Model-based system engineering (MBSE) uses digital representations, or ‘models’, instead of documents to support activities across the lifecycle of complex systems, including requirements definition, design, analysis, verification and validation. As part of the Engineering Support Services (ESS) that RHEA provides to the European Space Agency (ESA) in the domain of ground segment for ESA Earth observation (EO) missions, the team has developed a dedicated SysML profile and configured an MBSE platform to model payload data ground segments for operational and planned EO missions.

Ivan Famoso, RHEA System Engineer and ESS Area 1 Service Manager, explains the initial requirements, what it has delivered and the potential benefits of the approach.

What requirements led to the use of MBSE to model the ground segments of ESA EO missions?

The ground segments for ESA EO missions that are currently flying were designed at different points in time in response to the specific scientific needs of each mission and to be operational for a ‘limited’ period. However, the outstanding performances of many of these missions led to a significant extension of their operational lifetimes. This resulted in simultaneous operation of mission-specific ground segments supported by the availability of mission-generic data management services (for example data dissemination and data archival).

ESA expressed the need for a harmonized, high level, yet sufficiently detailed technical description of its ground segments for both operational and upcoming missions, to be delivered in a short timeframe, that would enable a preliminary assessment of the impact of potential system modifications.

Following a few iterations to better understand the customer’s needs, RHEA’s proposal to use MBSE based on the RHEASysML profile was favourably received by ESA. They recognized that this approach, combined with a thorough knowledge of the operational systems, would be able to provide an interactive, simple-to-understand representation of systems architecture, interfaces and dataflow. This would highlight complexities and help with identifying differences and similarities across systems.

How many missions were involved?

The exercise started with modelling the ground segments of four operational missions and three upcoming missions; furthermore, the approach was extended to selected mission-generic services.

Very rapidly, the MBSE approach was recognized as being very effective in representing the complexity of the systems and their incremental development over the years to onboard additional functionalities.

European Space Agency's FORUM satellite is an Earth observation mission
The European Space Agency’s FORUM satellite is among the missions benefitting from the use of MBSE for ground segment activities.

What was the timescale for development?

ESA’s requirement was to proceed very rapidly with a first descriptive phase providing the maximum possible amount of global information, with a possible refinement in terms of detailed analysis at a later stage. In practical terms, the model was set up starting with one mission, and following positive feedback from ESA it was extended to other missions.

This incremental approach, based on effective interactions between RHEA and ESA, was successful.

What were the main challenges and how did MBSE help solve them?

The main challenges were the limited time, the fact that the information needed by ESA was available in multiple documents, and also held within the knowledge of some key personnel, and that the documented design did not use a common ‘ontology’, or vocabulary. To address this, we defined a common ontology and translated it into a ‘metamodel’ [a SysML profile], and defined Excel templates compliant with the ontology to ingest the information automatically.

We trained key personnel, brought all the information into the model, and then starting from the ingested elements and relationships, we created, in a semi-automatic way, a graphical representation to provide a clear picture of the design to users of the model. The result was a harmonized view that allows a quick comparison between the mission and the repository, and can be queried to extract information.

How did the use of MBSE and models help to address ESA’s requirements?

The models developed in the context of this activity were able to capture the complexity of the analyzed systems and provide information at different levels. This allowed the level of granularity to be tuned during our analysis without flooding the user with details, hence responding well to the requirements set out by ESA.

Furthermore, these models can be looked at as a powerful means of knowledge transfer, extracting key information and making it available to a wide technical team. This helps to avoid any single points of failure in the engineering/operational knowledge chain, particularly for ground segments developed under specific contexts a long time ago and whose operational lifetime has been extended well beyond the initial one planned.

These elements were fully recognized and appreciated by ESA.

How could a model-based approach further support ground segment systems development?

As already mentioned, this approach has great potential when supported by a robust technical knowledge of the systems concerned and is very effective in accelerating engineering teamwork. In this sense, it was certainly appreciated by ESA as a design and analysis tool within the frame of the engineering services provided by RHEA.

Models provide an effective, straightforward way to access existing architectural solutions in a ‘smart’, interactive way at the desired level of granularity, which is extremely important to keep a solid reference along the whole operational phase.

In more generic terms, the MBSE approach has the potential to significantly streamline any engineering process, enabling truly interactive teamwork in identifying and implementing enhancements and synergies. Furthermore, the existence of digital models under configuration control could significantly reduce the need for ‘standard’ documentation, which would only be needed to complement the MBSE visuals.

In this context, the MBSE approach constitutes a relevant tool to enhance – in terms of quality and value added – the engineering services provided by RHEA within the ESS context.

Main image © EUMETSAT/ESA