1: Mission objective
A space mission is born out of the need for a product or service by private institutions or governmental agencies working in Space research. This need is formalised as a mission objective, taking into account high-level constraints. Some examples of primary objectives of famous missions follow.
- Gravity and steady-state Ocean Circulation Explorer (GOCE): to determine the Earth’s steady-state gravity field anomalies with an accuracy of 1e-5 m/s2 , and to assess geoid heights with accuracy between 1 to 2 cm, at length scales down to 100 km;
- Aelous: to provide accurate global measurements of winds from the surface up to 30 km;
- Gaia: to measure the positions and velocity of approximately one billion stars in our Galaxy, to determine their brightness, temperature, composition and motion through Space;
2: Mission concept and feasibility analyses
Once a mission objective is identified, engineers conduct mission analyses and identify constraints for the design of satellite subsystems and supporting ground infrastructure. A concurrent engineering approach is used to conduct trade-off studies, make decisions, and typically identify several stable mission architectures. After a mission concept is selected and the feasibility is confirmed, work starts on defining functional and operational requirements. During this phase, engineers’ decisions should account for required performance, implicit and explicit constraints, schedule, and risks.
3: Requirements definition
Functional requirements are flown down to the system level for both space and ground systems. These requirements are then decomposed and allocated to the subsystem level and interfaces between them. Even though this process is initiated using a top-down approach, reconciliation of system-level requirements with lower-level design development is commonplace. Engineers use system-level requirements to converge on the identification and definition of interfaces between all subsystems while using subsystem level requirements to identify specifications at the component level. This iterative process is very long and involves a multitude of stakeholders, and in most cases is even started even in the feasibility evaluation phase.
4: Satellite and mission design and development
Functional and operational mission requirements flown down to the satellite platform level are then taken down to the satellite subsystem level, and then from there to the components level to procure, design, and develop solutions for the satellite platform. Even during the design phase — and early during the Assembly, Integration and Testing phase — the software (SW) required to support the operation of the satellite subsystems should also be tested and verified. Engineers also investigate the characteristics of the selected orbit on the satellite design.
High-fidelity computer simulations typically simulate the satellite software and the space environment to detect and address possible failures and ensure that system-level requirements have been correctly brought down to the subsystem level. Defined interfaces must also be tested. These simulators and ‘reference facilities’ form part of the Ground Segment. Simulations for large satellites typically include sensor models (e.g., star trackers, sun sensors, earth sensors), actuators models (e.g., reaction wheels, thrusters, magnetic coils), and functional models of the power system management, telemetry control functions, the thermal management system, the fault detection and isolation system, and the propulsion system.
In Europe, reference facilities are present within ESA (European Space Agency) at ESTEC (European Space Research and Technology Center) which enable software-in-the-loop simulations, hardware-in-the-loop simulations, and provide several simulators for the environment, flight dynamics, onboard SW and data handling. Cosylab is in the process of analysing the critical design changes needed for integrating a next-generation control system in existing reference facilities hosted by ESA, making them adaptable to be able to host a range of missions.
5: Assembly, Integration and Testing (AIT)
The subsystem components are then assembled, integrated into their higher-level assemblies, and thorough testing must ensure the subsystems can withstand the operating space environmental conditions and the extreme acoustic and mechanical environment during launch. Engineers can conduct testing at various integration levels, which is highly dependent on a multitude of programmatic and technical factors. The goal of this phase is to ‘qualify’ the systems for flight. For more details on How to Design Space Instrumentation, please refer to a previously published article by my Cosylab colleague Diego Casadei.
Many tests have to be accomplished in a predefined order to ensure that the satellite and its subsystems survive the launch and the harsh space environment. For activities in this phase, interfaces and equipment to ground infrastructure have to be provided, and it is the ground segment that takes care of this. Also provided by the ground segment are the necessary facilities required to carry out the tests, including (but not limited to) anechoic chambers, thermal-vacuum chambers and solar simulators, shakers and acoustic chambers.
Apart from mechanical tests, the supporting software — both Space and ground SW — is thoroughly tested and validated. Of course, for the whole ground segment to work correctly and be compliant with the mission requirements, the ground segment must also be the subject of systematic tests. The final goal before launch is that of operationally testing the entire ground segment.