# High-Accuracy Clock Offsets Estimation Strategy of BDS-3 Using Using Multi-Source Observations
## 4 Conclusions
Although the common fluctuation will not affect the final position accuracy, it will affect the timing accuracy. It indicates that the common fluctuation in Figure 13 is caused by the fluctuation of BDT itself [2]! Therefore, the stability of the BDT's output signal limits the ability for BDS-3 to improve further the accuracy of BDS-3 satellite's clock offset parameters. It should be noticed that, despite the above results are inspiring for BDS-3, from the perspective of service stability, BDS-3 still needs longer time to check this strategy. What is more, in the bias estimation of the two-way estimation clock offsets, the inputs ODTS clock offsets are dual-frequency ionospheric free combination clock offsets (usually B1I/B3I). Therefore, the final two-way clock offsets are aligned with the one-way downlinks clock offsets with the specific combination frequency. However, for the users of other BDS-3 frequencies. They need to use the broadcast DCB and TGD, and ionospheric parameters to further correct their observation. However, because of their strong correlation and poor distribution of BDS-3 GNSS stations, it is also tricky for BDS-3 to realize their high-precision calculation under the condition that GNSS stations of BDS-3 are only distributed within China. Therefore, after optimizing and updating the clock offsets estimation strategy, the accuracy of the parameters of ionospheric and DCB/TGD will become the main point of growth of BDS-3 accuracy. Improving their accuracy is another key technology that BDS-3 needs to further breakthrough. Finally, we can see a good agreement between the one-way downlink clock offsets and the two-way estimated clock offsets, especially for the MEO satellites with higher ODTS clock offsets accuracy. It probably means that the unknown system errors between the ISL and L-band downlinks are not large. Considering that the two-way estimated clock offsets with less fluctuation than the current ODTS clock offsets in the timescale from the few hours to 1 day. We believe that if BDS-3 takes the two-way estimated clock offsets as the new observation in the GNSS orbit determination, BDS-3 will be able to enhance the accuracy of orbit determination further and accelerate the decouple and convergence time of GNSS parameters, which will be conducive to the study in other GNSS fields [20; 21; 22] and the improvement of the ability on BDS-3 PPP-B2b [32]. All of them deserve further study. J.Y., C.T., Y.Y. and X.H. ; Data curation, Y.X. and Z.Y. ; Formal analysis, J.L. ; Funding acquisition, S.Z. ; Investigation, Y.S., Y.L. and Z.Y. ; Methodology, J.Y., S.Z. and Y.S. ; Project administration, X.H. ; Resources, X.H. ; Software, J.Y. and Y.S. ; Supervision, Y.X., Y.Y. and X.H. ; Validation, J.L., Y.X., Y.L. and Z.Y. ; Visualization, J.Y. ; Writing--original draft, J.Y. ; Writing--review & editing, J.Y., Q.T. and X.H. All authors have read and agreed to the published version of the manuscript. Figure 13: Multi-day fitting residuals of BDS-3 satellite with highest-level-performance atomic clock onboard with respect to BDT (since 19 May 2022). The common fluctuation is caused by the fluctuation in BDT output signals. Funding: This research was supported by Shanghai Observatory's key cultivation project (N20210601003); National Natural Science Foundation of China (No. 12173072); Civil Aerospace \"14 th Five-Year\" Technology Pre-research Project (KJSP2020020203). Data Availability Statement: The data used in this contribution, including the ISL measurements, L-band two-way measurements, C-band two-way measurements, are provided by the Beijing Satellite Navigation Canter. The above data are available from the corresponding author upon request. The other data used in this contribution include the BDS-3 broadcast message provided by the Test and Assessment Research Center of the China Satellite Navigation Office and the precise clock data provided by the GBM, which can be downloaded from the internet. Conflicts of Interest: The authors declare no conflict of interest.