FAQ

What is EGNOS?

The European Geostationary Navigation Overlay Service (EGNOS) is Europe’s first venture into satellite navigation. It augments the positioning signals provided by the USA’s GPS global navigation satellite system and makes them suitable for safety critical applications such as flying aircraft or navigating ships through narrow channels.

Consisting of three geostationary satellites and a network of ground stations, EGNOS achieves its aim by transmitting a signal containing information on the reliability and accuracy of the positioning signals sent out by GPS. It allows users in Europe and beyond to determine their position to within 3 metres, compared with about 17 metres for GPS.

EGNOS has been developed through a tripartite agreement between the European Space Agency (ESA), the European Commission (EC) and Eurocontrol, the European Organisation for the Safety of Air Navigation. The agreement was signed in June 1998. It is Europe’s first activity in the field of Global Navigation Satellite Systems (GNSS) and is a precursor to Galileo, the full global satellite navigation system under development in Europe.

EGNOS entered its pre-operational phase in the summer of 2005.

The Open Service, for applications where human life is not at stake, such as personal navigation, goods tracking and precision farming, has been available since October 2009.

The Safety-of-Life Service, where human lives depend on the accuracy and integrity of the signals, became available for its primary purpose of aircraft navigation (beginning with vertical guidance for landing approaches) in March 2011.

EGNOS also provides a terrestrial commercial data service: EDAS (EGNOS Data Access Service).

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What is the difference between GPS and EGNOS?

GPS is a global navigation satellite system (GNSS) that allows anyone with a GPS receiver to navigate from anywhere in the world. EGNOS is a satellite based augmentation system (SBAS) that makes the GPS signal more precise and also provides an integrity signal about GPS performance by broadcasting error corrections to the users via three geostationary satellites over Europe. This means that without a GPS signal, EGNOS does not work.

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Who benefits from EGNOS?

Space navigation plays a key role in increasing safety in the transport of passengers and goods. EGNOS increases the accuracy and integrity of the GPS system, making it suitable for use by aircraft, ships, trains and other forms of transport. Other systems such as the USA’s WAAS are dedicated exclusively to air navigation.

Potential markets include:

  • Aviation: EGNOS is designed to assist navigation both en-route as well as during landing. The potential benefits will assist air traffic control to cope with increased traffic as well as improving performance while maintaining safety and reducing the infrastructure needed on the ground.
  • Maritime: EGNOS can be used for navigation operations, traffic management, sea port operations, casualty analysis  and offshore exploration and the exploitation of fisheries.
  • Land transport: EGNOS is one of the keys to managing land transport in Europe, whether it is by road, rail or inland waterways. It will increase both the capacity and the safety of land transport. Companies operating transport services need to know where their vehicles are at all times, as do other public services such as police, ambulance and taxi
  • Diverse potential uses: EGNOS can help farmers in aerial crop spraying or precision farming, fishermen to locate their catch and the police to detect fraud or run security patrols to monitor risky situations. EGNOS has been tested to help guide blind pedestrians and track racers in competitions such as the Tour de France.

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How does EGNOS work?

EGNOS provides the information needed to use the navigational signals from GPS for safety critical applications. It improves the accuracy of position measurements of GPS, informs users of the errors in the position measurements and warns of disruptions to the GPS 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 the GPS satellites, these three 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 ground segment consists of 34 Ranging and Integrity Monitoring Stations (RIMS), four Master Control Centres (MMC) and six Up Link Stations (ULS).

The RIMS measure the positions of each EGNOS satellite and compare accurate measurements of the positions of each GPS satellite with measurements obtained from the satellites’ signals. The RIMS then send this data to the master control centres, via a purpose built communications network.

The master control centres determine the accuracy of the GPS 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 widely spread across Europe. The up-link stations send the signal to the three EGNOS satellites, which then transmit it for reception by GPS users with an EGNOS-enabled receiver.

The GPS-like signals from the 3 transponders can be used as additional signals to fix a user’s position and also provide information about the accuracy of the position measurements delivered by GPS 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 GPS 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 at practically all times. At any one time, only one master control centre 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 failure.

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How does satellite navigation work?

The basic principle underpinning satellite positioning is the use of distance measurements at a precise moment in time between a receiver and several navigation satellites whose exact positions in space are known.

