Backup Hydrogen Maser Steering Algorithm for Galileo Precise Timing Facility

Two hydrogen masers (HM, a primary and a backup) are used in the precise timing facility (PTF) to provide the physical realization of Galileo System Time (GST), insuring the extremely high short-term stability required for the navigation functions, in particular to perform a reliable satellite clock modeling. In order to allow a smooth switch-over between backup and primary HMs in case of failure of the latter, without introducing the phase jump exceeding 30 ps in the value of GST, the 'backup HM steering algorithm' is developed, which acquires the phase difference between two HMs measured by a phase comparator and elaborates the steering correction applied to the backup HM via a Picostepper with a 0.1 ps resolution, to keep the backup HM in phase with the primary one. The algorithm is designed based on two functions: the outlier remover and the proportional-integral filtering controller. The overall backup HM steering system contributed by the phase comparator, the Picostepper and the algorithm is simulated using real HM to HM measurements, to verify the steering operability and the loop performance under various test cases including the nominal and degraded conditions with simulated anomalies or feared events (phase/frequency spikes, jumps and drift).