclid AOCS interfaces were defined between AOCS modes, sub-modes and functions, while for the case of Space Rider GNC, the main interfaces are between the Guidance, Navigation and Control logical blocks, for each of the mission phases.
The DJF trade-off process was described by generating diagrams and tables showing the different design alternatives, the trade-off criteria and corresponding weight, the qualitative assessment of each alternative and the quantitative evaluation of the different options. The trade-off diagrams can be easily maintained and allow a clear and dynamic definition of the design baseline. Design analyses were linked to external resources, to further complete the model information.
A bi-directional data sharing tool between IBM Rhapsody and MATLAB/Simulink was developed at the end of the project. This tool allows feeding Simulink simulators with data from Rhapsody models and extracting simulation results to update the common database package.
The paper will describe in detail the DDF and DJF modelling of Space Rider and Euclid. The expected benefits of this digitalisation are the traceability from requirements to design elements and verification evidence, the definition of clear and consistent interfaces and the direct access to information through model navigability. Additionally, the SysML models act as a standardized source of truth, which not only nurtures the reusability of their structure and contents, but also enables their connection to simulation environments.