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|YearOfPublication=2011
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The [[GALILEO Ground Segment]] comprises two control centres, a global network of transmitting and receiving stations implementing monitoring and control functions and a series of service facilities which support the provision of the Galileo services. 


The Galileo global component will provide the GALILEO Space segment: a 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 core of the [[GALILEO General Introduction|GALILEO ]] ground segment are the two Galileo Control Centres (GCC). Each control centre manages ''control'' functions supported by a Galileo Control Segment (GCS) and ''mission'' functions, supported by a dedicated Galileo Mission Segment (GMS): The GCS handles spacecraft housekeeping and constellation maintenance while the GMS handles navigation system control.<ref name="Galileo System">[https://www.gsc-europa.eu/galileo/system Galileo System at European Galileo Service Centre website ]</ref>


The [[GALILEO General Introduction|GALILEO ]] space segment will comprise 30 satellites in a Walker constellation with three orbital planes at 56° nominal inclination.<ref name="EsaGalileoweb">[http://www.esa.int/esaNA/galileo.html ESA Galileo web page]</ref> 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 GCS and GMS interfaces the satellites with a worldwide ground station network implementing control and monitoring functions <ref name="Galileo System">[https://www.gsc-europa.eu/galileo/system Galileo System at European Galileo Service Centre website ]</ref> <ref name="Galileo OS SDD">[https://www.gsc-europa.eu/system/files/galileo_documents/Galileo-OS-SDD.pdf Galileo Open Service - Service Definition Document ]</ref> :
* '''Galileo Sensor Stations (GSS)''', responsible for collecting and sending in real time the data measurements of Galileo SIS.
* '''Galileo Uplink Stations (ULS)''', responsible for the distribution and uplink of the mission data to the Galileo constellation.
* '''Telemetry, Tracking & Control stations (TT&C)''', responsible for collecting and sending the telemetry data that was generated by the Galileo satellites and also for the distribution and the uplink of the control commands that are necessary to maintain the Galileo satellites and constellation.
The ground segment also comprises a set of Medium-Earth Orbit Local User Terminals (MEOLUTs) serving Galileo’s Search and Rescue service.


==Constellation features==
[[File:Galileo_s_Global_Ground_Segment.jpg|left|400px|Galileo Ground Segment|thumb]]


[[File:Galileo Space Segment.jpg|left|thumb|400px|Galileo Space Segment]]
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.


There are good reasons for choosing such a structure for the Galileo constellation. With 30 satellites at such an altitude, there is a very high probability (more than 90%) that anyone anywhere in the world will always be in sight of at least four satellites and hence will be able to determine their position from the ranging signals broadcast by the satellites. The inclination of the orbits was chosen to ensure good coverage of polar latitudes, which are poorly served by the US GPS system.  
There are several services facilities that complement the core infrastructure <ref name="Galileo System">[https://www.gsc-europa.eu/galileo/system Galileo System at European Galileo Service Centre website ]</ref> <ref name="Galileo OS SDD">[https://www.gsc-europa.eu/system/files/galileo_documents/Galileo-OS-SDD.pdf Galileo Open Service - Service Definition Document ]</ref><ref name="Galileo SAR SDD">[https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo-SAR-SDD.pdf Galileo SAR Service Definition Document ]</ref> :
*The '''European GNSS Service Centre (GSC)''': located in Torrejón (Spain), the centre represents the link between the Galileo Initial OS (and in the future HAS) user communities and the Galileo system
From most locations, six to eight satellites will always be visible, allowing positions to be determined very accurately – to within a few centimeters. Even in high rise cities, there will be a good chance that a road user will have sufficient satellites overhead for taking a position, especially as the Galileo system will be interoperable with the US system of 24 GPS satellites.  
*The '''Geodetic Reference Service Provider (GRSP)''': responsible for the data processing that sustain the Galileo Control Centres in order to realise the Galileo Terrestrial Reference Frame (GTRF) consistently with the International Terrestrial Reference Frame (ITRF)
*The '''Time Service Provider (TSP)''': supports the GCC by realising the Galileo System Time (GST) in order to make it aligned to the Coordinated Universal Time (UTC).
*The '''Galileo Security Monitoring Centre (GSMC)''': located in St. Germain-en-Laye (France) and Swanwick (United Kingdom) these facilities are responsible for system security monitoring
*The '''SAR/Galileo Data Service Provider (SGDSP)''': located in Toulouse (France), this entity is responsible of the SAR/Galileo service operations coordination. There is also a MEOLUT Tracking Coordination Facility (MTCF) locate in the SAR/Galileo Service Centre.
*The '''Galileo Reference Centre (GRC)''': located in Noordwijk (The Netherlands), this facility is in charge of the performance monitoring and assessment of Galileo services. This facility is independent from the Galileo core infrastructure and its operations.[[File:Galileo_Ground_Segment_Architecture.png|right|300px|Galileo Ground Segment Architecture|thumb]]


