Editing Galileo (satellite navigation) (section)

== System description ==
=== Space segment ===
{{main|List of Galileo satellites}}

[[File:Galileo sat constallation.gif|thumb|upright=1.5|Constellation visibility from a location on Earth's surface]]

As of 2012,<ref>{{cite web|url=http://download.esa.int/docs/Galileo_IOV_Launch/Galileo_factsheet_2012.pdf |archive-url=https://web.archive.org/web/20121018063051/http://download.esa.int/docs/Galileo_IOV_Launch/Galileo_factsheet_2012.pdf |archive-date=2012-10-18 |url-status=live|publisher=ESA|title=Galileo fact sheet|date=15 February 2013|access-date=8 December 2015}}</ref> the system was scheduled to have 15 satellites operational in 2015 and reach full operation in 2020{{update after|2020|3|21}}<!-- did they ever achieve this 2012 objective? --> with the following specifications:
* 30 in-orbit spacecraft (24 in full service and 6 spares)
* Orbital altitude: {{cvt|23,222|km}} ([[Medium Earth orbit|MEO]])
* Orbital period: 14 hours and 5 minutes (every 17 revolutions, done in 10 sidereal days, [[Satellite revisit period|a satellite passes over the same location]])<ref>{{Cite web |title=Galileo Space Segment - Navipedia |url=https://gssc.esa.int/navipedia/index.php/Galileo_Space_Segment |access-date=2023-10-30 |website=gssc.esa.int}}</ref>
* 3 [[Orbital plane (astronomy)|orbital planes]], 56.0° [[Orbital inclination|inclination]], ascending nodes separated by 120.0° longitude (8 operational satellites and 2 active spares per orbital plane)
* Satellite lifetime: >12 years
* Satellite mass: {{cvt|675|kg}}
* Satellite body dimensions: {{cvt|2.7|×|1.2|×|1.1|m}}
* Span of solar arrays: {{cvt|18.7|m}}
* Power of solar arrays: 1.5&nbsp;kW (end of life)
* Power of navigation antennas: 155–265 W<ref>{{cite journal |last1=Bury |first1=Grzegorz |last2=Sośnica |first2=Krzysztof |last3=Zajdel |first3=Radosław |last4=Strugarek |first4=Dariusz |title=Toward the 1-cm Galileo orbits: challenges in modeling of perturbing forces |journal=Journal of Geodesy |date=2020 |volume=94 |issue=16 |page=16 |doi=10.1007/s00190-020-01342-2 |bibcode=2020JGeod..94...16B |doi-access=free}}</ref>

=== Ground segment ===
[[File:Galileo Control Centre at the DLR Oberpfaffenhofen site.jpg|thumb|Galileo Control Centre at the DLR Oberpfaffenhofen site]]
[[File:Galileo IOT L-band antenna at Redu ESA219283.jpg|thumb|Galileo In-Orbit Test (IOT) [[L-band]] (1,000 – 2,000 MHz) antenna at [[ESTRACK]] [[Redu Station]]]]

The system's orbit and signal accuracy is controlled by a [[ground segment]] consisting of:
* Two ground control centres, located in [[Oberpfaffenhofen]] and [[Fucino]] for Satellite and Mission Control
* Seven telemetry, tracking & control (TT&C) stations, located in [[Kiruna]], 2x [[Kourou]], [[Nouméa]], [[Sainte-Marie, Réunion|Réunion]], [[Redu]] and [[Papeete]]
* Ten mission data uplink stations (ULS), two per site, located in [[Svalbard]], [[Kourou]], [[Papeete]], [[Sainte-Marie, Réunion]] and [[Nouméa]]
* Several worldwide distributed reference sensor stations (GSS), including one in the [[Kerguelen Islands]] and [[Saint Pierre and Miquelon Satellite Station|Saint Pierre and Miquelon]]
* A data dissemination network between all geographically distributed locations
* One service centre, located in [[Madrid]], to help Galileo users.

=== Signals ===
The system transmits three signals: E1 (1575.42&nbsp;MHz), E5 (1191.795&nbsp;MHz) consisting of E5a (1176.45&nbsp;MHz) and E5b (1207.14&nbsp;MHz), and E6 (1278.75&nbsp;MHz):<ref name=ERNP>{{cite web |url=http://ec.europa.eu/DocsRoom/documents/33024|title=DocsRoom – European Commission|website=ec.europa.eu}}</ref>
 
{| class="wikitable" style="text-align:center; font-size:90%;"
|+ style="font-size:105%;" | Galileo FOC signals
|-
! Parameters
! E1-I !! E1-Q !! E5a !! E5b !! E6-I !! E6-Q
|-
| style="text-align:left;" | Carrier frequency (MHz)
| 1,575.42 || 1,575.42 || 1,176.45 || 1,207.14 || 1,278.75 || 1,278.75 
|-
| style="text-align:left;" | Modulation
| [[Composite Binary Offset Carrier|CBOC]] (6, 1, 1/11) || [[binary offset carrier modulation|BOCcos]] (15, 2.5) || AltBOC (15, 10) || AltBOC (15, 10) || [[binary phase-shift keying|BPSK]] (5) || [[binary offset carrier modulation|BOCcos]] (10, 5)
|}

