A Space-Based Autonomous Timekeeping Method Based on Onboard Atomic Clocks and Inter-Satellite Measurements

In global navigation satellite systems (GNSS), the system time reference is maintained by the ground control segment and kept traceable to UTC, enabling inter-system compatibility and interoperability. Advances in onboard atomic-clock stability and inter-satellite time transfer accuracy make it feasible for a constellation to autonomously realize a space-based time reference, with periodic traceability updates and steering via satellite-ground links to enhance resilient time maintenance. BeiDou-3 (BDS-3) carries high-performance onboard hydrogen masers and Ka-band inter-satellite links (ISL) for time transfer, providing stable frequency sources and high-precision time transfer capability for establishing a space-based time reference. Using in-orbit BDS-3 clock offset data, we propose a space-based autonomous timekeeping approach that combines high-precision ISL synchronization with timekeeping by a small ensemble of hydrogen masers, together with a space-ground cooperative strategy with BeiDou time (BDT). The approach first performs constellation-wide synchronization using ISL, then selects a timekeeping ensemble based on in-orbit clock performance to generate a space-based ensemble atomic timescale, denoted TA(SPACE); when satellite-ground links are available, TA(SPACE) is steered to BDT to maintain consistency with the ground time reference. Based on this space-based time reference, satellite clock offsets are predicted to generate clock-parameter products. Experiments show that, in the autonomous mode, the time offset between TA(SPACE) and BDT is kept within 25.06 ± 41.47 ns over 90 days, whereas in the space-ground cooperative mode, satellite-ground steering stabilizes the offset within 10 ns. The proposed approach provides a practical solution for constellation time maintenance under disruptions such as anomalous ground injection, improving the resilience and reliability of GNSS services.