What is Galileo?
Galileo will be Europe’s own global navigation satellite system, providing a highly accurate, guaranteed global positioning service under civilian control. It will be inter-operable with GPS and GLONASS, the two other global satellite navigation systems.
A user will be able to take a position with the same receiver from any of the satellites in any combination. By offering dual frequencies as standard, however, Galileo will deliver real-time positioning accuracy down to the metre range.
It will guarantee availability of the service under all but the most extreme circumstances and will inform users within seconds of a failure of any satellite. This will make it suitable for applications where safety is crucial, such as running trains, guiding cars and landing aircraft.
The first experimental satellite, GIOVE-A, was launched on 28 December 2005. The objective of this satellite is to characterize the critical technologies, which have already been developed under ESA contracts. GIOVE-A, has been placed in the first orbital plane from where it is being used to test the equipment on board and the functioning of ground station equipment. It has also permitted the securing of the Galileo frequencies within the International Telecommunications Union.
Initially, the performance of the two atomic clocks on-board was characterised. Then the signal generator was turned on to provide experimental signals with various modulation characteristics. Over the course of the test period, scientific instruments on board are measuring various aspects of the space environment around the orbital plane, in particular the level of radiation, which is greater than in low Earth or geostationary orbits.
A second experimental satellite (GIOVE-B) was launched in April 2008. GIOVE-B continued the testing begun by its older sister craft, but with the addition of a passive hydrogen maser and with a mechanical design more representative of the operational satellites.
Thereafter, four operational satellites - the basic minimum for satellite navigation in principle - will be launched in 2011 to validate the Galileo concept with both segments: space and related ground infrastructure. Once this In-Orbit Validation (IOV) phase has been completed, additional satellites will be launched to to reach the Initial Operational Capability. At this stage, The Open Service, Search and Rescue and Public Regulated Service will be available with initial performances. Then along the build-up of the constellation, new services will be tested and made available to reach the Full Operational Capability (FOC).
The fully deployed Galileo system consists of 30 satellites (27 operational + 3 active spares), positioned in three circular Medium Earth Orbit (MEO) planes at 23 222 km altitude above the Earth, and at an inclination of the orbital planes of 56 degrees with reference to the equatorial plane.
Once this is achieved, the Galileo navigation signals will provide good coverage even at latitudes up to 75 degrees north, which corresponds to the North Cape, and beyond. The large number of satellites together with the optimisation of the constellation, and the availability of the three active spare satellites, will ensure that the loss of one satellite has no discernible effect on the user.
Two Galileo Control Centres (GCCs) will be implemented on European ground to provide for the control of the satellites and to perform the navigation mission management. The data provided by a global network of twenty Galileo Sensor Stations (GSSs) will be sent to the Galileo Control Centres through a redundant communications network. The GCC’s will use the data from the Sensor Stations to compute the integrity information and to synchronise the time signal of all satellites with the ground station clocks. The exchange of the data between the Control Centres and the satellites will be performed through up-link stations. Five S-band up-link stations and 10 C-band up-link stations will be installed around the globe for this purpose.
As a further feature, Galileo will provide a global Search and Rescue (SAR) function, based on the operational COSPAS-SARSAT system. To do so, each satellite will be equipped with a transponder, which is able to transfer the distress signals from the user transmitters to the Rescue Co-ordination Centre, which will then initiate the rescue operation.
At the same time, the system will provide a signal to the user, informing him that his situation has been detected and that help is under way. This latter feature is new and is considered a major upgrade compared to the existing system, which does not provide feedback to the user.
Altogether Galileo will provide five levels of services with guaranteed quality which marks the difference from this first complete civil positioning system.
Last update: 11 May 2010
