Analysis of preliminary results from distributed autonomous timekeeping on Beidou-3 satellites

Autonomous timekeeping is necessary to maintain the navigation system time when the ground operation control system is unavailable. We use Ka-band inter-satellite two-way ranging data from Beidou-3 network satellites to carry out autonomous timekeeping tests. First, we introduce an algorithm for distributed autonomous timekeeping that uses Beidou-3 inter-satellite links and high-performance onboard passive hydrogen masers (PHMs). Using these tools, we analyze the main factors affecting the performance of autonomous timekeeping. Finally, the performance of autonomous timekeeping is evaluated using overall time drift and frequency stability, and this performance is compared with the timekeeping performance using broadcast clock offset parameters. Without autonomous corrections, use of the broadcast parameters will result in non-uniform drifts of satellite clocks versus Beidou system time (BDT) of over 300 ns after 30 days. Using the algorithm we present, the time reference within the constellation can be unified, and the timekeeping performance can be further improved through fine modeling of the satellite clocks. After 30 days of autonomous operation, the overall time drift of the constellation relative to BDT can be less than 1.3 ns (the last epoch), and the frequency stability is about 1.2E- 14 at 86,400 s interval.