=== Services ===
The Galileo system will have four main services:
;Open Service (OS):This will be available without charge for use by anyone with appropriate mass-market equipment; simple timing, and positioning down to 1 m {{snd}}for a double frequency receiver, best case.<ref>{{Cite web|url=https://gssc.esa.int/navipedia/index.php/Galileo_Open_Service_(OS)|title=Galileo Open Service (OS) – Navipedia|website=gssc.esa.int}}</ref>

;High Accuracy Service (HAS; resulting from the re-scope of the former Galileo Commercial Service): Accuracy to 20&nbsp;cm free of charge.<ref>{{Cite web|url=https://gssc.esa.int/navipedia/index.php/Galileo_High_Accuracy_Service_(HAS)|title=Galileo High Accuracy Service (HAS) – Navipedia|website=gssc.esa.int}}</ref>

;Public Regulated Service (PRS; encrypted): Designed to be more robust, with anti-jamming mechanisms and reliable problem detection. Limited to authorized governmental bodies.<ref>{{Cite web|url=https://gssc.esa.int/navipedia/index.php/Galileo_Public_Regulated_Service_(PRS)|title=Galileo Public Regulated Service (PRS) – Navipedia|website=gssc.esa.int}}</ref>

;Search and Rescue Service (SAR): The Galileo SAR Service is a Medium Earth Orbiting Search and Rescue [[International Cospas-Sarsat Programme#MEOSAR|(MEOSAR)]] service and part of the [[International Cospas-Sarsat Programme]].<ref name="Galileo SAR">{{cite web|url=https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo-SAR-SDD.pdf |archive-url=https://web.archive.org/web/20200229094524/https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo-SAR-SDD.pdf |archive-date=2020-02-29|title=SAR/GALILEO SERVICE DEFINITION DOCUMENT|publisher=European GNSS Service Centre|date=2 December 2021|access-date=2 December 2021|url-status=live}}</ref>

==== Quarterly Service Performance Reports ====
The European GNSS Service Centre provides public quarterly performance reports regarding the Open Service and Search and Rescue Service since 2017. Generally, the reported performance parameters measurements surpass the target values.<ref name="Performance Reports">{{cite web|url=https://www.gsc-europa.eu/electronic-library/galileo-service-performance-reports|title=Performance Reports|publisher=European GNSS Service Centre|date=30 November 2021|access-date=30 November 2021|url-status=dead|archive-date=30 November 2021|archive-url=https://web.archive.org/web/20211130154542/https://www.gsc-europa.eu/electronic-library/galileo-service-performance-reports}}</ref>
The Galileo April, May, June 2021 Quarterly Open Service Performance Report by the European GNSS Service Centre reported the UTC Time Dissemination Service Accuracy was ≤&nbsp;4.3 [[nanosecond]]s, computed by accumulating samples over the previous 12 months and exceeding the ≤&nbsp;30 ns target value. The Signal In Space Error (SISE) was also well within the ≤&nbsp;{{cvt|2|m}} target value for Single and (more accurate) Dual Frequency receivers.<ref name="Timing accuracy2">{{cite web|url=https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo-OS-Quarterly-Performance_Report-Q2-2021.pdf |archive-url=https://web.archive.org/web/20211020204407/https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo-OS-Quarterly-Performance_Report-Q2-2021.pdf |archive-date=2021-10-20|title=EUROPEAN GNSS (GALILEO) SERVICES OPEN SERVICE QUARTERLY PERFORMANCE REPORT APRIL - JUNE 2021|publisher=European GNSS Service Centre|date=1 July 2021|access-date=30 November 2021|url-status=live}}</ref><ref name="Timing accuracy">{{cite web|url=https://www.gsa.europa.eu/galileo/services/initial-services|title=Galileo Goes Live|publisher=European GNSS Agency|date=15 December 2016|access-date=1 February 2017|archive-date=15 January 2021|archive-url=https://web.archive.org/web/20210115130957/http://www.gsa.europa.eu/galileo/services/initial-services|url-status=live}}</ref> The Galileo navigation message includes the differences between Galileo System Time (GST), UTC and GPS Time (GPST) (to promote interoperability).<ref>{{Cite web |url=http://www.unoosa.org/pdf/icg/2012/Timescale-GALILEO.pdf |title=GNSS Timescale Description |access-date=5 October 2015 |archive-date=28 October 2020 |archive-url=https://web.archive.org/web/20201028210310/http://www.unoosa.org/pdf/icg/2012/Timescale-GALILEO.pdf |url-status=live }}</ref><ref>{{cite web|url=http://www.insidegnss.com/node/3560|title=ESA Adds System Time Offset to Galileo Navigation Message|website=insidegnss.com|access-date=5 October 2015|archive-date=28 March 2018|archive-url=https://web.archive.org/web/20180328231238/http://www.insidegnss.com/node/3560|url-status=dead}}</ref>
The Galileo April, May, June 2021 Quarterly Search and Rescue Service Performance Report by the European GNSS Service Centre reported the various performance parameters measurements surpassed their target values.
<ref name="Timing accuracy3">{{cite web|url=https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo-EnS-SAR-Quarterly-Performance_Report-Q2-2021.pdf |archive-url=https://web.archive.org/web/20210924134048/https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo-EnS-SAR-Quarterly-Performance_Report-Q2-2021.pdf |archive-date=2021-09-24|title=EUROPEAN GNSS (GALILEO) SAR SERVICE QUARTERLY PERFORMANCE REPORT APRIL – JUNE 2021|publisher=European GNSS Service Centre|date=1 July 2021|access-date=30 November 2021|url-status=live}}</ref>

