Satellite navigation systems rely in general on very stable satellite clock performances to allow accurate satellite clock behavior prediction, which is required to model accurately the satellite clocks at user level through the provided satellite clock model parameters.
Unpredictable events like contingent and sudden changes or jumps in the satellite clock frequency cannot be measured immediately by the ground monitoring segment, which is provides the satellite clock model parameters to the user, and therefore such events cannot be properly modeled by the user and impose an additional error due to the inaccurate satellite clock modeling accordingly.
Satellite navigation systems like GPS or the future European Galileo system currently use satellite frequency standards, i.e., satellite clocks, based on Rubidium technology that shows such unpredictable clock frequency jump behavior.
For Galileo, based on early test satellite measurement results with the experimentation satellites GIOVE-A and GIOVE-B, typical Rubidium clock (called RAFS) jumps occur up to 1-2 times per month with a frequency jump magnitude that might degrade the ranging accuracy by up to around 1 m to 10 m. This has a non negligible impact on the positioning service, and a major and significant impact on the future Galileo integrity service performance that is even jeopardizing the feasibility of those Galileo services.
Especially for integrity purposes where the user needs to know a priori the signal and related ranging quality with very high confidence, such unpredictable events must either be avoided, or immediately detected, in order to allow for user recovery actions, like removal of the affected satellite navigation signals from the data used for position detection.
Avoidance of the effect could be achieved by considering different satellite clock technologies, like passive hydrogen masers (PHM) as currently foreseen for the Galileo system in co-existence with RAFS onboard each satellite.
Detection of such events could be typically done at ground monitoring segment level, where each satellite is permanently tracked by several monitoring stations (up to 40 stations for Galileo).
Galileo integrity services can be secured only if one of the above mechanisms is implemented (i.e., avoidance or detection together with related recovery).