==Galileo satellites==
[[File:Galileo satellite system.jpg|right|thumb|300px|Galileo satellite]]


The Galileo constellation is composed of a total of 30 Medium Earth Orbit (MEO) satellites, of which 3 are spares, in a so-called Walker 27/3/1 constellation. Each satellite will broadcast precise time signals, ephemeris and other data. The Galileo satellite constellation has been optimised to the following nominal constellation specifications:<ref name="GALHLD"> [http://ec.europa.eu/dgs/energy_transport/galileo/doc/galileo_hld_v3_23_09_02.pdf Galileo Mission High Level Definition], v3, September 2002.</ref>


*circular orbits (satellite altitude of 23 222 km)
==Galileo Control Segment (GCS)==
*orbital inclination of 56°
*three equally spaced orbital planes
*nine operational satellites, equally spaced in each plane
*one spare satellite (also transmitting) in each plane
The Galileo satellite is a 700 kg/1600 W class satellite.


The image shows an artist's impression of a Galileo spacecraft in orbit with solar arrays deployed. The spacecraft rotates about its Earth-pointing axis so that the flat surface of the solar arrays always faces the Sun to collect maximum solar energy. The antennas, shown on the underside of the body in the picture, always point towards the Earth. The spacecraft body will measure 2.7 m x 1.1 m x 1.2 m and the deployed solar arrays span 13 m.
The Galileo Control Segment (GCS) is responsible for a large range of functions to support satellite constellation control and management of Galileo satellites. The scope of this functionality includes control and monitoring of the satellites and payload, planning and automation functions that allow safe and correct operations to take place, and the support of payload related operations by means of Telemetry Tracking and Control (TT&C) stations links.
The GCS provides the telemetry, telecommand and control function for the whole Galileo satellite constellation. Its functional elements are deployed within the Galileo Control Centres (GCC) and the six globally distributed Telemetry Tracking and Control (TT&C) stations. To manage this, the GCS uses the TT&C stations to communicate with each satellite on a scheme combining regular, scheduled contacts, long-term test campaigns and contingency contacts.


===Galileo Satellite components===
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 '''L-band antenna''' transmits the navigation signals in the 1200-1600 MHz frequency range.
The '''SAR (Search and Rescue) antenna''' picks up distress signals from beacons on Earth and transmits them to a ground station for forwarding to local rescue services.
The '''C-band antenna''' receives signals containing mission data from Galileo Uplink Stations. This includes data to synchronise the on-board clocks with a ground-based reference clock and integrity data which contains information about how well each satellite is functioning. The integrity information is incorporated into the navigation signal for transmission to users.
Two '''S-band antennas''' are part of the telemetry, tracking and command subsystem. They transmit housekeeping data about the payload and spacecraft to ground control and, in turn, receive commands to control the spacecraft and operate the payload. The S-band antennas also receive, process and transmit ranging signals that measure the satellite's altitude to within a few metres.
The '''infrared Earth sensors''' and the '''Sun sensors''' both help to keep the spacecraft pointing at the Earth. The infrared Earth sensors do this by detecting the contrast between the cold of deep space and the heat of the Earth's atmosphere. The Sun sensors are visible light detectors which measure angles between their mounting base and incident sunlight.  
   
