Model-based Systems Engineering in Preparation of Space Operations: The COMPASSO Mission On-Board the ISS as Use Case

Abstract

The COMPASSO mission of the German Aerospace Center plans the in-space demonstration of optical clocks and optical space to ground links. These technologies can increase the performance of Global Navigation Satellite Systems (GNSS) due to a more precise timing and a higher orbit determination accuracy. A Model-based Systems Engineering (MBSE) approach is used to manage the development process of the COMPASSO system. MBSE is based on formalized data models that describe the requirements, functions, architecture, and behavior of a system. The COMPASSO system design has been developed and modeled starting from the initial project idea and the high-level mission goals to a coarse system design followed by a logical and functional architecture and finally the physical system architecture. In this paper a workflow is proposed using the MBSE model to enable the mission operations team to gain insights how to operate COMPASSO during the design phase of the project and involve them in the design process. The focus is placed on the functional level of the COMPASSO model, as at this architecture layer all functionalities of the subsystems are modelled and interconnected. This functional architecture allows further analysis of the behaviour of the complete system. The mission objectives are the highest level of requirements and have been defined at the beginning of the project. They have been imported into the COMPASSO Capella model, where so-called functional chains have been derived from the mission objectives. Functional chains are defined in the logical model and describe a functional flow through the system to carry out specific functionalities. For every experiment that needs to be executed to achieve a mission objective, a functional chain has been defined in the COMPASSO model. Since at least one functional chain is available for each mission objective, a comprehensive analysis is possible to determine which subsystems and functions are involved at which point for each of the experiments to be executed and, ultimately, to achieve the defined mission objectives. Furthermore, state machines for the whole system are modelled, which enable, in combination with a self-developed state machine simulator a first analysis on how COMPASSO needs to be operated to carry out the planned experiments. This information serves as input for the mission operations team to start developing operational procedures and helps to save time by allowing early identification and resolve of inconsistencies, as they are involved in the design process at an early stage of the mission