You may have already heard that the Galileo EU satellite navigation constellation is out of service since last Thursday July 11th. Currently the Galileo constellation status page shows that the status of most Galileo satellites is not usable because of a service outage. The satellites not affected by this outage are E20, which was made unavailable on 2014-05-27, and E14 and E18, the Galileo eccentric satellites, which failed to achieve the circular nominal orbit and are only used for testing purposes.
The European GNSS Agency has given very little information regarding the causes of the problem. The available information boils down to NAGU 2019026, posted on 2019-07-13 20:15, which states that starting from 2019-07-12 01:50 the Galileo signals should not be used.
This has originated many rumours and confusion about the problem. It seems that the major cause was a failure in the Precise Timing Facility, which is in charge of the generation of a realization of the GST, the timescale used by Galileo. This has affected the OSPF, which is the service that generates the orbit and clock products (ephemeris) for the satellites. Thus, since Thursday, no new ephemeris are being computed and uploaded to the satellites.
Taking rumours aside, in this post I will look at some facts about the outage which can be learned by analizing the Galileo signal. Other people have published similar studies, such as the NAVSAS group from University of Torino.