Methods of calibration for GNSS one-way timing link and real-time GNSS system time monitoring

Abstract

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.