Europe launched another two satellites for its multi-billion-euro Galileo satnav system, a rival to America’s GPS.
The launch of satellites 11 and 12 in the Earth-orbiting constellation, brings Europe a step closer to providing initial navigation services next year.
The orbiter duo were taken aloft a Russian Soyuz rocket from Europe’s spaceport in Kourou, French Guiana. The satellites are due to orbit at an altitude of 23,222 kilometres (14,429 miles).
What is Galileo System?
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.
First launches
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.
Four pairs of FOC satellites have so far been launched by Soyuz from French Guiana, on 22 August 2014, 27 March 2015, 11 September 2015 and 17 December 2015.
Galileo services
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 per orbital plane, will ensure that the loss of one satellite should have no discernible effect on the user.
Ground infrastructure
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.
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.