Galileo: A Constellation of 30 Navigation Satellites
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.
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, together with advanced system simulators.
Then, the next two satellites will be launched into the third orbital plane. They will be followed by several launches with Ariane-5 or Soyuz from the Europe’s Space Port in French Guyana. The first services will be delivered when the constellation has reached its Initial Orbital Configuration.
When the 30 satellites are in space on all its three orbital planes, Galileo will be fully operational, providing its services to a wide variety of users throughout the world.