Backup Hydrogen Maser Steering System for Galileo Precise Timing Facility

Two hydrogen masers (HMs) are used in the Precise Timing Facility to provide the physical realization of Galileo System Time, insuring the extremely high short-term stability required for the navigation functions. In order to allow a smooth switch over between backup and primary HMs, the “backup HM steering algorithm” is developed. This acquires the phase difference measured between two HMs, computes a steering correction, and generates the steering correction to the backup HM via a PicoStepper with a 0.1-picosecond resolution. The algorithm design is based on outlier removal and a proportional-integral filtering controller. To verify the steering operability and the loop performance, the overall backup HM steering system is simulated using real HM-HM measurements, and with simulated anomalies (phase/frequency spikes, jumps, and drift).