The term ‘global navigation satellite system’ (GNSS) refers to a constellation of satellites providing signals from space transmitting positioning and timing data. By definition, a GNSS provides global coverage.
GNSS receivers determine location by using the timing and positioning data encoded in the signals from space. The USA’s NAVSTAR Global Positioning System (GPS) and Russia’s Global'naya Navigatsionnaya Sputnikovaya Sistema (GLONASS) are examples of GNSS.
Europe is in the process of launching its own independent GNSS, Galileo. Since 2011, four Galileo satellites have been launched and used as part of the In-Orbit Validation phase, allowing the first autonomous position fix to be calculated based on Galileo-only signals in March 2013. The Commission aims to have the full constellation of 30 Galileo satellites (which includes six in-orbit active spares) in operation before the end of this decade." Galileo will be interoperable with GPS and GLONASS. This interoperability will allow manufacturers to develop terminals that work with Galileo, GPS and GLONASS.
Satellite-based augmentation systems (SBAS) such as EGNOS are regional contributions to improve the performance of GNSS systems.
The performance of a satellite navigation system is assessed according to four criteria:
- Accuracy refers to the difference between the measured and the real position, speed or time of the receiver.
- Integrity refers to a system’s capacity to provide confidence thresholds as well as alarms in the event that anomalies occur in the positioning data.
- Continuity refers to a navigation system’s ability to function without interruption.
- Availability refers to the percentage of time during which the signal fulfils the accuracy, integrity and continuity criteria.
EGNOS improves the accuracy and the reliability of GPS information by correcting signal measurement errors and by providing information about the integrity of its signals.