Galileo in a vacuum
The next two Galileo satellites have completed harsh vacuum and temperature extreme testing prior to their 28 September launch.
The fourth satellite has completed 20 days of thermal vacuum testing at Thales Alenia Space, Rome. The third satellite completed the tests in May.
ESA explains that the two satellites are almost identical to the first two launched on 21 October 2011 - so they don’t need full-scale qualification tests because in-orbit testing has shown that the design performs to expectation.
Hence, only acceptance testing is being undertaken - checking workmanship, performance and readiness for launch.
Thermal vacuum testing involves placing each satellite into a vacuum chamber and pumping out all the air. Its external surfaces are then variously heated and cooled while the satellite is operated.
With no air in orbit to moderate temperatures, any part of a satellite in sunlight can become extremely hot, while those parts in shadow or facing deep space grow extremely cold - but critical systems must be kept within a set temperature range.
As an example, Galileo’s exterior laser retroreflector reached -110°C during the cold phase of testing - yet the navigation high-power amplifiers could be driven to over +40°C during the hot phase.
Galileo uses a variety of methods to maintain temperatures within range - including multi-layer insulation, heaters, heat pipes relying on evaporating ammonia to shift heat, and radiators to dump waste heat out to space.
A critical component is Galileo’s passive hydrogen maser atomic clock, at the heart of its navigation payload - it is cited as being within a second in 3 million years. But this requires extremely stable thermal conditions, needing an operating temperature regulated to within a single degree - though in practice a tenth of that can be achieved.
The passive hydrogen maser is mounted on a 3 mm-thick aluminium plate to help hold a uniform temperature, with waste heat radiated into space from the external satellite surface. And the clock and mounting plate are wrapped in multi-layer insulation and attached to the top panel of the satellite, which is kept permanently out of the Sun.
Details from ESA below.