On July 12th an incident affecting the European navigation system Galileo was reported on the news. For the user, the incident appeared so that Galileo satellites were not used for positioning. This was also seen on NLS' FinnRef network.
However, for the users of satellite positioning the user's receiver needs two types of data: observation data (radio signals) from the satellites and the current positions of the satellites. The precise satellite positions are called ephemerides.
In case of this Galileo incident, the last ephemerides broadcasted by the satellites were at 21:50 July 11th. According to the Galileo OS-SDD the validity of the ephemeris expires after four hours. After this time the ephemeris are not to be used. As a result, Galileo could not be used for navigation even though the satellite signals were received. The FinnRef reported normal signal strengths for all Galileo signals during this incident.
On the evening of July 16th it was observed on the FinnRef network that Galileo is usable again. However, on a closer inspection of several GNSS receivers, it was noticed that some of the receivers were behaving differently and not using Galileo for navigation. The reason for this different behavior was that apparently the Galileo OS-SDD guidelines for defining the usability status of Galileo ephemeris were not strictly followed. Galileo ephemeris validity should be checked from two data fields of the received ephemeris: satellite health flag and Signal In Space Accuracy (SISA).
Only the satellite health flag was again ok on Tuesday evening, while SISA flag was still reporting NAPA (No Accuracy Prediction Available) value. It was found out that some of the receivers only compare the health flag and pay no attention to the SISA flag. This handling is not fully following Galileo OS-SDD. Receivers that do not determine ephemeris validity based on both flags would acquire a positioning solution from the night of July 16th.
But on the evening of 17th, receivers of FinnRef network reported that all Galileo satellites are sending good values for both indicators. This was also officially confirmed by the provider of the Galileo service.
This Galileo incident serves as a good opportunity for the Galileo users and manufactures to review their implementation of Galileo in order to comply with the Galileo definitions. It has to be remembered that Galileo is providing only its initial services, which means that Galileo system is not claimed to have full operational capability.
Senior Research Scientist