Monitoring atomic clocks on board GNSS satellites

A method for monitoring atomic clocks on board Global Navigation Satellites System (GNSS) satellites is described to address the issue of clock related signal integrity in safety–critical applications of GNSS. The carrier-phase time transfer is employed in the clock monitoring method which enables tight tracking of the satellite onboard clocks and thus improves detectability of clock anomalies. Detecting onboard clock anomalies requires the ability to monitor clocks in real time, and a Kalman filter can then be utilized to estimate the phase offsets between the satellite clocks and ground clocks. This study, using the difference between the measured and predicted phase offset as a test statistic, sets a threshold for clock anomalies based on the prediction interval approach. Finally the validity of the monitoring method is examined by processing a set of real GNSS data that includes two recent incidents of clock anomalies in GNSS satellites.