EGNOS General Introduction

EGNOS
Title	EGNOS General Introduction
Edited by	GMV
Level	Basic
Year of Publication	2011

EGNOS (European Geostationary Navigation Overlay Service) is the European satellite-based augmentation service (SBAS) that complements the existing satellite navigation services provided by the US Global Positioning System (GPS). EGNOS provides the first European GNSS services to users. EGNOS constitutes together with Galileo the two major initiatives in Europe in terms of satellite navigation. Future evolutions such as EGNOS V3 are expected to augment both GPS and Galileo constellation data.

EGNOS Services

EGNOS Logo

EGNOS supports the following services:

• The Open Service (OS), freely available to the public over Europe. This service was officially started on 1 October 2009.
• The Safety of Life Service (SoL), that provides the most stringent level of signal-in-space performance to all communities of Safety of Life users over Europe. This service was officially started on 2 March 2011.
• The EGNOS Data Access System (EDAS) represents the provision of additional data for professional users not provided by the EGNOS signal broadcast by geostationary satellites but by other distribution channels.

EGNOS is meant to be part of a multi-modal inter-regional SBAS service, supporting a wide spectrum of applications in many different users’ communities. In particular the EGNOS service can support different applications in the transport domain (e.g. aviation, maritime and rail). Nevertheless, the main objective of the EGNOS SoL service is to support civil aviation operations down to Localizer Performance with Vertical guidance (LPV). This service is compliant with already well identified safety-critical aviation applications, as APV (Approach with vertical Guidance). The operational use of the EGNOS SoL service in other domains may require specific authorization by the relevant authorities in the applications sectors concerned.

EGNOS Architecture

EGNOS Functional architecture. Source: EGNOS User Support in ESSP website

The EGNOS Core architecture is composed of the following segments

• Ground segment , composed of 40 reference stations (RIMS - Ranging & Integrity Monitoring Stations) spread inside and outside Europe which monitor the GPS satellites; 2 control centres (MCC - Mission Control Centres) and 6 uplink stations (NLES - Navigation Land Earth Stations);
• Space segment, composed of 3 GEO satellites broadcasting EGNOS Signal In Space (SIS) over the service area;
• Support segment, composed of the Performance Assessment Check-out Facility (PACF) and the Application Specific Qualification Facility (ASQF);

In addition, the following elements are present in the EGNOS architecture, but are outside of the EGNOS Core:

• External Data Access Server, for the distribution to customers of EGNOS products enabling the EGNOS Data Access Service (EDAS).
• User segment, composed of the EGNOS receiver and user terminals.

EGNOS processing channel starts with each RIMS collecting raw data from the GNSS satellites and EGNOS GEO satellites in view and transmitting them to the MCCs. Then the Central Processing Facility (CPF) inside each MCC facility computes signal corrections in real time including ionospheric delays, GPS and GEO ephemeris and clock errors. Finally, the EGNOS signal and data are sent to the users via a GEO satellite link, with the NLES acting as uplink stations, and through the External Data Access Server.

EGNOS Performances

According to the Service Definition Documents for the different EGNOS services, they are presented in the following table.

Service	Service Coverage	Accuracy (95%)		Integrity				Continuity		Availability
		Lateral	Vertical	Integrity	TTA
Open Service	EU25 States (plus Norway and Switzerland) land masses	3m	4m	-		-		-	0.99
En-route and NPA	FIRs of ECAC 96	220m	-	1 – 1x10⁻⁷/h			Less than 6 seconds	<1 – 1x10⁻³ per hour in most of ECAC	<1 – 2.5x10⁻³ per hour in other areas of ECAC	0.999 in all ECAC
APV-I & LPV200	Land Masses of ECAC 96	3m	4m	1 – 2x10⁻⁷/approach			Less than 6 seconds	<1 – 1x10⁻⁴ per 15 seconds in the core of ECAC 1 –5x10⁻⁴ per 15 seconds in most of ECAC landmasses	0.99 in most of ECAC landmasses

However, actual EGNOS Performances may differ from the required ones. EGNOS is in a continuous process of enhancement with the aim of improving the performances and robustness of the system. Actual EGNOS performances are being monitored and analysed continuously by ESSP and ESA amongst other entities.

