Three geostationary satellites and a complex network of ground stations will carry out the task. The three satellites will send out a ranging signal similar to those transmitted by the GPS and GLONASS satellites. However, the signals will be more than another opportunity for users to fix a position. They will also provide information about the accuracy of position measurements delivered by GPS and GLONASS so that a train driver, for example, will be able to assess whether the position is accurate enough to rely on.

This information, or integrity data, will be modulated onto the ranging signal. It will include accurate information on the position of each GPS and GLONASS satellite, the accuracy of the atomic clocks on board the satellites and information on disturbances within the ionosphere that might affect the accuracy of positioning measurements. The EGNOS receiver, which is more sophisticated than a standard satellite navigation receiver, will de-code the signal to give a more accurate position than is possible with GPS or GLONASS alone and an accurate estimate of errors.
 

The EGNOS signal will be broadcast by two Inmarsat-3 satellites, one over the eastern part of the Atlantic, the other over the Indian Ocean, and the ESA Artemis satellite which will be launched into position above Africa later this year. Unlike the GPS and GLONASS satellites, these three will not have signal generators on board. A transponder will transmit signals up-linked to the satellites from the ground, where all the signal processing will take place. The sophisticated ground segment will consist of about 30 ranging and integrity monitoring stations (RIMS), four master control centres and six up-link stations.

The RIMS measure the positions of each EGNOS satellite and compare accurate measurements of the positions of each GPS and GLONASS satellite with measurements obtained from the satellites’ signals. The RIMS then send this data to the master control centres, via a purpose built communications network.

The master control centres determine the accuracy of GPS and GLONASS signals received at each station and determine position inaccuracies due to disturbances in the ionosphere. All the deviation data is then incorporated into a signal and sent via the secure communications link to the up-link stations, which are widely spread across Europe. The up-link stations send the signal to the three EGNOS satellites, which then transmit it for reception by GPS and GLONASS users with an EGNOS receiver.

Considerable redundancy is built into EGNOS so that the service can be guaranteed at practically all times. At any one time, only one master control centre will be “the master”, with another on stand-by to take over instantaneously should the first one fail. There is redundancy in the up-link stations, too. Only three are needed to operate EGNOS, one for each satellite. The other three are in reserve in case of failure.
 

What kind of satellite does EGNOS require?

EGNOS relies on the availability of geostationary satellites equipped with navigation payloads to broadcast a GPS look-alike signal containing integrity and wide-area differential corrections to users. The operational system uses three satellites to disseminate this data: Inmarsat III Atlantic Ocean Region–East (AOR-E) at 15.5ºW; Inmarsat III Indian Ocean Region (IOR) at 64ºE; and ESA ARTEMIS at 21.5ºE

Ground Segements

The EGNOS ground segment is composed of: the Master Control Centres; the Ranging and Integrity Monitoring Stations; the Navigation Land Earth Stations; the EGNOS Wide Area Network; and support facilities.
 

Once the EGNOS system is complete there will be four Master Control Centres (MCC), each having:

a central control facility for:

• monitoring and controlling EGNOS G/S
• mission monitoring and archiving ATC I/F

a central processing facility with a real-time software system developed to high software standards in order to:

• provide EGNOS WAD corrections
• ensure the integrity of the EGNOS system for users
• utilise independent RIMS channels to check corrections

RIMS Location

Ranging and Integrity Monitoring Stations
 
The EGNOS system will have 34 Ranging and Integrity Monitoring Stations (RIMS)

Technical data

• a GPS/GLONASS/GEO receiver
• an atomic clock
• the EGNOS RIMS network is based on TCP/IP, Framerelay and VSAT

Main functions

• perform pseudorange code/phase measurements towards SVis (GPS L1 and L2 + GEO/GLO L1)
• demodulate SIS messages
• mitigate local multipath and interference
• support the detection of anomalies in signals from space (e.g. EWF GPS/GEO)
• packet and transmit data to the MCCs via FEE/EWAN
• provide BITE and M&C capabilities
• provide time offset UTC(k)/ENT (UTC RIMS)

Navigation Land Earth Stations
 
The first stage EGNOS Advanced Operational Capability architecture foresees seven Navigation Land Earth Stations (NLES), five of which will uplink EGNOS messages to the Inmarsat III Atlantic Ocean Region – East (AOR-E) and Indian Ocean Region (IOR) satellites, and two of which will uplink EGNOS messages to the Artemis satellite.

The main functions of the NLES will be to:

• generate a GPS-like signal and transmit this to a GEO transponder

• synchronise this signal to EGNOS time (ENT) at the output of the GEO L1-band antenna

• control the code/carrier coherency

• transmit the GIC and WAD messages to satellites in geostationary orbit.

EGNOS Wide Area Network
 
The EGNOS Wide Area Network (EWAN) links all the EGNOS components together.
 
 
Support facilities
 
Support facilities are made up of the Performance Access Check out Facility (PACF) and the Application Specific Qualification Facility or ASQF.

The PACF is a single unique centralised facility that provides non-critical operations support, engineering support and some maintenance and logistics support capabilities for the EGNOS operations system. ASQF is a set of tools that provide the technical means through which user-specific applications are qualified in the domain of operations.

For more reading on the current GPS setup, check out one of our other articles How Does GPS Work.