EGNOS enables users with an EGNOS-enabled GPS receiver:
- to improve positioning accuracy;
- to have integrity data for validating the signals transmitted by GPS satellites:
- to benefit from accurate and reliable synchronisation with UTC;
- to improve availability.
An EGNOS-enabled receiver can provide location accuracy to within three metres, compared to the 17-metre accuracy provided by a standard GPS receiver.
Moreover, EGNOS provides extremely good stability over time, as shown in the following graph (blue line). The accuracy of GPS can be very variable (pink line), even though its overall performance is satisfactory. Using EGNOS makes it possible to overcome the variations in GPS’ occasional positioning error.
EGNOS monitors the GPS constellation and is able to detect GPS satellite faults and assign a confidence level to the data transmitted to a user.
Despite its great accuracy, the reliability of data supplied by the GPS system is not guaranteed. For example, a malfunction of the atomic clock on-board a GPS satellite may lead to very significant positioning errors. Some GPS applications will therefore have to be used with caution.
The following graph shows EGNOS’ capacity to detect GPS faults, such as that which occurred in June 2006 in the active atomic clock on GPS satellite SVN30. The fault quickly led to errors in navigation of more than 1.6 km, observed at the Grasse, France EGNOS station. EGNOS detected this anomaly almost instantaneously and informed all its users via a navigation message.
EGNOS’s availability is usually calculated in relation to the percentage of time when the confidence levels are below their threshold values. These values are set for each type of operation by specified alarm limits.
For civil aviation procedures called ‘Approach with Vertical Guidance 1’ (APV-1), EGNOS is currently available over its service area for 99% of the time.
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