Two Galileo satellites will be launched on 18:15:00 GMT (20:15:00 CEST) on 12 October. They will complement two other Galileo satellites which are already in orbit. This follow-up launch marks a major step for Europe's own satellite navigation system. Four is the minimum number of satellites needed to achieve a navigational fix on the ground, with one satellite each to measure latitude, longitude, altitude and provide a time reference.
Galileo is Europe’s own global navigation satellite system, providing a highly accurate, guaranteed global positioning service under civilian control. It is inter-operable with GPS and Glonass, the two other global satellite navigation systems. So once this second pair of satellites has been commissioned and tested, the quartet will form a completely operational mini-constellation that will be used to validate the Galileo system.