=== Concept ===
[[File:ESA Galileo Passive Hydrogen Maser.jpg|thumb|Space Passive Hydrogen Maser used in Galileo satellites as a master clock for an onboard timing system]]
[[File:Rb atomic clock Galileo satellite.jpg|thumb|Prototype Rb atomic clock for a Galileo satellite made in 2002]]
Each Galileo satellite has two master passive [[hydrogen maser]] [[atomic clock]]s and two secondary [[Rubidium standard|rubidium]] atomic clocks which are independent of one other.<ref>{{cite web |url=https://safran-navigation-timing.com/product/imaser-3000/|title=Passive Hydrogen Maser (PHM)|work=safran-navigation-timing.com|date=11 December 2018}}</ref><ref>{{cite web |url=https://safran-navigation-timing.com/solution/atomic-clocks-and-oscillators/|title=Rb Atomic Frequency Standard (RAFS)|work=safran-navigation-timing.com|date=11 December 2018}}</ref> As precise and stable space-qualified atomic clocks are critical components to any satellite-navigation system, the employed quadruple [[Redundancy (engineering)|redundancy]] keeps Galileo functioning when onboard atomic clocks fail in space. The onboard passive hydrogen maser clocks are four times more precise than the onboard rubidium atomic clocks, with an estimated drift of just 1 second every 3 million years. A timing error of just 1 [[nanosecond]] would correspond to a positional error of 30 cm on Earth's surface. These clocks provide the accurate timing signals necessary for calculating signal travel time to the receiver.<ref name="Galileo’s clocks">{{cite web|url=http://www.esa.int/Our_Activities/Navigation/Galileo/Galileo_s_clocks|title=Galileo's clocks|publisher=European Space Agency |access-date=16 January 2017}}</ref><ref name="About errors">{{cite web|url=http://www.esa.int/Our_Activities/Navigation/What_about_errors|title=What about errors|publisher=European Space Agency|access-date=16 January 2017}}</ref><ref name="clock failures">{{cite web|url=https://www.bbc.com/news/science-environment-38664225|title=Galileo satellites experiencing multiple clock failures|publisher=British Broadcasting Corporation (BBC)|date=17 January 2017|access-date=18 January 2017}}</ref> The Galileo satellites are configured to run one hydrogen maser clock in primary mode and a rubidium clock as hot backup. Under normal conditions, the operating hydrogen maser clock produces the reference frequency from which the navigation signal is generated. Should the hydrogen maser encounter any problem, an instantaneous switchover to the rubidium clock would be performed. In case of a failure of the primary hydrogen maser the secondary hydrogen maser could be activated by the ground segment to take over within a period of days as part of the redundant system. A 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 hydrogen 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 NSU information is used to calculate the position of the receiver by [[Trilateration|trilaterating]] the difference in received signals from multiple satellites.

The onboard passive hydrogen maser and rubidium clocks are very stable over a few hours. If they were left to run indefinitely, though, their timekeeping would drift, so they need to be synchronized regularly with a network of even more stable ground-based reference clocks. These include active hydrogen maser clocks and clocks based on the [[Caesium standard|caesium frequency standard]], which show a far better medium and long-term stability than rubidium or passive hydrogen maser clocks. These clocks on the ground are gathered together within the parallel functioning Precise Timing Facilities in the Fucino and Oberpfaffenhofen Galileo Control Centres. The ground based clocks also generate a worldwide time reference called Galileo System Time (GST), the standard for the Galileo system and are routinely compared to the local realisations of UTC, the UTC(k) of the European frequency and time laboratories.<ref>{{cite web|url=http://www.spectratime.com/uploads/pdfs/gstpg_ptti06.pdf|title=38th Annual Precise Time and Time Interval (PTTI) Meeting GALILEO SYSTEM TIME PHYSICAL GENERATION|access-date=28 July 2017|archive-url=https://web.archive.org/web/20170729010652/http://www.spectratime.com/uploads/pdfs/gstpg_ptti06.pdf|archive-date=29 July 2017|url-status=dead}}</ref>

For more information of the concept of global satellite navigation systems, see [[GNSS]] and [[GNSS positioning calculation]].