Centimeter-level clock synchronization and space-borne timescale generation for BDS-3 using inter-satellite link measurements

Because modernized global navigation satellite systems (GNSSs) carry high-performance atomic clocks onboard, a new problem has arisen regarding whether the clock estimation approach is good enough to reflect the true performance of onboard atomic clocks. This paper proposes a new and novel centimeter-level clock synchronization approach for BDS-3 satellites based on two-way comparisons using inter-satellite link measurements. The proposed approach estimates satellite clock offsets by adjusting satellite clock offsets obtained from inter-satellite ranging measurements, and measurement noise is greatly reduced. The noise level of clock offsets estimated by the new approach is 0.7 cm, which is 41% of that of clock offsets estimated by ISL direct two-way comparisons. Compared to the ODTS approach, the new approach efficiently eliminates orbit errors in clock offsets. On this basis, the paper presents a space-borne timescale (SPBT) using high-performance atomic clocks onboard BDS-3 satellites. The frequency stability of the SPBT is 8.6E-16 at a 1-day interval; this result is superior to ground GNSS timescales such as BDT. Based on the SPBT, this paper evaluates the performance of the BDS-3 onboard atomic clock. The 2-h and 24-h prediction uncertainties for the BDS-3 PHM are 0.04 and 0.12 m, and the frequency stabilities are 1.6e-14 and 3.4e-15 at 10,000-s and 1-day intervals, respectively. The 2-h and 24-h prediction uncertainties of the BDS-3 RAFSs are 0.05 and 0.37 m, and the frequency stabilities are 1.9E-14 and 8.1E-15 at 10,000-s and 1-day intervals. This study contributes to autonomous navigation and signal-in-space accuracy improvements for GNSSs.