Characterization of biases between BDS-3 and BDS-2, GPS, Galileo and GLONASS observations and their effect on precise time and frequency transfer

With the development of the Chinese BeiDou Navigation Satellite System (BDS), from a regional system BDS-2 to a global system BDS-3, more satellites can now be employed in the domain of globally precise time and frequency transfer. However, if the current BDS-3 and BDS-2, Global Positioning System (GPS), Galileo and GLObal NAvigation satellite system (GLONASS) are combined for multi-global navigation satellite system (multi-GNSS) time transfer, the characteristics of inter-system biases (ISBs) remain unclear. In this study, we analyzed the characteristics of ISBs between BDS-3 and BDS-2, GPS, Galileo and GLONASS, and revealed that their ISB series is not constant and that all the daily ISB series of the multi-GNSS show some systematic characteristics among different stations. Therefore, three stochastic models for ISBs of white noise process, random constant process and random walk process expressed as 'White', 'Hour' and 'Walk', respectively, were employed to assess the performance of the time and frequency transfer using the time link accuracy and frequency stability indicators. The results show that the models for the 'Hour' and 'Walk' schemes are notably better than that for the 'White' scheme by 10.4% and 16.8%, respectively, for the least noisy time link of WTZZ-BRCH. Furthermore, the improvements in the 'Hour' and 'Walk' schemes are 2.1% and 3.5%, respectively, for the TP01-BRCH time link, when compared with that of the 'White' scheme. With respect to frequency stability, the 'Hour' and 'Walk' schemes also perform better in terms of frequency stability than the 'White' scheme at different time intervals. Similarly, the improvements over the 'White' scheme are 5.05% and 4.97% for the TP01-BRCH time link and are 24.01% and 32.48% for the WTZZ-BRCH time link.