What is Galileo?
Galileo is Europe’s own global navigation satellite system, providing a highly accurate, guaranteed global positioning service under civilian control. It is interoperable with GPS and Glonass, the US and Russian global satellite navigation systems.
By offering dual frequencies as standard, Galileo is set to 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 any satellite failure, making it suitable for safety-critical applications such as guiding cars, running trains and landing aircraft.
On 21 October 2011 came the first two of four operational satellites designed to validate the Galileo concept in both space and on Earth. Two more followed on 12 October 2012.
This 'In-Orbit Validation' (IOV) phase has been followed by additional 'Full Operational Capability' (FOC) satellite launches.
Two pairs of FOC satellites have so far been launched by Soyuz from French Guiana, on 22 August 2014 and 27 March 2015. The next launch is planned for September 2015, with another due at the end of the year.
In future the number of Galileo satellites that can be inserted into orbit with a single launch will double from two to four, as a specially customised Ariane 5 launcher enters operation.
Galileo services are designed with quality and integrity guarantees – this marks the key difference of this first complete civil positioning system from the military systems that have come before.
The fully deployed Galileo system will consist of 24 operational satellites plus six in-orbit 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 to the equator.
Initial services will be made available by the end of 2016. Then as the constellation is built-up beyond that, new services will be tested and made available, with system completion scheduled for 2020.
Once this is achieved, the Galileo navigation signals will provide good coverage even at latitudes up to 75 degrees north, which corresponds
to Norway's North Cape - the most northerly tip of Europe - and beyond. The large number of satellites together with the carefully-optimised constellation design, plus the availability of the three active spare satellites, will ensure that the loss of one satellite should have no discernible effect on the user.
Two Galileo Control Centres (GCCs) have been 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 Galileo Sensor Stations (GSSs) are sent to the Galileo Control Centres through a redundant communications network. The GCCs 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 is performed through up-link stations.
As a further feature, Galileo is providing a global Search and Rescue (SAR) function, based on the operational Cospas-Sarsat system. Satellites are therefore equipped with a transponder, which is able to transfer the distress signals from the user transmitters to regional rescue co-ordination centres, which will then initiate the rescue operation.
At the same time, the system will send a response signal to the user, informing him that his situation has been detected and that help is on the way. This latter feature is new and is considered a major upgrade compared to the existing system, which does not provide user feedback.
Preparation for Galileo
Experimental satellites GIOVE-A and GIOVE-B were launched in 2005 and 2008 respectively, serving to test critical Galileo technologies, while also the securing of the Galileo frequencies within the International Telecommunications Union.
Over the course of the test period, scientific instruments also measured 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.
The four operational Galileo satellites launched in 2011 and 2012 built upon this effort to become the operational nucleus of the full Galileo constellation, followed by the first four Galileo FOC satellites.