EGNOS Ground Segment

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

The EGNOS Ground Segment comprises a network of Ranging Integrity Monitoring Stations (RIMS), four 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 (ESSP SAS).

EGNOS RIMS

The main function of the RIMS is to collect measurements from GPS satellites and to transmit these raw data every second to the Central Processing Facilities (CPF) of each MCC. The initial confi guration includes 34 RIMS sites located over a wide geographical area.

In order to improve the performance of the EGNOS system and enlarge the area where the EGNOS services can be used, an extension of the monitoring network is expected soon which will see the deployment of additional RIMS in La Palma, (Spain), Athens (Greece) and Alexandria (Egypt). A further extension is also planned in a slightly longer timeframe that should improve the EGNOS performance in the southern parts of the service area. Figure 3 shows the geographical distribution of the RIMS already in operation and the RIMS current under deployment.

EGNOS CPF

The Central Processing Facility (CPF) is a module of the Mission Control Centres that uses the data received from the network of RIMS stations to:

• Elaborate clock corrections for each GPS satellite in view of the network of RIMS stations. These corrections are valid throughout the geostationary broadcast area (i.e. wherever the EGNOS signal is received).
• Elaborate ephemeris corrections to improve the accuracy of spacecraft orbital positions. In principle, these corrections are also valid throughout the geostationary broadcast area. However, due to the geographical distribution of the EGNOS ground monitoring network, the accuracy of these corrections will degrade when moving away from the core service area.
• Elaborate a model for ionospheric errors over the EGNOS service area in order to compensate for ionospheric perturbations to the navigation signals.

This function requires a dense network of monitoring stations. For this reason, the ionospheric model broadcast by EGNOS is not available for the whole geostationary broadcast area but is only provided for a region centred over Europe.

These three sets of corrections are then broadcast to users to improve positioning accuracy.

In addition, the CPF estimates the residual errors that can be expected by the users once they have applied the set of corrections broadcast by EGNOS. These residual errors are characterised by two parameters:

1. User Differential Range Error: this is an estimate of the residual range error after the application of clock and ephemeris error correction for a given GPS satellite.
2. Grid Ionospheric Vertical Error: this is an estimate of the vertical residual error after application of the ionospheric corrections for a given geographical grid point.

These two parameters can be used to determine an aggregate error bounded by the horizontal and vertical position errors. Such information is of special interest for Safety of Life users but may also be benefi cial to other communities needing to know the uncertainty in the position determined by the user receiver.

The CPF includes a large number of monitoring functions designed to detect any anomaly in GPS and in the EGNOS system itself and is able to warn users within a very short timeframe (less than 6 sec) in case of an error exceeding a certain threshold. These monitoring functions are tailored to the Safety Of Life functions and will not be further detailed in this document.

Critical Data Flow description

In order to meet the time to alarm requirements, the EGNOS data flow that is separated in two cycles shall be completed in less than the 6 sec timeframe. These two cycles are:

1. Processing cycle: RIMS A data collection and message generation.
2. Check cycle: RIMS B and C data collection and correction check.

Processing cycle: This cycle elaborates the correction and integrity information. The first step is the collection of GPS/GLONASS/GEO raw data through the channel A of the 34 RIMS stations. These data consists in:

• Pseudo range measurements (code+phase)

NLES

The messages elaborated by the CPF at the Master MCC are transmitted to the NLESs. The NLESs (two for each GEO for redundancy purposes) transmit the EGNOS message received by the CPF to the GEO satellites for broadcast to users and to ensure the synchronisation with the GPS signal.

CCF

The EGNOS system is controlled through a Central Control Facility (CCF) located in each of the Mission Control Centres. These facilities are manned on a 24/7 basis in order to ensure permanent service monitoring and control.