EGNOS Architecture

EGNOS
Title	EGNOS Architecture
Edited by	GMV
Level	Basic
Year of Publication	2011

The goal of EGNOS (European Geostationary Navigation Overlay Service) is to augment GNSS 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. Current EGNOS only augments GPS satellites on L1 signal.

These augmentations are obtained by providing orbit and clock corrections for GNSS satellites and also correcting 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.

The EGNOS architecture is very complex and highly redundant. It is currently composed by more than 40 elements deployed in more than 20 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 (Wide Area Differential) 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.

For the next generation of the EGNOS augmentation system (EGNOS V3), the GSA requested the complete overhaul of the EGNOS ground segment, which was becoming obsolete. This modernization programme will see the deployment of EGNOS V3 in ground stations at more than 50 sites in Europe, and surrounding countries.

The GSA also requested the development of new EGNOS capabilities to support the augmentation of a second GPS signal (L5) and of the Galileo signals E1-E5. Another requirement is that the system is made more robust, to deal with the growing number of users numbers and to reflect their increasing dependence on EGNOS and its ground applications - in some countries (e.g. France) instrument landing systems (ILS) are being decommissioned on several airports because the civil aviation authorities have decided instead to rely on EGNOS.

The current EGNOS augments the accuracy of GPS signals across Europe and informs users of their current reliability level within six seconds. EGNOS belongs to a family of systems called Satellite Based Augmentation Systems (SBAS).

EGNOS V3 is planned to provide the aviation community with advanced Safety of Life (SoL) services as well as new services to maritime and land users. EGNOS V3 will provide augmented operational SoL services over Europe that improve the accuracy and availability of user positioning services from existing Global Navigation Satellite Systems (Galileo and GPS) and provides crucial integrity messages to EGNOS users with alerts within a few seconds in case of system degradation, consolidating EGNOS’ position as one of the leading edge GNSS Systems in the future.

EGNOS V3 will thus offer improved SoL services performances (where people’s lives are potentially at stake) over Europe to Civil Aviation community and new applications for maritime or land users, and will improve robustness against increasing security risks, in particular cyber-security risks.

EGNOS V3 will ensure a full continuity of service for the next decade and will be within the first operational SBAS systems implementing the dual frequency and multi constellation world standard, with both GPS and Galileo, replacing EGNOS V2 which has been in operation since 2011.

Architecture Overview

EGNOS functional architecture

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

• The Ground Segment computes precise differential corrections and integrity bounds and makes all these information available to users through a broadcast by the Space Segment.
• The Space segment, using GEO satellites, provides redundant data transmission channel to broadcast toward EGNOS user messages containing differential corrections with the associated integrity information.
• The User Segment is made of EGNOS receivers which enable EGNOS users to accurately compute their position.
• The Support Segment contains off-line facilities supporting activities such as performance analysis, troubleshooting, maintenance and qualification.

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). This configuration 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, the others broadcasting test signal. Since it is only necessary to track a single GEO satellite link to benefit from the EGNOS SoL, this secures a switching capability in case of interruption and ensures a high level of continuity of service. The configuration of the GEOs in operation does not change frequently but possible updates are nevertheless reported to users by the EGNOS Service Provider.

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.

The GSA’s request for EGNOS V3 development is part of its overall EGNOS modernization program that also includes renewal of the space segment. As part of this program, the GSA contracted Eutelsat for the preparation and service provision phases of the EGNOS GEO-3 payload, to be hosted on the EUTELSAT 5.

EGNOS Ground Segment

The EGNOS Ground Segment comprises a network of Ranging Integrity Monitoring Stations (RIMS), two Mission Control Centres (MCC), six Navigation Land Earth Stations (NLES), and the EGNOS Wide Area Network (EWAN) which provides the communication network for all the components of the Ground Segment. Two additional facilities are also deployed as part of the ground segment to support system operations and service provision, namely the Performance Assessment and Checkout Facility (PACF) and the Application Specific Qualification Facility (ASQF), which are operated by the EGNOS Service Provider.

The EGNOS system is a widely distributed and redundant system. Data flows from one subsystem to another subsystem have different level of criticality.

EGNOS Data flow

The main EGNOS functions are carried out by the Ground Segment through the following critical subsystems: the RIMS stations, the CPF units and the NLES stations. The Ground Segment is a periodic synchronous and pipelined system. The synchronization of subsystems located in widely separated geographic places is referred to the GPS time by means of GPS receivers providing a 1 PPS (one Pulse Per Second) synchronization pulse to the associated subsystem.

Another feature of the EGNOS Ground Segment is that all Monitoring and Control (M&C) functions performed by humans (non-automatic functions) are centralized and implemented in the CCF subsystems, which is outside of the critical data flow. Operators on duty in another CCF (hot backup) are ready to take over the system monitoring and control if the master CCF fails. Remaining two CCF (cold backup) can be reactivated if the master one fails.

In the EGNOS data flow representation, the real time critical data flow is indicated in red whereas the non-critical data flow is indicated in green.

The sub-systems involved in the processing of the critical data are responsible for the achievement of the main EGNOS system performance (accuracy, integrity, continuity, time to alarm, and service coverage) while the sub-systems involved in the management of non-critical data perform the Monitoring and Control (M&C) and archive functions.

The EGNOS Ground Segment includes the following support facilities (EGNOS Support Segment):

• 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 of specification and validation, and 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.

Due to the implementation of EGNOS V3, the RIMS and NLES performance engineering have to be updated in order to be responsible for:

• Monitoring the proper implementation of the RIMS and NLES specifications imposed by EGNOS V3 system.
• Characterizing the RIMS and NLES performances as achieved by the implemented RIMS and NLES (incl. characterization of RIMS and NLES environment) used as input for EGNOS V3 system performance analysis.
• Maintaining and updating RIMS and NLES specifications as needed to support EGNOS V3 system performance analysis (this includes survey of specifications provided as CFI by the customer (e.g. scintillation specifications, RIMS and NLES RFI environment specifications, User Rx standards (e.g. new L5 DFMC SBAS MOPS), survey of NLES specification provided by the Customer).

EGNOS User Segment

The EGNOS User Segment consists of the GNSS receivers that enable their users to accurately compute their positions using EGNOS corrections. It is important to bear in mind that a GNSS receiver only monitors signals sent by the satellites and does not establish any contact with them. Therefore, a GNSS receiver cannot be used by a third party to find out a user’s position without his knowledge.

To receive EGNOS signals an EGNOS compatible receiver is required. There are many receivers available on the market from a variety of manufacturers. An EGNOS receiver is like a GNSS receiver but with special software inside that allows the receiver to lock onto the code used by the EGNOS satellites and compute the EGNOS corrections to the GNSS signals. Apart from this, an EGNOS receiver is just like a GNSS receiver. This means that it can pick up GPS signals as well. An EGNOS receiver is the same size as a GPS receiver and uses the same type of antenna.

To test the EGNOS receiver, special prototypes have been developed with extensive capabilities to log and analyze data.

EGNOS V3 will provide enhanced performances and robustness in the whole GEO broadcast area for those users equipped with standard space based augmentation system (SBAS) receivers capable of both Galileo and GPS. Moreover the baseline V3 system architecture will be modular and upgradeable in time in order to progressively accommodate and support a very wide span of brand new GNSS services for various user communities.