BDS-2/BDS-3 combined precise time-frequency transfer with different inter-system bias estimation strategies and different analysis centres products

BeiDou-3 global navigation satellite system (BDS-3) has officially provided positioning, navigation, and timing (PNT) services on 31 July 2020. The difference between BDS-2 and BDS-3 in signal modulation mode, signal characteristics, and satellite clock datum leads to the possible existence of inter-system bias (ISB) between BDS-2 and BDS-3, which becomes the key to affect the time-frequency transfer of BDS. In this contribution, taking the observations of BRUX, CEBR, USUD, WTZZ, and YEL2 stations as an example, five analysis centres (ACs) precise products (for instance, CNT, COM, ESA, GBM, and WUM) are used to evaluate the BDS-2 BII/B3I combined BDS-3 B1I/B3I, B1I/B2a, B1C/B2a, and B1C/B2a time-frequency transfer performance with ignore ISB (ISBNO), estimating ISB as constant (ISBCT), estimating ISB as a random walk (ISBRW) and estimating ISB as white noise (ISBWN) four kinds of estimation strategies. The results show that the ISBCT estimation strategy has the better performance than ISBNO, ISBRW, and ISBWN estimation strategies in BDS-2/BDS-3 combined time-frequency transfer; compared with ignoring ISB strategy, the standard deviation and frequency instability of ISBCT estimation strategy is decreased by 9.16% and 12.83% on average, respectively. Meanwhile, an ISBCT estimation strategy is recommended when using CNT, COM, ESA, and WUM precision products for BDS-2/BDS-3 combined time-frequency transfer.