The next two satellites in the Galileo European navigation have been subjected to an aggressive vacuum conditions and extreme temperatures in preparation for launch, scheduled for September 28. The fourth satellite Galileo completed earlier this month 20 days of thermal vacuum tests on the location of Thales Alenia Space on the outskirts of Rome, Italy. The third had passed the same test a month earlier.
"These satellites are virtually identical to the two released on October 21 last year," explains ESA engineer Nigel Watts. "For this reason, it is necessary to carry out a campaign of full certification. Thanks to the orbit test campaign, we know that the design meets our expectations."
Launch of the first Galileo satellites
"What we are doing is a campaign of acceptance: check the quality of manufacturing, performance and preparation for the launch of these new satellites."
During the thermal vacuum tests, each satellite is placed inside a sealed chamber from which all the air is extracted and then heat and cools the external surfaces of the satellite while checking its operation.
In orbit, the absence of air capable of moderate temperatures, the surfaces exposed to sunlight become very hot while staying in the shade, or are directed into deep space, cooled quickly. However, critics of satellite systems must be kept within a certain temperature range to ensure proper operation.
"To get an idea, the outside of the laser retroreflector Galileo reached -110 ° C during the cold phase of the trial," said Guido Barbagallo, Galileo thermal engineer. "On the other hand, high power amplifiers navigation system came up to +40 ° C during the warm phase."
Like most satellites, Galileo uses various methods to maintain the temperature within the proper range, such as multilayer insulation blankets, heaters, caloductos (devices that evaporate and condense ammonia to transfer heat between different areas of the satellite), and radiators that remove excess heat.
Galileo in orbit
The passive hydrogen maser of Galileo, in the heart of the navigation system is an atomic clock with a stability of a second in three million years. To ensure such precision, you need extremely stable thermal conditions: operating temperature must remain constant, with a maximum deviation of one degree. However, the thermal control system can regulate it to the nearest tenth of a degree.
"The passive hydrogen maser is mounted on an aluminum plate 3 mm thick which helps maintain a uniform temperature. Excess heat is radiated into space through the outer surface of the satellite, "says Guido. The atomic clock and the plate on which is mounted are covered with multilayer insulation blankets. The assembly is installed on the top panel of the satellite, which is permanently isolated from direct sunlight.