The satellites emit electromagnetic waves which are propagated through space at the speed of light. It is then possible to calculate the distance separating the satellite from the receiver by determining the time a wave takes to travel from satellite to receiver using a mathematical formula: d = c * t, where d is the distance, c the speed of light and t the time it takes for the wave to travel from satellite to receiver.

To estimate the time that signals take to travel between a given satellite and the receiver, the receiver compares a unique code linked to the satellite’s navigation signal with a copy of the same code generated by the receiver itself. Since the time interval between the codes corresponds to the transit time, this can then be used to calculate the distance, or ‘pseudorange’. The use of ‘pseudo’ in this term is because this distance does not correspond to the geometric distance between satellite and receiver due to the bias between the time reference used by the GPS system and that used by the receiver. With at least three distance measurements to three different satellites it is theoretically possible to determine the position of the receiver if and only if the receiver’s clock is perfectly synchronised with those on board the satellites.

Since a 1 millisecond difference between a satellite clock and receiver clock can produce a 300km positioning error, this clock bias must be compensated for. That is why distance measurements are made to a fourth satellite in order to calculate the bias.

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How much should I pay for EGNOS? What services are available?

The signal from the EGNOS Services are free to anyone with an EGNOS receiver. Users can rely on the EGNOS Safety-of-Life Service for flying final approach procedures when all certificates are valid (aircraft, receiver, pilot training). In order for your airport to publish a procedure, the local ANSP is recommended to enter into a contractual relationship with the EGNOS Service Provider to ensure that liability issues are covered in the case of an incident or accident.

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Is EGNOS interoperable with other SBAS?

Although all satellite-based augmentation systems (SBAS) are regional systems, it is important to ensure that they are compatible and that SBAS providers cooperate with each other and coordinate their actions.

In addition to EGNOS, three other regional systems of note exist:

  • WAAS (Wide Area Augmentation System): The United States Federal Aviation Administration leads the development of  WAAS, which covers the US and Canada.
  • MSAS (MTSAT Satellite-Based Augmentation System): The Japanese Civil Aviation Bureau is implementing the MSAS which will cover the Flight Instrument Rules region of Japan.
  • GAGAN: The Indian Space Research Organization (ISRO) along with the Airport Authority of India (AAI) has worked on a joint programme to implement GAGAN, a satellite-based augmentation system that uses GPS/GLONASS over Indian airspace.

Compatibility makes each system more effective and ensures that all the systems can be integrated into a seamless worldwide navigation network. Cooperation on SBAS is currently coordinated through the Interoperability Working Groups for EGNOS and MSAS and EGNOS and WAAS. Interoperability tests are regularly organised.

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What is EGNOS deployment?

The deployment of EGNOS covers a large area and involves various countries and partner organisations.

The elements that make up the EGNOS system include: 34 ranging and integrity monitoring stations (RIMS) which pick up GPS signals; four master control centres (MCCs) to process the data delivered by the RIMS; and six uplink stations to send the signal to three geostationary satellites, which then relay the data back to users with EGNOS-enabled receivers on the ground.

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Who designed and developed EGNOS?

The development of EGNOS arose from a tripartite agreement between the European Space Agency (ESA), the European Commission (EC) and Eurocontrol (the European Organisation for the Safety of Air Navigation). The agreement was signed in June 1998.

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Who operates EGNOS?

The European Commission has assigned the operational management and maintenance of EGNOS to ESSP (European Satellite Services Provider), a company founded in 2001 by seven key European air navigation service providers (ANSPs) based in Toulouse, France.  and founded by seven air navigation service providers: AENA (Spain), DFS (Germany), DSNA (France), ENAV (Italy), NATS (UK), NAV (Portugal) and Skyguide (Switzerland).

In 2009, ESSP was designated by the European Commission as the  EGNOS system operator and EGNOS Safety-of-Life (SoL) service provider, in particular for civil aviation.

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Who owns EGNOS?

European citizens own EGNOS.

The European Commission took over ownership of EGNOS’s infrastructure from the European Space Agency on behalf of the European Community on 1 April 2009.

EGNOS and Galileo, the future global navigation satellite system, are now part of Europe’s GNSS programmes managed by the European Commission.

