How does EGNOS work?

EGNOS provides the information needed to use navigation signals from GPS satellites for safety critical applications. It improves the accuracy of position measurements from a few metres to around one metre, informs users of the errors in the position measurements and warns of disruption to a satellite signal within six seconds.

Three geostationary satellites and a complex network of about forty ground stations carry out this task. The three satellites send out a ranging signal similar to those transmitted by the GPS satellites. However, these signals are more than another opportunity for users to fix a position. They also provide integrity information about the accuracy of position measurements delivered by GPS so that a pilot, for example, can assess whether the position is accurate enough to rely on.

This information, or integrity data, is modulated onto the ranging signal. It includes accurate information on the position of each GPS satellite, the accuracy of the atomic clocks on board the satellites and information on disturbances within the ionosphere that might affect the accuracy of positioning measurements. The EGNOS receiver decodes the signal to give a more accurate position than is possible with GPS alone, and an accurate estimate of errors.

EGNOS

Space Segment

The EGNOS Space Segment consists of three geostationary satellites that relay correction and integrity data generated by the Ground Segment to users across Europe and the surrounding areas.

The first generation of EGNOS relied on two Inmarsat-3 satellites, one over the eastern part of the Atlantic, the other over the Indian Ocean, and ESA’s Artemis satellite above Africa.

The second and current generation of EGNOS relies on three satellites: two satellites for Safety-of-Life provision (SES-5 and Eutelsat 5WB) and one satellite for testing (ASTRA 5B).

The next generation of the system, called EGNOS V3, will enhance the accuracy, integrity and availability of GNSS signals, including Galileo, across multiple transport sectors, and will represent another leap forward in safety and precision. EGNOS V3 will introduce Eutelsat HB 13G, which will replace SES-5.

EGNOS V3 will increase the robustness of EGNOS services, with the adoption of dual frequency (which uses two radio signals in parallel to offset ionospheric interference) and multi-constellation systems (which draw on satellites from various constellations to improve signal availability).

How EGNOS works

Ground segment

The EGNOS Ground Segment is responsible for monitoring GNSS signals from various constellations, computing corrections and ensuring system integrity.

A network of Ranging and Integrity Monitoring Stations (RIMS) across Europe and neighbouring regions gathers the signals and navigation messages of the GNSS satellites and of each EGNOS satellite. There are 38 of these stations for the current version of EGNOS, and EGNOS V3 will have 45. The RIMS send this raw data to the Mission Control Centres (MCC), via a purpose-built communications network.

Because the locations of the RIMS are known precisely, the two MCC can calculate the correction and integrity messages. They do this by determining the accuracy of GPS signals received at each station (by comparing the known location of the RIMS to the location given by GPS) and the 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 six uplink stations, called Navigation Land Earth Stations (NLES), which are spread across Europe. The uplink stations send the signal to the EGNOS satellites, which then transmit it for reception by GPS users with an EGNOS-enabled receiver.

Considerable redundancy is built into EGNOS so that the service can be guaranteed at practically all times. At any one time, one MCC is ‘the master’, with another on stand-by to take over instantaneously should the first one fail. There is redundancy in the uplink stations, too. Only three are needed to operate EGNOS, one for each satellite. The other three are in hot backup in case of failure.

The EGNOS Wide Area Network (EWAN) provides the communication network that ensures the secure, real-time data exchange between all components of the Ground Segment.

Support segment

The Support Segment provides facilities that maintain and assess the performance of EGNOS. The Performance Assessment and Checkout Facility (PACF) analyses the system’s performance, troubleshoots problems, and assists with maintenance and upgrades. The Application Specific Qualification Facility (ASQF) provides tools and support for the qualification, validation and certification of EGNOS applications, particularly in the aviation sector.

User segment

The User Segment comprises EGNOS-compatible GNSS receivers and is used across various sectors, including aviation, maritime, agriculture and surveying.

EGNOS Services

The EGNOS Open Service is freely available to any user with a, EGNOS-enabled receiver. It is used to improve precision farming, support mapping and surveying by improving positioning in real-time, aid fleet management systems by enhancing driver safety and route optimisation and enhance navigation on mobile devices for use in maps, ride-hailing and travel apps.

The EGNOS Safety of Life service provides higher accuracy, integrity and continuity in GNSS signals, for use when human lives are at stake. It is used for landings at airports across Europe, decreasing the environmental impact of flying due to lower fuel use and reducing the noise levels around airports. It is also used in the maritime industry to support safety of life operations.

The EGNOS Data Access Service, offered on a controlled access basis, provides ground-based access to EGNOS data through the internet to customers requiring enhanced performance for professional use.

Who benefits from EGNOS?