Autonomous Time Synchronization Using BeiDou Inter-satellite Link Ranging

Inter-satellite time transfer is implemented through inter-satellite links (ISLs) with the system of bidirectional ranging. With the influence of the transmission delay of ranging signals and the relative movements of satellites, the bidirectional ranging measurements are not simultaneous. The time-tags are required to be corrected to a reference epoch with the estimated orbits and clocks to get the inter-satellite clock deviations. The algorithm of BeiDou inter-satellite time synchronization is introduced. And the synchronization performance of BeiDou-3 satellites has been analyzed with real ISL measurements and post-processed precise orbit and clock deviation products. The precision of time synchronization is superior to 0.2 ns. Besides time transfer between BeiDou satellites, the ISLs are available for precise time transfer from BeiDou satellites to extended users whose positions and clock deviations can be solved with GNSS receivers with larger errors compared to BeiDou satellites. The influence of positioning and timing precisions used in the time-tag correction on the synchronization precision has been analyzed in theory and with simulations. The results have demonstrated the precision of two-way time synchronization is free of continuous orbit and clock deviation errors which are varying slowly. And it is feasible that precise time transfer with two-way links between Beidou satellites and extended users in space.