   
The '''laser retro-reflector''' allows the measurement of the satellite's altitude to within a few centimetres by reflecting a laser beam transmitted by a ground station. The laser retro-reflector is used only about once a year, as altitude measurements via S-band antenna ranging signals are otherwise accurate enough.  
The TTC Stations include 13-meter 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, [https://www.nasa.gov/directorates/heo/scan/services/networks/tdrs_main TDRSS], and [http://www.esa.int/Our_Activities/Telecommunications_Integrated_Applications/Artemis ARTEMIS] data relay applications, are 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 allows non-ESA TTC stations to be used.
The '''space radiators''' are heat exchangers that radiate waste heat, produced by the units inside the spacecraft, to deep space and thus help to keep the units within their operational temperature range.  


===Interior: payload===
==Galileo Mission Segment (GMS)==
[[File:GMS_Fucino.JPG|300px|left|thumb]]


A '''passive hydrogen maser clock''' is the master clock on board the spacecraft. It is an atomic clock which uses the ultra stable 1.4 GHz transition in a hydrogen atom to measure time to within 0.45 ns over 12 hours.  
The Galileo Mission Segment (GMS) consists of facilities deployed in the two Galileo Control Centres (GCCs) plus a series of Mission Up-Link Stations (ULS) and Galileo Sensor Stations (GSS) deployed at remote sites located around the world.
The GMS is responsible for the determination and uplink of navigation data messages needed to provide the navigation and timing data. For this purpose, it uses a global network of Galileo Sensor Stations (GSS)  <ref name="Galileo System">[https://www.gsc-europa.eu/galileo/system Galileo System at European Galileo Service Centre website ]</ref> 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 is duplicated for redundancy.
   
   
A '''rubidium clock''' will be used should the maser clock fail. It is accurate to within 1.8 ns over 12 hours.
The GMS communicates with the Galileo satellites through a global network of Mission Up-Link Stations (ULS), installed at five sites, each of which hosts a number of 3-meter antennas. ULSs operate in the 5 GHz Radionavigation Satellite (Earth-to-space) band.  
[[File:Rubidium_clock.jpg|left|thumb|250px|Rubidium clock]]
 
The spacecraft has four clocks, two of each type. At any time, only one of each type is operating. Under normal conditions, the operating maser clock produces the reference frequency from which the navigation signal is generated. Should the maser clock fail, however, the operating rubidium clock will take over instantaneously and the two reserve clocks will start up. If the problem with the failed maser clock is unique to that clock, the second maser clock will take over from the rubidium clock after a few days when it is fully operational. The rubidium clock will then go on stand-by or reserve again. In this way, by having four clocks, the Galileo spacecraft is guaranteed to generate a navigation signal at all times.  
   
   
The '''clock monitoring and control unit''' provides the interface between the four clocks and the navigation signal generator unit (NSU). It passes the signal from the active master clock to the NSU and also ensures that the frequencies produced by the master clock and the active spare are in phase, so that the spare can take over instantly should the master clock fail.  
The GMS uses the GSS network in two independent ways. The first is the Orbitography Determination and Time Synchronisation (OD&TS) function, which provides 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 are up-loaded into that satellite nominally every 100 minutes using a scheduled contact via a Mission Up-link Station.  
   
   
The '''navigation signal generator, frequency generator and up-conversion units''' are in charge of generating the navigation signals using input from the clock monitoring unit and the up-linked navigation and integrity data from the C-band antenna. The navigation signals are converted to L-band for broadcast to users.
The OD&TS operation thus monitors the long-term parameters due to gravitational, thermal, ageing and other degradations.
The '''remote terminal unit''' is the interface between all the payload units and the on-board computer.
 
===Interior: service module===
'''SADM'''  is the drive mechanism that connects the solar arrays to the spacecraft and rotates them slowly so that the surface of the arrays can remain perpendicular to the Sun's rays at all times.
The '''gyroscopes''' measure the rotation of the spacecraft.
The '''reaction wheels''' control the rotation of the spacecraft. When they rotate, so does the spacecraft. It rotates twice per orbit to allow the solar arrays to remain parallel to the Sun's rays.
The '''magneto bar''' modifies the speed of rotation of the reaction wheels by introducing a torque (turning force) in the opposite direction.
The '''power conditioning and distribution unit''' regulates and controls power from the solar arrays and batteries and distributes it to all the spacecraft's subsystems and payload.
The '''on-board computer''' controls all aspects of spacecraft and payload functioning.
 
