Abstract: Precise timing is an important part of Global Navigation Satellite Systems (GNSS). Each of GNSS has independent internal reference timescales. The residual offset of these timescales may reach tens of nanoseconds. In order to realize the interoperability and compatibility of these systems, it is necessary to account the offset among these systems. The theory of monitoring GNSS time offset by the Signal-in-Space at the single monitoring station is analyzed. The offset between GPST and GLONASST in 130 days is taken into account by using the data from a GPS/GLONASS combined receiver, then the monitoring result is estimated/tested by the related data published by BIPM through the circular T bulletin. The test shows that the max residual offset between monitoring result and circular T bulletin data in the 130 days is 8 nanoseconds. It turns out the algorithm is effective and usable.

Key words: Timing, Interoperability and compatibility, Monitoring GNSS time offset, Measurement error