With the completion of the BeiDou global navigation satellite system (BDS-3), the BeiDou Navigation Satellite System Signal In Space Interface Control Document Precise Point Positioning Service Signal PPP-B2b (Version 1.0) was officially announced, and BDS-3 officially broadcast PPP-B2b correction to broadcast ephemeris through geostationary earth orbit (GEO) satellites to provide precise point positioning services for users in the Asia–Pacific region. This study comprehensively analyzes the application of the PPP-B2b product to time transfer and positioning. On a daily basis, the PPP-B2b positioning accuracy after convergence is calculated using the four ionosphere-free (IF) combinations in static and simulated kinematic modes: BDS B1I/B3I, BDS B1C/B2a, BDS B1I/B3I + GPS, and BDS B1C/B2a + GPS. Observations of time laboratories including the National Time Service Center of the Chinese Academy of Sciences (NTSC) and the Telecommunication Laboratories (TL) are employed to conduct zero-baseline common clock difference (CCD) time comparison experiments and long-baseline time comparison experiments using the PPP-B2b product and the GBM product. The results indicate that the PPP-B2b position accuracy in static mode by only BDS is 1.5/2.7/3.9 cm, and by GPS + BDS is within 1.5/2.5/3.5 cm in North, East, and Up directions, respectively. Regarding simulated kinematic PPP-B2b, the average root mean square (RMS) values of the position errors in the North, East, and Up directions for the combination of BDS B1I/B3I + GPS and BDS B1I/B3I are 3.4/5.8/7.6 cm and 3.8/6.6/7.8 cm, respectively. Simultaneously, the average RMS values of position errors using BDS B1C/B2a + GPS and BDS B1C/B2a are 3.6/4.9/8.1 cm and 4/6.1/8.5 cm. In the time comparison study, the results of zero-baseline CCD using the PPP-B2b product and the GBM product are within the fluctuation range of 0.1 ns, respectively. Particularly, the long-baseline time comparison difference between results employing the PPP-B2b product and the GBM product is within the range of ±0.5 ns.

2020 saw the official completion of the BDS-3 and the start of the PPP-B2b signal-based real-time precise point positioning (PPP) service to users in China and the neighboring areas. In this work, the quality of PPP-B2b products is first evaluated and compared with real-time products from the CNES and the differential code bias (DCB) from the Chinese Academy of Science (CAS). Then, a detailed performance evaluation of the PPP time transfer based on the PPP-B2b service (B2b-RTPPP) is conducted. Three solutions, namely, GPS-only (G), BDS-3-only (C), and GPS + BDS-3 (GC) B2b-RTPPP solutions, are compared and assessed. The results suggest that for the PPP-B2b products, BDS-3 satellites have better orbit and clock offset quality than GPS satellites, while the opposite is true for CNES products. The quality of the PPP-B2b orbit and clock offset is poorer than those of the CNES. The PPP-B2b DCB shows excellent agreement with the CAS DCB. The accuracy of the B2b-RTPPP solutions is sub-nanosecond level. The accuracy of B2b-RTPPP time transfer with DCB correction is approximately improved by 64% compared with that without DCB correction. The GC B2b-RTPPP solution has the greatest frequency stability, while G B2b-RTPPP solution has the poorest. Considering that the receiver may be blocked, the B2b-RTPPP time transfer performance is also evaluated at different cut-off elevation angles. As the angle increases, the B2b-RTPPP time transfer performance decreases. Additionally, the short-term frequency stability remains constant at different cut-off elevation angles, but deteriorates in the long term, especially when the angle is 40°.

In 2020, users in and around China had access to a new real-time precise point positioning (RTPPP) service with the BeiDou global navigation satellite system (BDS-3) B2b (PPP-B2b) signal. In this study, the quality of PPP-B2b products is first assessed and compared with the CNAV1 broadcast ephemeris. Then a mode of BDS-3 PPP time transfer with PPP-B2b (B2b-RTPPP) is developed and evaluated in static and kinematic modes. The results demonstrate that the discontinuity of orbit is improved by applying the PPP-B2b, and its root mean square errors (RMSEs) are 0.081 m, 0.165 m, and 0.107 m in the radial, along-track, and cross-track components, respectively. The standard deviation (STD) of the PPP-B2b clock offset is 0.08 ns, greatly better than that of the broadcast clock offset. For time transfer, the type A uncertainty of the B2b-RTPPP solution is approximately 0.1 ns in static mode, and approximately 0.3 ns in kinematic mode. The B1I/B3I B2b-RTPPP time transfer solution performs better than the B1C/B2a solution. For frequency stability, the modified Allan deviation (MDEV) of the B2b-RTPPP solution is comparable to that of the post-processing solution. The B1C/B2a combination has better short-term frequency stability than B1I/B3I, while the long-term frequency stability of B1C/B2a is worse. In addition, the contribution of the differential code bias (DCB) to B2b-RTPPP time transfer is also investigated. The type A uncertainty of the B2b-RTPPP solution without DCB correction is worse than that of the B2b-RTPPP solution with DCB correction. The B1C/B2a B2b-RTPPP solution without DCB correction has better frequency stability than the B1I/B3I B2b-RTPPP solution.