==Launching and phases==
 
ESA will launch the first four operational satellites using two separate launchers. The first two satellites will be placed in the first orbital plane and the second in the second orbital plane. These four satellites, plus part of the ground segment, will then be used to validate the Galileo system as a whole, together with advanced system simulators. Then, the next two satellites will be launched into the third orbital plane. They will be followed by several launches with Ariane-5 or Soyuz from the Europe’s Space Port in French Guyana. The first services will be delivered when the constellation has reached its Initial Orbital Configuration.
When the 30 satellites are in space on all its three orbital planes, Galileo will be fully operational, providing its services to a wide variety of users throughout the world. 
 
The assembly of the first four satellites in the future constellation, which
will be launched in 2011-2012 as ESA has confirmed, is currently being completed.
The creation of the terrestrial element of the infrastructure is continuing in parallel;
this involves choosing sites and building a large number of stations spread across a number of countries and regions of the world: Belgium, France, Italy, Germany,
Spain, the Netherlands, the United Kingdom, New Caledonia, Réunion, French
Guyana, Tahiti, Sweden, Norway, the United States, Antarctica (Troll, Adélie Land), etc.


Work on the deployment phase was launched in 2008 and is proceeding actively.
==Control Centres Components==
This work has been divided up essentially into six packages, each of which is the
subject of a public procurement procedure. Competitive dialogue with the tendering
firms is a key element in the procedures which have been launched.
As a result, the first four contracts, with a total value of around € 1 250 million, were awarded in 2010; they are for the packages covering system engineering support, satellite construction (with an initial order for 14 satellites), launchers (for the launch of 10 satellites, but with options for additional launches) and operations,
respectively. The other two packages, relating to ground infrastructure, will be
awarded in 2011. The contracts for additional equipment and facilities will also need to be awarded in the course of 2011.


The public procurement contracts already awarded are enabling the Commission to
The Galileo Control Centres include components of the two Galileo segments, GCS and GMS.  
adjust its approach to meet the deadline of 2014. Accordingly, the development and deployment phases will continue in parallel until 2012, when the development phase will be completed, and the exploitation phase for the first services will start in 2014. A first stage will be the partial commissioning of the infrastructure (initial
operational capability, or IOC) as from 2014-2015 and the provision of the open
service, the search and rescue service and the PRS. At this stage, however, accuracy and availability will not yet have reached their optimum levels.


The European GNSS has one prime advantage over other systems: it is the only one designed for civilian purposes and under civilian control. It has other potential
The main GMS facilities are the following ones:
advantages which should not be disregarded, such as its commercial service, which might make it possible to authenticate signals and further improve the accuracy of the open service. Finally, its open service complements, and is interoperable with, the American GPS, so that the combined use of the two systems will afford a degree of reliability and accuracy which is likely to fulfil most users' needs worldwide in the mass applications market. However, most of these benefits will materialise only after the full infrastructure is complete. Galileo's Full Operational Capability (FOC) should be achieved in 2019-2020. It might change, depending on availability of financing, technical problems and industrial performance.<ref name="Mid-term review">[http://ec.europa.eu/enterprise/newsroom/cf/_getdocument.cfm?doc_id=6321 Mid-term review of the European satellite radio navigation programmes]</ref>
*OSPF: Orbit determination and Synchronization 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 facility needed to multiplex all the messages either generated within the GCC or received by external entities, into a single data stream to be sent to each ULS in order to be uploaded to spacecrafts.
*PTF: Precision Timing Facility, responsible for the generation of a physical realization of Galileo System Time (GST) which is provided to all elements for time synchronization purposes.  
*GACF: Ground Assets Control Facility, monitoring and controlling all the elements of the GMS in real time.
*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 and calibration data for the real-time elements.
*MTPF: Maintenance and Training Platform, which contains the instances of all elements and support equipment for maintenance and training purposes.  
*GMS KMF: GMS Key Management Facility, that supports security aspects and data protection (generation of encryption keys, encryption/decryption process,...).
*SPF: Service Product Facility, which is dedicated to the implementation of the exchange gateway between the GCC and the external world.


