Galileo takes its place on top of the world
Preparing for this year's first launch of Galileo navigation satellites, ESA is about to inaugurate the most northerly node in its worldwide network of Galileo ground stations. Svalbard is positioned more than 78° north, less than a thousand kilometres from the North Pole.
Located on Spitsbergen, the largest island of Norway's Svalbard archipelago, the Kongsberg Satellite Services, Svalbard Satellite Station – SvalSat for short – already serves as a ground station to numerous polar-orbiting Earth observation missions, including ESA's Envisat, Goce, SMOS and ERS-2.
For Galileo, SvalSat will serve as a Sensor Station (GSS) to check the timing and positioning accuracy of Galileo signals as well as an Uplink Station (ULS), to transmit correction messages to the satellites as needed, sharpening the overall accuracy of Galileo navigation services.
Typically ground stations incorporate robust air conditioning to compensate for electrical equipment pumping out heat, but that is unnecessary in Svalbard’s Arctic conditions.
The road to Spitsbergen's main settlement of Longyearbyen is regularly blocked by severe weather – a helicopter pad provides back-up access for the 23-strong team of engineers who operate the site around the clock – and there are more polar bears than people on the island as a whole.
SvalSat was established by the Norwegian Space Centre in 1997. However, since 2002 the station is owned and operated by Kongsberg Satellite Services (KSAT).
Svalbard is one of the three Galileo ground stations to be hosted by Norway – another is at the Norwegian Troll Antarctic base with a third one to be built on the island of Jan Mayen in the North Atlantic. Those three stations are – or will be – built, maintained and operated by KSAT, employing their specialised knowledge of these harsh polar environments.
KSAT is the world's largest ground station supporting polar orbiting satellites. The extensive KSAT network of ground stations has active contact with around 50 satellites about 10.800 times per month. KSAT is therefore providing multi-mission satellite support to satellite owners and operators as well as users of global satellite data services
Galileo on the ground
There is a lot more to Galileo than just satellites in space. The worldwide ground-based infrastructure ESA has put in place is essential to ensure the continued reliability of the Galileo signal’s time and positioning information. Satellite navigation relies on the user’s receiver calculating the time and place in space that the signal was transmitted to an extremely high level of accuracy.
But onboard atomic clocks can still drift, as can the orbits of the satellites themselves, perturbed by gravitational influences and the slight but significant push of sunlight itself.
So ESA's network of ground stations continuously checks each satellite's clock for any drift, as well as performing radio-ranging on the satellites to identify any orbital perturbation. If corrections are needed, they can be embedded into the latest version of the Galileo signal that is transmitted up to the satellites to be rebroadcast. The entire Galileo system amounts to one important ensemble of extremely accurate clocks that can inform its user if it is running slow or fast.
The Galileo ground segment breaks down as follows:
- Ground Mission Segment (GMS) Control Centre – deployed in Fucino Control Centre in Italy, must provide cutting edge navigation performance, processing navigation performance at high speed around the clock with data collected from a worldwide network of sensors stations.
- Ground Control Segment (GCS) Control Centre – deployed in Oberpfaffenhofen in Germany, needs to perform monitoring and control for the Galileo constellation of satellites, with a high degree of operational automation. In the future, both control centres will host the GMS and GCS to work together in hot back-up configuration with real-time data synchronisation.
- Tracking and Telecommand Stations – two in total, deployed at Kiruna in Sweden and Kourou in French Guiana to ensure connectivity with the satellites as part of the GCS.
- Uplink Stations – Svalbard is the most northerly of a network of stations to uplink the updated navigation data as part of the GMS. The other stations are located at Kourou (French Guiana), Reunion Island, Nouméa (New Caledonia) and Papeete (French Polynesia)
- Sensor Stations – A global network providing worldwide coverage for collecting signals broadcast by orbiting satellites to be used for clock synchronisation and orbitography measurements in the GMS.