EGNOS Architecture

EGNOS
Title	EGNOS Architecture
Author(s)	GMV.
Level	Basic
Year of Publication	2011

The goal of EGNOS (European Geostationary Navigation Overlay Service) is to augment the [GPS Introduction|GPS]] in order to improve the navigation performances in terms of accuracy and integrity (with the required levels of availability and continuity of service) over the European Civil Aviation Conference (ECAC) Region and to be expandable over neighbouring regions.

These augmentations are obtained by providing a GPS-like Geostationary Ranging service (R-GEO), by broadcasting precise Wide Area Differential corrections to correct for GPS, GLONASS and GEO orbit and clock errors and also to correct the ionospheric delays affecting the signal paths to the users. The complex message broadcast by EGNOS GEOs also include Integrity data which allow to bound the remaining errors with a high level of confidence. EGNOS has deployed its Advanced Operational Capability (AOC) using two existing INMARSAT-III navigation transponders (AOR-E and IOR-W) plus a navigation transponder on board of Artemis satellite.

The EGNOS architecture is very complex and highly redundant. It is composed by 47 elements deployed onto 41 sites in 22 Countries. EGNOS has been designed to meet the international SBAS standards and performs the following tasks:

• Collect GPS/GLONASS/GEO signals and data.
• Estimate the integrity data and WAD corrections for the service area.
• Transmit to the users, via the GEO satellites, a GPS-like signal, augmented with integrity and correction messages.
• Verify the correctness of these integrity and correction messages.

Architecture Overview

The EGNOS system is directly decomposed in its four segments, a Ground Segment, a Space Segment, a User segment and a Support Segment:

EGNOS functional architecture

• The Ground Segment is the real-time part of EGNOS. It computes precise differential corrections as well as the GEO ranging and makes all these information available to users through a broadcast by the Space Segment.
• The Space segment, using three GEO satellites, provides redundant data transmission channel to broadcast toward EGNOS users messages containing differential corrections with the associated integrity information. The GEO satellites additionally provide GPS-like ranging signal that is permanently steered and synchronised by the ground segment.
• The User Segment is made of EGNOS receivers that enable EGNOS users to accurately compute their position.
• The Support segment contains off-line facilities listed hereafter:
  • PACF: Performance Assessment and Check-out Facility, provides support to EGNOS management in such area as performances analysis, troubleshooting, operational procedures as well as upgrade specification and validation, support to maintenance.
  • ASQF: Application specific qualification facility provides civil aviation and aeronautical certification authorities with the tools to qualify validate and certify the different EGNOS applications.
  • DVP: Development & Verification platform is used to validate and verify EGNOS requirements during the design phase. It contains simulation facilities, a real-time testbed and an assembly, integration and verification platform. The DVP is incorporated in PACF.

EGNOS Space Segment

The EGNOS Space Segment comprises 3 GEO satellites broadcasting corrections and integrity information for GPS satellites in the L1 frequency band (1575,42 MHz). At the date of publication the 3 GEOs used by EGNOS are:

GEO Name	PRN Number	ID(NMEA)	Orbital Slot
ARTEMIS	124	37	21.5E
INMARSAT AOR-E	120	33	15.5W
INMARSAT IOR-W	126	39	25E

This space segment confi guration provides a high level of redundancy over the whole service area in case of a geostationary satellite link failure. The EGNOS operations are handled in such a way that, at any point in time, typically two of the three GEOs broadcast an operational signal. Since it is only necessary to track a single GEO satellite link to benefi t from the EGNOS SoL, this secures a switching capability in case of interruption and ensures a high level of continuity of service.

It is intended that the EGNOS space segment will be replenished over time in order to maintain a similar level of redundancy. The exact orbital location of future satellites may vary, though this will not impact the service offered to users. Similarly, different PRN code numbers may be assigned to future GEOs. However, all SBAS user receivers are designed to automatically detect and use any code in a pre-allocated set reserved for SBAS. Such evolutions will therefore be transparent for end users and will not necessitate any human intervention or change of receiving equipment.