When Galileo, Europe's own global satellite navigation system, is fully operational, there will be 30 satellites in Medium Earth Orbit (MEO) at an altitude of 23 222 kilometres. Ten satellites will occupy each of three orbital planes inclined at an angle of 56° to the equator. The satellites will be spread evenly around each plane and will take about 14 hours to orbit the Earth. One satellite in each plane will be a spare; on stand-by should any operational satellite fail.
Planners and engineers at ESA had good reasons for choosing such a structure for the Galileo constellation. With 30 satellites at such an altitude, there is a very high probability (more than 90%) that anyone anywhere in the world will always be in sight of at least four satellites and hence will be able to determine their position from the ranging signals broadcast by the satellites. The inclination of the orbits was chosen to ensure good coverage of polar latitudes, which are poorly served by the US GPS system.
From most locations, six to eight satellites will always be visible, allowing positions to be determined very accurately – to within a few centimetres. Even in high rise cities, there will be a good chance that a road user will have sufficient satellites overhead for taking a position, especially as the Galileo system will be interoperable with the US system of 24 GPS satellites.
So how do you build up such a constellation of satellites and ensure that each one is in precisely the correct position at any time? This delicate operation will take place in stages.
ESA launched an experimental satellite at the end of 2005, on board a Soyuz launcher. Galileo satellites have magneto-torquers and reaction wheels to help maintain them in the correct orbit, but they do not have engines to manoeuvre themselves into the right orbit in the first place. Therefore, it is essential for the launcher to place the satellite directly into the correct position.
The first Galileo satellite, 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. The test campaign is planned to last two-and-a-half years.
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) is scheduled to be launched in late 2007. GIOVE-B will continue 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.
Long lead items for a third experimental satellite (GIOVE-A2) have been ordered so as to be ready for launch, if needed, in the second half of 2008. GIOVE-A2 is meant to maintain the International Telecommunications Union (ITU) frequency filing that was secured by its predecessor and facilitate further development of ground equipment, should anything happen to GIOVE-A or B since ITU regulations do not allow a broadcast gap of more than two years.
The new satellite will incorporate some enhancements over GIOVE-A, allowing additional signals to be generated and received on the ground. The aim is to keep on providing early in-orbit experimentation with the common baseline L1 open service signals recommended by the European Union and the United States. In the future, these open service signals will provide free of charge position and timing competitive with other GNSS systems.
Next, ESA will launch the first four operational satellites using two separate launchers. The first two satellites will be placed in the first orbital plane and the second in the second orbital plane. These four satellites, plus part of the ground segment, will then be used to validate the Galileo system as a whole, using advanced system simulators. Then, the next two satellites will be launched into the third orbital plane.
Once the Galileo system has been validated, the final stage will be to build up the rest of the constellation by completing it on all its three orbital planes. This will then require several launches with Ariane-5 or Soyuz from the Europe’s Space Port in French Guyana. Galileo will then be fully operational, providing its services to a wide variety of users throughout the world.
http://www.esa.int/esaNA/ESAAZZ6708D_galileo_0.html
