How does EGNOS work?
How does EGNOS work?
EGNOS provides the information needed to use GNSS navigational signals for safety critical applications. It improves the accuracy of GNSS position measurements, informs users of the errors in the position measurements and warns of disruptions to the satellite signals.
The EGNOS signal is transmitted by three geostationary satellites: two Inmarsat-3 satellites, one over the eastern part of the Atlantic the other over the Indian Ocean, and the ESA Artemis satellite above Africa. Unlike GPS satellites, these satellites do not have signal generators on board. A transponder transmits signals up-linked to the satellites from the ground, where all the signal processing takes place.
The EGNOS ground segment consists of 34 Ranging and Integrity Monitoring Stations (RIMS), four Master Control Centres (MCC) and six Up Link Stations (ULS).
The RIMS measure the positions of each EGNOS satellite and compare accurate measurements of the positions of each GNSS satellite with measurements obtained from the satellites’ signals. The RIMS then send this data to the MCCs via a purpose-built communications network.
The MCCs determine the accuracy of the GNSS signals received at each station and determine position inaccuracies due to disturbances in the ionosphere. All the deviation data is then incorporated into a signal and sent via the secure communications link to the up-link stations, which are spread across Europe. The up-link stations send the signal to the three EGNOS satellites, which then transmit it for reception by users with an EGNOS-enabled GNSS receiver.
The GNSS-like signals from the three transponders can be used as additional signals to fix a user’s position. They also provide information about the accuracy of the position measurements delivered by GNSS so that users can assess whether the position is accurate enough to rely on.
The information, or integrity data, is modulated onto the ranging signal. It includes accurate information on the position of each GNSS satellite, the accuracy of the atomic clocks on board the satellites and information about disturbances within the ionosphere that might affect the accuracy of the positioning measurements.
Considerable redundancy is built into EGNOS so that the service can be guaranteed practically all the time. At any one time, only one MCC will be ‘the master’, with another on stand-by to take over instantaneously should the first one fail. There is redundancy in the up-link stations, too. Only three are needed to operate EGNOS, one for each satellite. The other three are in reserve in case of a failure.