Galileo Ground Segment

GALILEO
Title	Galileo Ground Segment
Author(s)	GMV
Level	Basic
Year of Publication	2011

The GALILEO Space Segment will be complemented by the GALILEO Ground Segment, which will comprise two control centres and a global network of transmitting and receiving stations.

The core of the GALILEO ground segment will be the two control centres (GCC). Each control centre will manage control functions supported by a dedicated Galileo Control System (GCS) and mission functions, supported by a dedicated Galileo Mission System (GMS). The GCS will handle spacecraft housekeeping and constellation maintenance while the GMS will handle navigation system control.

Galileo Control System (GCS)

Galileo Ground Segment

The Galileo Control System (GCS) is responsible for satellite constellation control and management of Galileo satellites. It provides the telemetry, telecommand and control function for the whole Galileo satellite constellation. Its functional elements are deployed within the Galileo Control Centers (GCC) and the five globally distributed Telemetry Tracking and Control (TT&C) stations. To manage this, 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.

A hybrid Communication Network interconnects the remote stations (ULS, GSS, and TT&C stations) with the GCC by different means of standard and special radio, wired data and voice communication links, assuring the communication between all the sites. The two Ground Control Centres (GCCs) constitute the core of the Ground Segment. There are two redundant elements located at Fucino (Italy) and Oberpfaffenhofen (Germany).

The TTC Stations will include 13-metre antennas operating in the 2 GHz Space Operations frequency bands. During normal operations, spread-spectrum modulation, similar to that used for Tracking and Data Relay Satellite System, 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.

Galileo Mission System (GMS)

The Galileo Mission System (GMS) is responsible for the determination and uplink of navigation and integrity data messages needed to provide the navigation and UTC time transfer service. For this purpose, it will use a global network of nominally 30 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, 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 failures or changes.

Control Centers Components

The Galileo Control Centers include components of both Galileo systems, GCS and GMS.

• The main GMS facilities :
  • OSPF: Orbit determination and Synchronisation Processing Facility, in charge of the determination of satellite navigation parameters, i.e., ephemeris computation, satellite clock prediction, and determination of the Signal-in-Space accuracy (SISA).
  • MGF: Message Generation Facility, which is the multiplexing facility needed to multiplex all the messages either generated within the GCC (navigation and integrity) or received by external entities, into a single data stream to be sent to each ULS in order to be uploaded to spacecrafts.
  • MUCF: Mission Uplink Control Facility ,which is responsible for the on-line and off-line mission monitoring and control including the Galileo overall long-, mid- and short-term mission planning and uplink scheduling.
  • MSF:Mission Support Facility, used to the off-line support functions including the computation of configuration data for the real-time elements.
  • MTPF:Maintenance and Training Platform, which contains replicas of all elements and support equipment for maintenance and training purposes.
  • PTF: Precision Timing Facility, responsible for the computation of the GST and thus providing a reliable and stable coordinated time reference for the Galileo system.
  • IPF: Integrity Processing Facility, responsible for the determination of the integrity flags for each satellite in real time.
  • GACF: Ground Assets Control Facility, monitoring and controlling all the elements of the GMS in real time.
  • SPF: Service Product Facility, which is dedicated to the implementation of the exchange zone between the GCC and the external world.

• Belonging to GCS:
  • SCF: Satellite Control Facility, that controls all Galileo satellites and uploads the navigation data messages generated in the OSPF, providing the data interfaces for the reliable and timely upload of integrity messages.