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Will EGNOS be discontinued when Galileo enters into operation?

No, the EGNOS service will continue to be provided by the Galileo Service Provider along with the Galileo service.

Like GPS, Galileo is a global navigation satellite system.

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Can EGNOS provide an accurate time reference?

The second mission of EGNOS is the real time distribution of Coordinated Universal Time (UTC ) for the benefit of time/frequency users. EGNOS uses a system time known as ENT (EGNOS Network Time), linked to UTC, notably through the installation of an EGNOS ground station on the site of the Observatoire de Paris, which itself provides UTC reference time for France.

All the differential corrections broadcast by EGNOS are referenced according to ENT. Thus, the time obtained by the user when he calculates his position using EGNOS data is also referenced in ENT, not in GPS time.

In addition, EGNOS also broadcasts a specific message containing several parameters allowing the receiver to estimate a UTC. The user then has a precise, reliable time directly synchronised with UTC. The accuracy obtained relative to UTC is less than 50 nanoseconds.

Note: UTC (Coordinated Universal Time) represents a time scale which serves as international reference time. It is close to Universal Time, UT, directly linked to the Earth’s rotation and differs from International Atomic Time (TAI) by an integral number of seconds.

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Can the EGNOS service area be extended?

The design allows for extending services to other areas in the broadcast area of the geostationary satellites. Such extensions can be made by building additional monitoring stations and without interrupting the EGNOS signal. Africa, South America and Asia could benefit from such extensions.

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How does an SBAS (Satellite Based Augmentation System) system work?

SBAS systems improve the performances of GPS to make it useable for safety critical services. This is accomplished by providing, by means of separate (geostationary) signals, a set of corrections that improve the position and time calculation performed by the user’s satellite receiver.

EGNOS provides these corrections for GPS. Generally speaking, an SBAS system is based on the principle of the spatial and temporal correlation of measurement errors that arise when making distance measurement from a space born source.

The difference between the theoretical and the real measurement performed in a known position can be found, with similar values, in other real measurements performed near a known position. In other words, this principle assumes that the distance measurements made in a small geographical area may be affected by the same errors. Once you know the measurement error in one place, it can be used as a correction for the distance measurements made in nearby places.

In a scenario where several reference points are available, a wide-area correlation law, which models the difference in distance measurements, can be derived. These data are collected by a network of reference stations, processed and then transmitted to the users by means of geostationary satellites, using a signal with the same frequency as GPS (L1=1575.42 MHz) and a different data format.

The information contained in the navigation message modulated on L1, the additional ranging capability offered by the geostationary satellites and the complexity of ground processing and checks are able to improve the accuracy, the integrity and the reliability of GPS.

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Is EGNOS signal encrypted or affected by interference?

The EGNOS signal is neither encrypted nor should it normally be affected by interference. There is a constant monitoring on EGNOS signal to prevent interference that may significantly impact the system’s operations.

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Is there a list of SBAS receivers?

Yes. It is available on this site.

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What is the difference between the ESTB and EGNOS architecture?

ESTB (EGNOS System Test Bed) is a reduced version of EGNOS using dedicated monitoring stations and processing devices. Since February 2000, ESTB has been offering an experimental signal to help navigation equipment manufacturers and application developers test their products. The experimental signal also allows users to become familiar with EGNOS. The ESTB also allows tests to be conducted so EGNOS can be expanded its services outside of Europe.

EGNOS and ESTB are two independent systems making use of their own ground segment infrastructure and different geostationary satellites. EGNOS and ESTB broadcast two different signals.

The ESTB signal cannot and does not provide the availability and integrity that EGNOS provides. Therefore it can not be used in any safety critical applications.

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Where can I find general information on SBAS? What is the reference documentation on SBAS developments?

There are several SBAS developments ongoing: EGNOS in Europe, WAAS in USA., MSAS in Japan, and others (Canada and India).

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Will a receiver be able to process EGNOS, MSAS and WAAS signals?

Yes, all receivers compliant with RTCA/DO-229C MOPS will work with any SBAS. All SBAS service providers meet on a regular basis to ensure signal compatibility and system interoperability.

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Updated: Jul 14, 2016