Editing Galileo (satellite navigation) (section)

=== 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]].