Developing and operating a complex system like a space system implies many activities involving many parties who may be widely-distributed, both geographically as well as in time. Successful development and operation of space systems therefore requires successful information sharing during the whole lifecycle of the space system. This is an integral part of the system engineering process and can only be achieved by realising interoperability between all involved parties.
Interoperability is often described in terms of data exchange according to specific data formats and communication protocols, i.e. from a syntactical viewpoint. While agreements on how to exchange the data are required for interoperability, syntactic agreements alone are not sufficient to guarantee a complete understanding of the information that is exchanged. For this understanding, semantic interoperability, addressing the “what” in the exchange is required.
In 2011, the European Cooperation for Space Standardization (ECSS, www.ecss.nl) has published a technical memorandum, the “space system data repository” (ECSS-E-TM-10-23A). In this technical memorandum, the difficulties that are encountered with respect to sharing and reusing knowledge during the development and operations of large systems are addressed. The memorandum looks at semantic modelling and semantic interoperability for defining a mechanism whereby structured information can be reliably and efficiently exchanged between all involved parties, at all levels of the system decomposition, within and across all system life cycle phases.
The FAMOUS-2 project aims at developing a solution to support semantic interoperability, in line with the concept of space system data repository as defined in the ECSS-E-TM-10-23A.
No semantic interoperability without syntactic interoperability
Interoperability efforts typically focus on the physical data model used in the exchange. That is, a standard manner for achieving syntactic interoperability is to specify a set of syntax rules that are to be obeyed by the data that is exchanged. In other words, syntactic interoperability implies that the structure in which the data is exchanged is defined. This means that the domain-specific schema, for which the data is an instance shall be known by all parties involved. However, the generic schema that specifies the rules to which the domain-specific schema has to conform shall also be known such that there is no mismatch of interpretation of the domain-specific schema. In essence, for syntactic interoperability:
- the data that is to be exchanged needs to be known,
- the domain-specific schema to which the data complies needs to be known, and
- the generic schema to which the domain-specific schema complies needs to be known (also known as the meta-schema).
In the context of the FAMOUS-2 research project the Knowledge Triangle as shown in Figure 1 is used to differentiate between the three levels described above.
Figure 1: Exchange of data requires the data itself (level I), the domain-specific schema (level II) and the generic schema (level III).
Semantic interoperability focuses on the “what”, the requirements that have to be fulfilled when exchanging information, both by the receiver as well as the sender of the data. For this, it is required to model the semantics. This is achieved through modelling at conceptual level. Modelling at conceptual level should be independent of any design or implementation concerns. The aim of modelling at conceptual level is to express the meaning of the terms and concepts used by the different parties involved and to find the correct relationships between the different concepts. A conceptual model has as main aim to remove ambiguity as well as any risk of misunderstanding. Therefore, within the context of the FAMOUS-2 research project, the FAMOUS modelling method is used to develop conceptual models.
The FAMOUS method is a method that consists of both a structural as well as a structuring part. The structural part specifies all the concepts that are of interest (the language) while the structuring part specifies how, when and by whom the different concepts are used and/or created. In the FAMOUS-2 research project, the method is extended with means to:
- formally specify derivation rules,
- identify existential dependency relationships between different object types,
- formally specify dynamic rules,
- provide extensive support for data types, units, quantities and dimensions in accordance to QUDV,
- identify possible conceptual nodes in a hierarchical view on the conceptual schema and their functional parts, and
- provide support for interoperability through a global conceptual model.
Global conceptual model
The approach suggested in the FAMOUS-2 project for semantic interoperability is the development of a global conceptual model that can be used as single point of reference for the development of product-specific local models.
Modelling at global level, in line with the single “ECSS system” development approach, implies:
- standardizing at global level the “what” and
- offering means to tailor that “what” recursively from projects to systems, elements, subsystems, whereby tailoring means adding specific needs and selecting the subset of the global conceptual model of interest.
Using the FAMOUS conceptual modelling approach to develop a global conceptual schema, each party involved in the development of the space system (and thus involved in the exchange) can extract its own “local” conceptual schema from this global conceptual schema by:
- base-lining through identifying that subset of the global conceptual schema that applies in the local context,
- tailoring the selected subset by potentially strengthening the requirements, by e.g. adding additional derivation rule for mapping from the global to the local view, and
- extracting the tailored local view from the global conceptual model such that it can be used as the starting point for further development in the local context.
Figure 2: From global conceptual schema to local conceptual schemas.
A local conceptual schema is the starting point for further development. Since FAMOUS-2 aims at supporting semantic interoperability, it also provides means to support the required syntactic interoperability. This is achieved by providing the ability to derive logical models from the conceptual model and physical models from the logical models. The types of models supported by FAMOUS-2 are depicted in Figure 3.
Figure 3: The conceptual model as the basis for logical models which in turn form the basis for physical models.
The overall architecture of FAMOUS-2 can be found in the publications section.