Metrological traceability in dynamic scenarios timing

Abstract

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.