Research on Quad-Frequency PPP-B2b Time Transfer

Carrier phase time transfer which is a crucial technique in universal time coordinated (UTC) calculation is implemented through precise point positioning (PPP). Since August 2020, the Beidou global navigation satellite system (BDS-3) has provided users with the precise satellite product which is an essential external input in the PPP implementation, named the PPP-B2b product, through three geostationary earth orbit (GEO) satellites instead of a network in the Asia-Pacific area. The PPP-B2b product can be considered to solve the instability problem caused by network interruption in traditional PPP time transfer. Currently, the fact that the PPP-B2b time transfer using dual-frequency (DF) ionosphere-free combination can achieve sub-nanosecond accuracy has been proven. Considering the BDS-3 can provide users with a wide range of frequency signals for PPP; meanwhile, the multi-frequency PPP will improve the accuracy of time transfer and accelerate the convergence. This improvement can be attributed to an increase in the number of observation equations due to the utilization of multiple frequencies. To promote the application of real-time PPP-B2b time comparison in UTC calculation, a quad-frequency (QF) PPP-B2b time transfer model is proposed and investigated. Compared to DF PPP-B2b time transfer, the accuracy of the QF time transfer model was verified from long-baseline time links and zero-baseline common clock difference (CCD). Results showed that the QF PPP-B2b time transfer had smoother CCD results and fluctuated within 0.1 ns, compared to the DF PPP-B2b model. Taking the PPP time comparison using the GBM product as a reference, the results for all long-baseline links show that the residuals of the QF PPP-B2b time comparison truly fluctuate within 1 ns.