EGNOS Signal Structure

The EGNOS Signal Structure is compliant with the ICAO SARPs for SBAS as stated in the EGNOS Open Service Service Definition Document and the EGNOS Safety of Life Service Definition Document. The EGNOS SIS interface characteristics comprise carrier and modulation radio frequency, message structure, protocol and content of the EGNOS message (detailed in the EGNOS Message Format definition).

EGNOS uses the same frequency (L1 1575.42 MHz)) and ranging codes as GPS, but has a different data message format. Sixteen different message types have so far been defined to broadcast integrity data and Wide Area Differential (WAD) corrections. The message schedule follows a 6-second duty cycle in line to the standards (ICAO SARPs ).

Integrity is provided at two levels:

• use/don’t use flags for satellites and for ionospheric grid points;
• two parameters – UDRE and GIVE – that are statistical estimates of the satellite and ionospheric errors remaining after applying the WAD corrections. These are used to compute a certified error bound for the position solution in an integrity assessment.

Fast and slow WAD corrections model the temporal de-correlation of the different error sources. The fast corrections model rapidly changing error sources including satellite clock errors. The slow corrections model more slowly changing error sources including long-term satellite clock drift and ephemeris errors. Ionospheric delays are provided at pre-defined grid points.

At user level, the receiver estimates corrections for satellite clock and ephemeris errors using the fast and slow satellite data messages. It has to account for both range-rate effects between successive fast corrections and performance degradation if a message is missed. The UDRE term characterizes statistically the residual range errors after having applied the fast and slow clock and ephemeris corrections.

The receiver predicts also ionospheric delays for each range in three steps:

• it estimates where the view line from satellite to receiver pierces the ionospheric layer;
• the vertical delay at the pierce point is then interpolated from the surrounding grid points which have been estimated by the system;
• and finally the estimated delay is applied to the range measurement. The GIVE term is applied to the range vector to characterize statistically the residual ionospheric errors.

Tropospheric errors may be mitigated using a simple model related to the receiver’s position and the day of year.

For more information, please refer to the EGNOS Signal Structure article.

SBAS Interoperability

SBAS Interoperability refers to the ability of two or more SBAS systems to be used together to provide better capabilities at user level than those achieved by relying solely on one of the systems.

The SBAS interoperability has always been a pre-requisite for delivering a global seamless safety-of-life service. This was recognized early on by SBAS developers and air traffic services providers, and they have worked closely together to co-ordinate their activities at the International Civil Aviation Organization (ICAO) and in the Interoperability Working Group (IWG). One of their key activities has been to assist ICAO and the Radio Technical Commission for Aeronautics (RTCA) in the development of standards: Standards and Recommended Practices (SARPS) for system developers and Minimum Operational Performance Standards (MOPS) for receiver manufacturers.

The combination of SBAS interoperability and expansion concepts should allow the provision of a truly global and seamless navigation service.

EGNOS Future and Evolutions

EGNOS Applications

The European Commission (EC) intends to ensure the future of EGNOS services for a minimum period of 20 years, as from its first declaration date, with 6 years advance notice in case of significant changes in the Services provided. In this context, the budget should be secured to operate the system and manage the system obsolescence. Moreover, major EGNOS system evolutions towards a multi frequency and multi constellation configuration are currently being assessed with the objective to have them operational by 2025.:

• 2011-2030: En-route / NPA / APV1 / LPV200 service based on augmentation of GPS L1 only. The Safety Of Life (SoL) service is being offered by EGNOS from early 2011 on a regional basis and this will be guaranteed up to 2030 in compliance with ICAO SBAS SARPS.
• 2020+: It is planned that EGNOS will experiment a major evolution by 2025, EGNOS V3, including the fulfilment of the SBAS L1/L5 standard, expansion to dual-frequency, and evolution toward a multi-constellation concept.

To support this mission roadmap, EGNOS needs to evolve. This evolution is divided into minor updates of the current EGNOS version, EGNOS V2, and a major evolution leading to the provision of new services, EGNOS V3.

The minor evolutions in the current EGNOS version are performed in a regular basis at an approximate pace of an update per year, and aim at solving infrastructure obsolescence issues, at supporting the LPV200 service beyond APV1 and at improving the operation of the system.

The major evolution requires a full dedicated engineering cycle starting from the definition of the mission of the system highly coupled with a technical feasibility analysis in coordination with the evolution of the SBAS standards.