ABOUT EGNOS

Contents

The need for EGNOS Introducing EGNOS EGNOS System Description EGNOS Industrial Consortium EGNOS Performances Explained EGNOS Integrity Plots

The need for EGNOS

The current capabilities of GPS and GLONASS constellations, although adequate for some user communities, present some shortfalls. The lack of civil international control is considered an important issue by European institutions. In addition, some user communities have a need for enhanced performance. In particular, the Civil-Aviation requirements for the precision approach phases of flight cannot be met by GPS or GLONASS only. Other modes of transport can also benefit from “augmentation” techniques, resulting in improved GPS/GLONASS performance. For example, in the maritime domain, port approaches cannot be performed solely with GPS/GLONASS receivers due to the insufficient performances of these systems in high traffic density environments.

EGNOS (European Geostationary Navigation Overlay Service) has been designed to broadcast a GPS-like ranging signal in Europe with embedded corrections, providing improved performances over GPS and GLONASS. With EGNOS, all navigation receivers will benefit from enhanced accuracy, availability and continuity over GPS and GLONASS. An accuracy of around 1 metre is foreseen. EGNOS will also provide integrity information about the GPS/GLONASS signals, giving an alarm within 6 seconds in case of faulty positioning information).


Click on the image to view an animated description of EGNOS!
(animation courtesy of AENA)

EGNOS is the European Satellite-Based Augmentation System (SBAS) that complements GPS and GLONASS. The other inter regional SBAS are the United States WAAS and the Japanese MSAS (MTSAT Satellite - based Augmentation System). More SBAS are planned, such as GAGAN in India. As these systems are designed to be interoperable, the same aircraft should be able in the future to fly across the world using SBAS as means of navigation. Furthermore, SBAS enabled receivers purchased in United States or Japan are interoperable and can be used without any hardware or software modifications in Europe.

The EGNOS coverage area will be Western Europe, but could be readily extended to include other regions within the broadcast area of the geostationary satellites, such as Africa, Eastern European countries, and Russia. EGNOS is the first element of the European satellite-navigation strategy and a major stepping-stone towards Galileo, Europe’s own global satellite navigation system for the future.

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EGNOS System Description

EGNOS is a European-wide, distributed system incorporating a ground network of redundant infrastructure. The EGNOS system is composed of four segments:

The space segment, corresponding to the 3 geostationary satellites which broadcast EGNOS data over Europe
The ground segment: the core of the system
The user segment: the provision of EGNOS Signal In Space and data to users.
The support segment: support to infrastructure operators and EGNOS users

Legend
Ranging and Integrity Monitoring Stations (RIMS), reference stations
Master Control Centres (MCC)
Navigation Land Earth Stations (NLES), uplink stations
Performance Assessment and Check out Facility (PACF), system support facility
Application Specific Qualification Facility (ASQF), user support facility

The entire system is composed of 45 ground elements deployed over 23 countries. The 33 reference stations (RIMS) collect the GPS signals, deliver them to the 4 control centres (MCC) that process and correct the position errors. Then the 6 uplink stations (NLES) send the corrected information to the 3 geostationary satellites, which in their turn relay the corrected signal back to the user segment.

The EGNOS space segment is represented in the following table:

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EGNOS Industrial Consortium

Alcatel Space is in charge of coordinating the deployment activities of EGNOS on behalf of ESA. The industrial consortium in charge of deployment activities includes the following companies: Alenia Aerospazio, Alenia Marconi, Airsys, ARCS, Astrium, British Telecom, Cap Gemini Ernst & Young, DNV, Edisoft, EuroTelematik, GMV, Indra, Kongsberg Seatex, Laben, Lightning, Logica, NDSatCom, NovAtel, NLR, Sener, Speng, SSI, Telespazio, Thales, Vitrociset.

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EGNOS Performances Explained

EGNOS will provide enhanced performances over GPS according to four parameters: accuracy, availability, continuity and integrity. Although the concept of position accuracy is easy to understand, the other three notions are frequently misunderstood. They are defined as follows by the system requirements:

Integrity - Integrity relates to the trust which can be placed in the correctness of the information supplied by the Navigation System, it includes the ability of the system to provide timely warnings to the user when the system or data provided by the system should not be used for navigation.

Continuity - Continuity of Navigation service is defined as the probability that the accuracy and integrity requirements will be supported by the Navigation System throughout a flight operation or flight hour given that they are supported at the beginning of the flight phase and that the flight operation is initiated and predicated to be supported all along the flight phase. Satellite outages predicted at least 48 hours in advance of the outage do not contribute to a loss of continuity.

Availability - Availability of the Navigation Service is the probability that the Positioning service and the Integrity monitoring service are available and provide the required accuracy, integrity and continuity performances.
Availability is computed, at any point within the service volume, as the percentage of the time during which the service is available over the lifetime of the system, taking into account all the outages whatever their origins.
The service will be declared available when accuracy, integrity and continuity requirements are estimated to be met.

EGNOS Integrity Plots

Thanks to the professional receiver installed at ESSP premises, it is possible to show a concrete example of EGNOS benefits.

The model for the charts below was developped by the WADGPS Laboratory of the Stanford University in California, USA, for the WAAS system. They have been incorporated by EUROCONTROL in their Pegasus software, used to validate EGNOS performances.

The above charts are used to display SBAS integrity performance. In this case, they are showing the compliance to Integrity performance requirements in the vertical axis. The eastings display the Vertical Position Error and the northings the Vertical Protection Level. The Integrity risk is defined as the probability that the error exceeds the alert limit. The chart is divided in several areas, in which the measurements (epochs) are displayed according to their integrity level. The quality of the signal is demonstrated when all epochs are shown in the "APV-I" zone, as it is the case on the chart above, on the right.

Below are charts obtained from measurements made by the ESSP with a Septentrio PolaRx2 receiver, in Brussels.
Date of measurements: January 30th 2004.