There is lot more to Galileo than just its satellites in space. A global ground network is essential to ensure the continued reliability of the time and positioning information embedded within the signals from orbit.
Keeping satellites on time and in line
Satellite navigation relies on the receiver deriving the time and point in space that a signal was transmitted to an extremely high level of accuracy. This information is embedded within the satellite signal itself. But a satellite’s onboard atomic clocks can still drift – and just a billionth of a second clock error corresponds to a 30 cm increase in ranging error. A second's clock error would put users 300 000 km off target. So a network of ground stations continuously checks each satellite’s clock against Galileo System Time. Accurate to 28 billionths of a second, GST is generated by the Precise Timing Facilities at the Galileo Control Centres in Fucino, Italy and Oberpfaffenhofen, Germany, which are in turn cross-checked for alignment to the International Coordinated Universal Time by a group of European timing laboratories.
Satellite orbits drift as well, nudged by the gravitational tug of Earth’s slight equatorial bulge and by the Moon and Sun. Even the slight but continuous push of sunlight itself can affect satellites in their orbital paths. So the ground stations placed around the world picking up Galileo signals perform radio-ranging in reverse on the satellites transmitting them, to pinpoint their current position and identify any orbital drift. The information on the satellites’ clock performance and positions is gathered so that a correcting message can be uplinked to the satellites for rebroadcast to users in the satellite signals themselves. Closing the loop in this way means that optimal performance can be maintained over time. The quality and reliability of each individual Galileo signal is also checked.
In order to keep all running a series of control and monitoring centers are deployed and in operation throughout Europe and the world. The figure above depicts the main components of this network, including the actual MEO (Medium Earth Orbit) Satellites, the LEOP (Launch and Early Orbit Phase) and IOT (In-Orbit Testing) centers, etc. And the map below shows where most of these centers and stations are located
Main source for this article ESA GALILEO IOV (In Orbit Validation)
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