Welcome back Galileo

In the last days the Galileo system was often on the news for a service outage. This post will try to explain what we experienced as Galileo users, curious of investigating technical aspects from a “scientific/educational” point of view.

First of all, it’s really important to stress that Galileo is still in a pilot phase. The so called “Full Operational Capability” should be declared not before  2020. It means that the Galileo services are already available to users worldwide as “initial services”, with  the awareness that the whole infrastructure is under monitoring for the detection of technical issues before the system becomes fully operational.

Some background:

  • On July 11th, the Galileo Service Centre published a first NOTICE ADVISORY TO GALILEO USERS – NAGU https://www.gsc-europa.eu/notice-advisory-to-galileo-users-nagu-2019025 saying that: UNTIL FURTHER NOTICE, USERS MAY EXPERIENCE SERVICE DEGRADATION ON ALL GALILEO SATELLITES. THIS MEANS THAT THE SIGNALS MAY NOT BE AVAILABLE NOR MEET THE MINIMUM PERFORMANCE LEVELS DEFINED IN THE SERVICE DEFINITION DOCUMENTS AND SHOULD BE EMPLOYED AT USERS’ OWN RISK. THE NOMINAL SERVICE WILL BE RESUMED AS SOON AS POSSIBLE.
  • The following NAGU https://www.gsc-europa.eu/notice-advisory-to-galileo-users-nagu-2019026 declared a service outage and was followed by a service note from GSC https://www.gsc-europa.eu/system/files/galileo_documents/Galileo-service-notice-02.pdf explaining the situation: “A technical incident in the Galileo ground infrastructure is affecting the functioning of the Galileo system, as a result of which there is a temporary interruption of some of the Galileo initial services. The navigation messages for all satellites are expired since 01:50 (UTC) on 12/07/2019, and according to the Service Definition Documents, users shall not use expired Galileo navigation messages. This incident does not affect the Galileo Search and Rescue service used for locating and helping people in distress situations. The cause of the technical incident is identified and recovery actions are implemented to ensure that the nominal service is resumed as soon as possible while safeguarding quality of the services.

 

As Galileo users, we were curious to understand what was going on, and we started to investigate. We knew that the Galileo satellites Signals in Space (SISs) were not usable until further notice. On July 15th and 16th, we used our software receiver to analyze the Signals in Space (SISs) and make some measurements (all the info here http://www.navsas.eu/node/610). Our conclusion was that, yes, the Galileo service outage was expected and widely notified by the GSC, but there was an unexpected finding: the SIS status was not changed to “Unhealthy” for any of  the satellites. Fortunately, standard position quality checks and the age of data read from the satellites messages were enough for user receivers to temporarily exclude the Galileo SISs from the use for navigation.

The day after, the 17th (see http://www.navsas.eu/node/611), we noticed that the ephemerides in most Galileo signals visible over our lab in Torino (Italy) seemed to be valid again and then usable by GNSS receivers. We also found out that the Signal-In-Space Accuracy(SISA) index broadcast by all the satellites was set to 255, i.e., the accuracy status was “No Accuracy Prediction Available (NAPA)”. In this condition, one of our commercial receivers provided the PVT fix without further warnings, while others, namely the smartphones, were still excluding the Galileo satellites from their PVT fix.

With the use of external ephemerides to process the Galileo signals during the “system outage”, we get the confirmation, as suggested by some experts, that, during the outage, only the ephemerides updates were affected by problems, while the other SIS components were apparently sound, usable and “healthy”.

Finally today, July 18th, Galileo came back!

Since the early morning (in Torino) the Signal-In-Space Accuracy (SISA) index has exhibited the value 107, which corresponds to a SISA value equal to 3.12 m, as reported in our last news http://www.navsas.eu/node/613. Indeed, the new NAGU from the GSC, released a few hours after our observations, says that the Galileo service has been restored, even if users may still experience service instability.

We thank all the Galileo experts, the users, the researchers, the journalists and the students that contacted us in these days for their interest in our research.

The NavSAS is a joint research group of LINKS Foundation and Politecnico di Torino.

 

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