The main GCS facilities are:
*SCCF: Spacecraft & Constellation Control Facility, that performs the on-line monitoring and control of the satellites, both for routine and critical operations.
*SCPF: Spacecraft & Constellation Planning Facility, which handles the problem of scheduling regular contact (once per orbit) with all satellites in the constellation to support routine operations and special extended contacts to support critical operations.
*FDF: Flight Dynamics Facility, responsible for non-nominal orbit determination (GMS provides nominal) and manoeuver planning.
*OPF: Operations Preparation Facility, responsible for preparation and configuration control of all operational databases and procedures, including those that are destined for automated execution.
*CMCF: Central Monitoring & Control Facility, that supports the monitoring and control of all GCS ground assets, including the TT&C stations, GCC resident facilities and networks.
*GCS KMF: GCS Key Management Facility,  that supports security aspects and data protection (generation of encryption keys, encryption/decryption process...).
*CSIM: Constellation Simulator, which is used for validation of operational process, training and anomalies investigation.


==Notes==
==Credits==
Edited by GMV, using information from the Galileo - Open Service – Service Definition Document and the European GNSS Service Centre website as indicated through the references.
<references group="footnotes"/>
<references group="footnotes"/>
==References==
==References==
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[[Category:GALILEO]]
[[Category:GALILEO]]
[[Category:GALILEO Architecture]]
[[Category:GALILEO Ground Segment]]

Revision as of 10:10, 18 March 2020


GALILEOGALILEO
Title Galileo Ground Segment
Edited by GMV
Level Basic
Year of Publication 2011
Logo GMV.png

The GALILEO Ground Segment comprises two control centres, a global network of transmitting and receiving stations implementing monitoring and control functions and a series of service facilities which support the provision of the Galileo services.

The core of the GALILEO ground segment are the two Galileo Control Centres (GCC). Each control centre manages control functions supported by a Galileo Control Segment (GCS) and mission functions, supported by a dedicated Galileo Mission Segment (GMS): The GCS handles spacecraft housekeeping and constellation maintenance while the GMS handles navigation system control.[1]

The GCS and GMS interfaces the satellites with a worldwide ground station network implementing control and monitoring functions [1] [2] :

  • Galileo Sensor Stations (GSS), responsible for collecting and sending in real time the data measurements of Galileo SIS.
  • Galileo Uplink Stations (ULS), responsible for the distribution and uplink of the mission data to the Galileo constellation.
  • Telemetry, Tracking & Control stations (TT&C), responsible for collecting and sending the telemetry data that was generated by the Galileo satellites and also for the distribution and the uplink of the control commands that are necessary to maintain the Galileo satellites and constellation.

The ground segment also comprises a set of Medium-Earth Orbit Local User Terminals (MEOLUTs) serving Galileo’s Search and Rescue service.

File:Galileo s Global Ground Segment.jpg
Galileo Ground Segment


There are several services facilities that complement the core infrastructure [1] [2][3] :

  • The European GNSS Service Centre (GSC): located in Torrejón (Spain), the centre represents the link between the Galileo Initial OS (and in the future HAS) user communities and the Galileo system
  • The Geodetic Reference Service Provider (GRSP): responsible for the data processing that sustain the Galileo Control Centres in order to realise the Galileo Terrestrial Reference Frame (GTRF) consistently with the International Terrestrial Reference Frame (ITRF)
  • The Time Service Provider (TSP): supports the GCC by realising the Galileo System Time (GST) in order to make it aligned to the Coordinated Universal Time (UTC).
  • The Galileo Security Monitoring Centre (GSMC): located in St. Germain-en-Laye (France) and Swanwick (United Kingdom) these facilities are responsible for system security monitoring
  • The SAR/Galileo Data Service Provider (SGDSP): located in Toulouse (France), this entity is responsible of the SAR/Galileo service operations coordination. There is also a MEOLUT Tracking Coordination Facility (MTCF) locate in the SAR/Galileo Service Centre.
  • The Galileo Reference Centre (GRC): located in Noordwijk (The Netherlands), this facility is in charge of the performance monitoring and assessment of Galileo services. This facility is independent from the Galileo core infrastructure and its operations.
    Galileo Ground Segment Architecture


Galileo Control Segment (GCS)

The Galileo Control Segment (GCS) is responsible for a large range of functions to support satellite constellation control and management of Galileo satellites. The scope of this functionality includes control and monitoring of the satellites and payload, planning and automation functions that allow safe and correct operations to take place, and the support of payload related operations by means of Telemetry Tracking and Control (TT&C) stations links. The GCS provides the telemetry, telecommand and control function for the whole Galileo satellite constellation. Its functional elements are deployed within the Galileo Control Centres (GCC) and the six globally distributed Telemetry Tracking and Control (TT&C) stations. To manage this, the GCS uses the TT&C 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 include 13-meter 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, are 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 allows non-ESA TTC stations to be used.