Single point positioning (SPP) can meet the requirements of the majority of real-time time transfer applications. Meanwhile, a single-frequency (SF) receiver is cheaper than a dual-frequency receiver. However, SPP performance can be greatly affected by large pseudorange observation noise. Phase smoothing the pseudorange is an effective approach to reduce pseudorange noise. Since the classical phase-smoothed pseudorange algorithm does not account for the effect of ionosphere delay, we propose a BDGIM-based phase-smoothed pseudorange algorithm to eliminate the ionospheric delay and apply it to BeiDou Navigation Satellite System (BDS-3) SPP time transfer. In this paper, we first evaluate the performance of the BeiDou global ionospheric delay correction model (BDGIM) and compare it with that of the BeiDou Klobuchar model to determine if it is practical to incorporate the BDGIM into our suggested method. The performance of the BDGIM is better than that of the Klobuchar model. The mean RMS value of the BDGIM is 2.6 Total Electron Content Unit (TECU). The average ionospheric correction rate of the BDGIM is 75.5%. Then, we investigate the performance of the improved SF SPP time transfer. The performance of the improved SPP time transfer is much better than that of the traditional SPP time transfer. Compared with the traditional time transfer, the average Type A uncertainty of the improved time transfer is 2.08 ns, which is reduced by about 11.1% from the time transfer without it. Regarding frequency stability, the modified Allan deviations of the improved time transfer are 1.43E-12 and 1.68E-13 at 960 s and 61,440 s, with improvements of 51.2% and 59.9%, respectively.

Soliton microcomb (SMC) arouses wide interest in high-precision distance measurement by virtue of its characteristics in high repetition rate and on-chip capability. However, the high repetition rate brings challenges to frequency locking and signal processing. In this study, we propose a microwave photonic mixing method to simplify the signal processing in SMC-based synthetic wavelength interferometry. In order to achieve this method and obtain the equivalent low-frequency signal, multi-order sidebands with the same frequency interval can be generated and subsequently mixed in the optical domain using an electro-optic modulator (EOM). The microwave photonic mixing method not only decreases the required modulation frequencies for injection locking and frequency-downshifting, but also enables the use of a low-bandwidth photodetector for detection. In the ranging demonstration, the residuals are less than 5 μm compared with a referenced laser interferometer (LI) over a 90 mm distance, and the stability test shows an Allan deviation of 868 nm at an average time of 10 s. It is believed that this microwave photonic mixing synthetic wavelength interferometry for SMC-based distance measurement showcases potential in LIDAR, micro-nano manufacturing, inter-satellite ranging, etc.

Chip-scale soliton microcombs, particularly those operating at low, electronically detectable repetition rates (≤ 26.5 GHz), are highly promising for portable metrology. However, their practical deployment has been critically hindered by poor robustness against intracavity noise and environmental perturbations. Here, we overcome this limitation by proposing a multimodal locking architecture that actively and simultaneously stabilizes all three fundamental parameters in a soliton microcomb: the pump frequency, the cavity resonance, and the repetition rate. This architecture is confirmed both theoretically and experimentally. Implemented on a Si ₃ N ₄ microresonator with an FSR of 24.96 GHz, this approach enables robust soliton generation and sustains record-long, collapse-free operation for over 48 h. More importantly, it maintains soliton robustness with exceptional resilience to environmental shocks, under temperature variations exceeding 10°C and vibration accelerations beyond ±4 g. The microcomb’s metrological utility as a precise optical ruler is further validated through optical frequency calibration and frequency-sweeping-based absolute ranging demonstration. This work provides a critical solution for robust and field-deployable low-repetition-rate soliton microcombs, paving the way for their use in portable optical clocks, high-precision ranging, time-frequency transfer and spectroscopic sensing.