Research on Single-Frequency PPP-B2b Time Transfer

Time transfer based on global navigation satellite system (GNSS) using precise point positioning (PPP) is a critical technique in universal time coordinated (UTC) calculation. PPP relies on precise satellite orbit and satellite clock offset products obtained through network solutions. Since August 2020, the Beidou global navigation satellite system (BDS-3) has provided GNSS users with the PPP product using PPP-B2b signal through three geostationary earth orbit (GEO) satellites instead of a network in the Asia-Pacific area. Network interruption will cause PPP reconvergence and terminate the time transfer, which hinders the application of PPP time transfer. The real-time PPP-B2b product broadcast by GEO satellites can be considered to solve the problem of interruption caused by network fluctuation in the field of real-time PPP. To promote the application of real-time PPP time comparison in UTC calculation and develop PPP-B2b application in time transfer, single-frequency (SF) PPP-B2b time transfer is investigated. Meanwhile, the treatment of ionospheric delay of SF PPP-B2b is crucial, and a GRAPHIC SF PPP-B2b model is proposed and validated in order to keep PPP-B2b network-independent. Data from two international atomic time (TAI) time-keeping laboratories and two international GNSS service (IGS) stations were used to analyze the accuracy in SF time comparison applications using the PPP-B2b product from common clock difference (CCD) and long-baseline time links. Results show that the SF PPP-B2b CCD sequences can be concentrated within 0.2 ns. The long-baseline time difference between SF time transfer using PPP-B2b product and dual-frequency (DF) time transfer using the final product fluctuated within 2 ns. In addition, the standard deviation (STD) values of PPP-B2b time comparison are mainly distributed at 0.5 ns. Meanwhile, only BDS and $\text{BDS}+\text{GPS}$ can achieve similar SF PPP-B2b time transfer accuracy.