The Galileo System will comprise global, regional and local components.
The global component is the core of the system, comprising the satellites and the required ground segment.
The regional component of Galileo may comprise a number of External Region Integrity Systems (ERIS), implemented and operated by organisations, countries or groups of countries outside Europe to obtain integrity services independent of the Galileo System, in order, for example, to satisfy legal constraints relating to system guarantees.
Local components may be deployed for enhancing the performance of Galileo locally. These will enable higher performance such as the delivery of navigation signal in areas where the satellite signals cannot be received. Value-added service providers will deploy local components.
The Galileo global component will provide the constellation of Galileo satellites, each of which will broadcast navigation timing signals together with navigation data signals which will contain not only the clock and ephemeris correction data essential for navigation but also integrity signals which provide a global space-based augmentation service.
The space segment will be complemented by the Galileo ground segment, which will comprise a pair of control centres and a global network of transmitting and receiving stations.
The Galileo space segment will comprise 30 satellites in a Walker constellation with three orbital planes at 56° nominal inclination. Each plane will contain nine operational satellites, equally spaced, 40° apart, plus one spare satellite to replace any of the operational satellites in case of failures.
The orbit altitude of 23 222 km results in a repeat a constellation repeat cycle of ten days during which each satellite has completed seventeen revolutions.
The altitude of the satellites has been chosen to avoid gravitational resonances so that, after initial orbit optimisation, station-keeping manoeuvres will not be needed during the lifetime of a satellite. The altitude chosen also ensures a high visibility of the satellites.
The position constraints for individual satellites are set by the need to maintain a uniform constellation, for which it is specified that each satellite should be within +/- 2° of its nominal position relative to the adjacent satellites in the same orbit plane and should be within 2° of the orbit plane.
The in-plane accuracy is equivalent to a relative tolerance of over 1000 km but requires very careful adjustment of the satellite velocity to ensure that the orbit period of all the satellites is kept precisely the same. The across-track tolerance allows the inclination and RAAN of each satellite to be biased at launch so that natural drifts remain within the tolerance without the need for orbit plane changes requiring major expense of fuel.
The spare satellite in each orbit plane ensures that in case of failure the constellation can be repaired quickly by moving the spare to replace the failed satellite. This could be done in a matter of days, rather than waiting for a new launch to be arranged which could take many months.
The satellites are designed to be compatible with a range of launchers providing multiple and dual launch capabilities.
Ground control segment
The core of the Galileo ground segment will be the two control centres. Each control centre will manage 'control' functions supported by a dedicated Ground Control Segment (GCS) and 'mission' functions, supported by a dedicated Ground Mission Segment (GMS). The GCS will handle spacecraft housekeeping and constellation maintenance while the GMS will handle navigation system control.
The GCS will use a global network of nominally five TTC stations to communicate with each satellite on a scheme combining regular, scheduled contacts, long-term test campaigns and contingency contacts.
The TTC Stations will be large, with 13-metre antennas operating in the 2 GHz Space Operations frequency bands. During normal operations, spread-spectrum modulation (similar to that used for TDRSS and ARTEMIS data relay applications) will be used, to provide robust, interference free operation. However, when the navigation system of a satellite is not in operation (during launch and early orbit operations or during a contingency) use of the common standard TTC modulation will allow non-ESA TTC stations to be used.
Mission control segment
The Galileo Mission Segment (GMS) will use a global network of nominally thirty Galileo Sensor Stations (GSS) to monitor the navigation signals of all satellites on a continuous basis, through a comprehensive communications network using commercial satellites as well as cable connections in which each link will be duplicated for redundancy. The prime element of the GSS is the Reference Receiver.
The GMS communicates with the Galileo satellites through a global network of Mission Up-Link Stations (ULS), installed at five sites, each of which will host a number of 3-metre antennas. ULSs will operate in the 5 GHz Radionavigation Satellite (Earth-to-space) band.
The GMS will use the GSS network in two independent ways. The first is the Orbitography Determination and Time Synchronisation (OD&TS) function, which will provide batch processing every ten minutes of all the observations of all satellites over an extended period and calculates the precise orbit and clock offset of each satellite, including a forecast of predicted variations (SISA - Signal-in-Space Accuracy) valid for the next hours. The results of these computations for each satellite will be up-loaded into that satellite nominally every 100 minutes using a scheduled contact via a Mission Up-link Station.
The second use of the GSS network is for the Integrity Processing function (IPF), which will provide instantaneous observation by all GSSs of each satellite to verify the integrity of its signal. The results of these computations, for the complete constellation, will be up-loaded into selected satellites and broadcast such that any user will always be able to receive at least two Integrity Messages.
The Integrity messages will comprise two elements. The first is as an “Integrity Flag”, which warns that a satellite signal appears to exceed its tolerance threshold. This flag will be generated, disseminated and broadcast with the utmost urgency, so that the Time-to-Alert, being the period between a fault condition appearing at a user's receiver input and the Integrity Flag appearing there will be no more than six seconds, and will be re-broadcast a number of times. The second element of the Integrity Message comprises Integrity Tables, which will be broadcast regularly to ensure that new users or users who have missed recent signal (for example when travelling through a tunnel) will be able to reconstitute the system status correctly.
The OD&TS operation thus monitors the long-term parameters due to gravitational, thermal, ageing and other degradations, while the IPF monitors short-term effects, due to sudden failure or change.
The Galileo Global Component will also include a set of Test User Receivers.