When so much effort has been devoted to a spacecraft, designed to be operational for over ten years, and when the hopes and expectations of its users are so great, everything must be done to ensure reliability of the spacecraft and the success of the mission.
The satellite and its scientific instruments are designed and tested at every stage in their development as well as during the final construction. Its programme needs continuous functional testing and repeated environmental tests. For the launch, the satellite must withstand the severe conditions of its ride into space.
Most ESA spacecraft have a two-step verification. During the first, the Structural and Thermal Model (STM) and the Electrical Model (EM) of the satellite are submitted to a variety of tests. Several of these tests take place in an environment tougher than the expected real conditions of space and launch. This is the design qualification phase. When all the requirements were met, the programme concentrates on the flight model of the satellite. During its final integration, tests are once again carried out, but in an acceptance phase. At this stage, the design is no longer in doubt; but it is now a question of ensuring that everything is built according to the design that had been qualified, and that it has been assembled properly.
There are three kinds of environmental tests on the spacecraft, its modules and science instruments.
- Thermal vacuum tests, where the satellite or equipment is placed in a vacuum chamber with a sun simulator that reproduced the intensity of sunlight, which recreated the environment of space with its extreme variations in temperature.
- Vibration tests, where the spacecraft is progressively shaken at different strengths on a vibtrating table, or 'the shaker'. The conditions created are up to 25% more severe than those expected at lift-off. The spacecraft resonance is also measured to evaluate how its different parts would react to set frequencies, including those that it would probably encounter at launch.
- Acoustic tests, where the complete spacecraft is submitted in a reverberating chamber to very intense noise similar to that it would encounter during launch.
The electrical tests verify the integrity of all the circuits on board the satellite, but also the electromagnetic compatibility with the launcher and its ground installations. We cannot have equipment on the spacecraft creating interference that could jeopardise the launch. Compatibility tests are conducted to verify that the internal electric circuits of the spacecraft do not interfere with each other.
The environmental tests that lead up to launch are not the whole story. We must ensure that a satellite will work properly in space once it has been launched. More tests accompany each stage of development. For example, we must test that:
- electrical signals generated by one part of the spacecraft are received and understood where they are meant to be used;
- software covers all operating modes and flight procedures;
- the spacecraft sends the right information to the ground and processes received information correctly;
- the spacecraft can navigate and point in the desired direction in space.
All these tests are usually carried out at different locations: at ESTEC, at the prime contractor facilities, at the suppliers of individual electronic equipment, or at the various dedicated space environment and optical test facilities, such as the Centre Spatial de Liège (CSL). Qualification and acceptance phases can take over two years, but once the fully assembled satellite arrives at ESTEC, it can be given final tests right up to the moment it is packed for its journey to the launch site.
Last update: 9 July 2003