The metrological traceability of time and frequency has been established and demonstrated in static stations, such as, the metrological traceability to UTC of UTC(k) in the frame established by CCTF key comparison: CCTF-K001.UTC. In dynamic scenarios, most mobile stations use the GNSS one-way time transfer to achieve time synchronization. However, the time reference of the output of the receiver is not clearly identified, which means the time and frequency values obtained by the user will not be metrologically traceable (This is attributable to the time broadcast by GNSS, which includes both the GNSS system time (GNSST) and the prediction of UTC broadcast by the GNSS system.). Mobile stations such as sailing ships, self-driving cars, and high-speed railway trains have put forward a demand for high-precision time synchronization. To ensure the reliability and legitimacy of the time and frequency values obtained by the mobile stations, a study on metrological traceability in dynamic scenarios timing should be conducted.
The construction of the traceable link from the mobile station to the ‘reference’ is an important part of this study (The ‘reference’ can be any station that achieves traceability to UTC). A time transfer link is designed, which is constructed by two calibrated GNSS time transfer receivers located separately at the mobile station and the ‘reference’. The real-time position of the mobile station is calculated to generate the result of the 30-second difference between the mobile station and GNSST. The 30-second calculation of the difference between the ‘reference’ and GNSST is performed at the ‘reference’ as well, which then is transmitted to the mobile station via the network every 30 seconds. Through the differentiating procedure, the real-time time transfer result between the mobile station and the ‘reference’ is obtained. In this way, it can be ensured that the time and frequency values obtained by the mobile station are derived from a clear and definite
reference.
We construct a platform for the establishment of traceability of time in dynamic scenarios and conduct the experiments on this platform. A trolley, a car traveling on a city road, and a railway train are the mobile stations, and our lab which keeps an atomic time scale TS(BJTU) is employed as the ‘reference’. The real-time time transfer was successfully achieved between the mobile stations and the ‘reference’. The time transfer results calculated based on the post-processing Precise Point Positioning (PPP) technology are used for the evaluation of link noise level. In three dynamic time transfer scenarios, the noise level distributes between 2 ns and 7 ns. Further analysis of the uncertainty of time transfer in dynamic scenarios will be conducted to accomplish this study.
Kun Liang, Baoying Wei· 150th anniversary of the Met...· 0 citations
The method of GNSS one-way timing has been most commonly used in many sectors that rely on the real-time time and frequency values. The GNSS receiver which can provide real-time signals (e.g. 1 PPS and 10 MHz) employs a time and frequency standard. To make the systems in these sectors more stable and reliable, the collaboration of multiple systems and the introduction of new technologies impose a demand for higher precision in time synchronization, for instance, 65 ns time synchronization for Multiple-Input Multiple-Output and 10 ns for positioning error of 3 meters in the 5th Generation Mobile Communication Technology for Railways conducted currently. The limitations of GNSS one-way timing, which constrain the advancement of the systems are revealed, include: (1) the output signals with hardware delays of the uncalibrated receiver provide inaccurate time information; (2) the time and frequency values obtained from the receiver are not traceable to UTC; (3) the sources of time for the output signals of different receivers are multiple (e.g. BDS system time (BDT) and GPS system time (GPST)). Therefore, the following research is conducted: (1) calibration method for GNSS one-way timing receiver; (2) the real-time monitoring method for the GNSS system time（GNSST）, differences among different GNSS system time and the predictions of UTC by different GNSS (UTC(GNSS)).
The calibration scheme for the GNSS one-way timing receiver is designed and the cross-validation experiments for absolute and differential calibration based on a self-developed calibration validation system are conducted. The calibration results of the receiver are cross-validated through step-by-step, integrity, and differential calibration methods. The uncertainty of the step-by-step calibration, the differential calibration, and the differential calibration are less than 6.80 ns, 6.70 ns, and 9.50 ns, respectively. The uncertainty levels of three calibration methods for one-way timing receiver meet the requirement of the Beidou Open Service Specification for one-way timing accuracy (20 ns).
An atomic time scale TS(BJTU) kept by our laboratory is selected as the monitoring reference since the real-time time link has been constructed between TS(BJTU) and UTC(NIM). A calibrated receiver referenced to TS(BJTU) acquires navigation messages. The real-time difference between TS(BJTU) and GNSST can be obtained from the parameter REFSYS in CGGTTS files, which is calculated every 16 minutes. At the same time, the difference between GNSST and UTC(GNSS) is calculated based on the UTC parameters decoded from navigation messages. The differences among GNSSTs are acquired by real-time differentiating, which is conducted by a procedure. Based on the real-time monitoring methods proposed above, a monitoring platform is built. The monitoring experiment is conducted on the platform. From the monitoring results of one month, the difference of -12.0 ns appears between BDS-2 system time and GPST. The differences among UTC(GNSS), BDS-3 system time, and other GNSS system time will be shown in the poster. The calibration method of GNSS one-way timing receiver, the differences between BDS-3 system time and GPST, TS(BJTU)-UTC(GPS), TS(BJTU)-UTC(BDS-3), and the evaluation of the performance of real-time monitoring will be shown detailed in the poster.
Kun Liang, Baoying Wei, Yufeng Li· 150th anniversary of the Met...· 0 citations
