EGNOS is what we call a satellite-based augmentation system (SBAS) covering a large area, this in contrast to a ground-based augmentation system (GBAS) that covers relatively small areas.

The EGNOS satellite-based augmentation system improves the accuracy of GNSS signals over Europe and Northern Africa. Receivers capable of receiving information from both the navigation satellites and correction information from the augmentation system, often called differential, allows the user to determine their position within two metres, compared with about twenty metres for GNSS alone. The system consists of transponders aboard three geostationary satellites (Inmarsat) over the eastern Atlantic Ocean and Europe, linked to a network of about forty ground stations and four control centres. The ground stations (see inset map for their locations) receive signals from the GNSS satellites and the control centres calculate the accuracy and reliability of these signals. The results are relayed to users via the geostationary satellite transponders.

EGNOS can be looked upon as the European counterpart of the American wide-area augmentation system (WAAS) and the Japanese multifunctional satellite augmentation system (MSAS). Russia is also developing a similar constellation which they are calling their system for differential correction and monitoring (SDCM), to become operational around 2011. The EGNOS signal can be received all over Europe, Northern Africa and the eastern part of the Atlantic Ocean and is free of charge.

This page shows an artist's impression of four Inmarsat-3 communication and navigation satellites in orbit, with coloured circles showing the area on Earth covered by transmissions beamed from each. The Inmarsat 3 satellites are located in equatorial geostationary orbit and are used to relay signals from mobile telephones. They also form part of the EGNOS.

I would like to apologise for the number of abbreviations in this feature, but modern technology seems to breed them!