Trial by vacuum brings next Galileo satellites closer to launch The fourth Galileo In-Orbit Validation flight model satellite, FM4, pictured at the start of thermal vacuum testing at Thales Alenia Space Italy's facility in Rome in May 2012. The third Galileo flight model, FM3, had already undergone this testing. Image:
"These two satellites are almost identical to the first two Galileo satellites that were launched last 21 October," explained ESA's Nigel Watts.
"So we don't need to carry out full-scale qualification tests because we already know from our in-orbit test campaign that the design performs to our expectations.
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. Critical systems must be kept within a set temperature range, however.
"Meanwhile, the navigation high-power amplifiers could be driven to more than 40°C during the hot phase."
Like most satellites, Galileo's uses a variety of methods to maintain its temperature range, including multi-layer insulation, heaters, heat pipes relying on evaporating ammonia to shift heat, and radiators to dump waste heat out to space.
But it requires extremely stable thermal conditions to achieve this.
Its operating temperature needs to be regulated within a single degree, though in practice a tenth of that can be achieved.
The atomic clock and the mounting plate are wrapped in multi-layer insulation and attached to the top panel of the satellite, which is itself kept permanently out of the Sun.