EGNOS performances are measured in terms of four parameters: Accuracy, Integrity, Continuity and Availability.

 

The definitions of these terms are:

 

Accuracy: GNSS position error is the difference between the estimated position and the actual position. For an estimated position at a specific location, the probability should be at least 95 per cent that the position error is within the accuracy requirement. (ICAO SARPS)

Integrity: is a measure of the trust which can be placed in the correctness of the information supplied by the total system. Integrity includes the ability of a system to provide timely and valid warnings to the user (alerts) when the system must not be used for the intended operation (or phase of flight). (ICAO SARPS)

Continuity: is the capability of the system to perform its function without unscheduled interruptions during the intended operation. It relates to the capability of the navigation system to provide a navigation output with the specified accuracy and integrity during the approach, assuming that it was available at the start of the operation. (ICAO SARPS) Availability: is characterised by the portion of time the system is to be used for navigation during which reliable navigation information is presented to the crew, autopilot, or other system managing the flight of the aircraft. (ICAO SARPS)

When referring to EGNOS performances, it is usual to consider those attainable by an error free receiver using all the GPS satellites in view at location and with no multipath effects. This is necessary to provide a valid reference as the actual performances will depend in any case of the type of receiver, number of satellites used (usually less than all in view at location) and the local multipath effects.

Nominal performances: Vertical accuracy is improved from 15 meters to 4 meters. Horizontal accuracy is improved from 10 meters to 3 meters. Observed performances: Vertical accuracy is improved from 4-5 meters to around 2 meters. Horizontal accuracy is improved from 2-3 meters to around 1 meter. On top, EGNOS includes an integrity function i.e. a warning in case of an issue with the system, emitted within 6 seconds of the malfunction and repeated.

For the time being, EGNOS offers full coverage (APV-1 99%) over Austria, Belgium, Denmark, France, Germany, Luxemburg, Netherlands, Slovenia, Switzerland, Croatia, Bosnia and partial coverage over Czech Republic, Finland, Hungary, Ireland, Italy, Poland, Portugal, Slovakia, Spain, Sweden, United Kingdom, Norway, Montenegro and Serbia

For most of the applications of EGNOS, the receiver will select the EGNOS satellite from those available at that moment (typically the one providing better performance). In these cases, the change of GEO will be transparent to the end user. In the particular cases where the GEO used to compute the EGNOS corrections is manually selected, it should be set to an available one.

The EGNOS Signal in Space is being broadcast from two satellites for redundancy purposes only. This redundancy ensures that although during the normal operation of the system a low number of discontinuity events can be expected, the simultaneous outage of both satellites is extremely remote. This redundancy mitigates as well the masking effects of one of the GEOS by obstacles or other sources.