Galileo Mission Segment (GMS)

GMS Fucino.JPG

The Galileo Mission Segment (GMS) consists of facilities deployed in the two Galileo Control Centres (GCCs) plus a series of Mission Up-Link Stations (ULS) and Galileo Sensor Stations (GSS) deployed at remote sites located around the world. The GMS is responsible for the determination and uplink of navigation data messages needed to provide the navigation and timing data. For this purpose, it uses a global network of Galileo Sensor Stations (GSS) [1] 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 is duplicated for redundancy.

The GMS communicates with the Galileo satellites through a global network of Mission Up-Link Stations (ULS), installed at five sites, each of which hosts a number of 3-meter antennas. ULSs operate in the 5 GHz Radionavigation Satellite (Earth-to-space) band.

The GMS uses the GSS network in two independent ways. The first is the Orbitography Determination and Time Synchronisation (OD&TS) function, which provides 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 are up-loaded into that satellite nominally every 100 minutes using a scheduled contact via a Mission Up-link Station.

The OD&TS operation thus monitors the long-term parameters due to gravitational, thermal, ageing and other degradations.

Control Centres Components

The Galileo Control Centres include components of the two Galileo segments, GCS and GMS.

The main GMS facilities are the following ones:

  • OSPF: Orbit determination and Synchronization 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 facility needed to multiplex all the messages either generated within the GCC or received by external entities, into a single data stream to be sent to each ULS in order to be uploaded to spacecrafts.
  • PTF: Precision Timing Facility, responsible for the generation of a physical realization of Galileo System Time (GST) which is provided to all elements for time synchronization purposes.
  • GACF: Ground Assets Control Facility, monitoring and controlling all the elements of the GMS in real time.
  • 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 and calibration data for the real-time elements.
  • MTPF: Maintenance and Training Platform, which contains the instances of all elements and support equipment for maintenance and training purposes.
  • GMS KMF: GMS Key Management Facility, that supports security aspects and data protection (generation of encryption keys, encryption/decryption process,...).
  • SPF: Service Product Facility, which is dedicated to the implementation of the exchange gateway between the GCC and the external world.

The main GCS facilities are:

  • SCCF: Spacecraft & Constellation Control Facility, that performs the on-line monitoring and control of the satellites, both for routine and critical operations.
  • SCPF: Spacecraft & Constellation Planning Facility, which handles the problem of scheduling regular contact (once per orbit) with all satellites in the constellation to support routine operations and special extended contacts to support critical operations.
  • FDF: Flight Dynamics Facility, responsible for non-nominal orbit determination (GMS provides nominal) and manoeuver planning.
  • OPF: Operations Preparation Facility, responsible for preparation and configuration control of all operational databases and procedures, including those that are destined for automated execution.
  • CMCF: Central Monitoring & Control Facility, that supports the monitoring and control of all GCS ground assets, including the TT&C stations, GCC resident facilities and networks.
  • GCS KMF: GCS Key Management Facility, that supports security aspects and data protection (generation of encryption keys, encryption/decryption process...).
  • CSIM: Constellation Simulator, which is used for validation of operational process, training and anomalies investigation.

Credits

Edited by GMV, using information from the Galileo - Open Service – Service Definition Document and the European GNSS Service Centre website as indicated through the references.

References

  1. ^ a b c d Galileo System at European Galileo Service Centre website
  2. ^ a b Galileo Open Service - Service Definition Document
  3. ^ Galileo SAR Service Definition Document
Retrieved from "https://gssc.esa.int/navipedia/index.php?title=Galileo_Ground_Segment&oldid=14876"