Abstract
The BeiDou navigation satellite system (BDS) tracks medium earth orbit (MEO) satellites using only regional tracking stations in China. As a result, the broadcast clock accuracy of the MEO satellites decreases rapidly during the invisible arcs because of the lack of available observations. The inter-satellite link (ISL) technology of the third generation of BDS (BDS-3) can be used to extend the visible arcs of MEO satellites and to measure the relative inter-satellite clock in nearly real time. We propose a broadcast clock approach for BDS-3 by integrating observations from regional tracking stations and ISLs. The clock error between satellites is obtained through centralized estimation based on ISLs. The Ka-band hardware delay is calibrated by taking the double difference between ISL-centralized clock and the Multi-satellite Precise Orbit Determination clock. The deviation between the ISL-centralized clock and the BeiDou time is obtained using only one Two-way Satellite Time Comparison station or anchor station. To validate the algorithms, we analyze clock estimation and prediction accuracy, hardware delay stability, and time synchronization accuracy. The results show that the frequency stability of the BDS-3 onboard passive hydrogen maser (PHM) and rubidium atomic frequency standard (RAFS) is competitive to those of the GPS IIF RAFS and Galileo FOC PHM and better than those of GPS IIR RAFS. The root-mean-square error of the 2-h clock prediction is better than 0.25 ns, and the validation result relative to the post-processed precise clock product is better than 0.4 ns. The time synchronization accuracy of better than 1 ns can be obtained based on only one TSTC station or an anchor station, and the standard deviation of Ka-band hardware delay is about 0.12 ns. It is believed that the ISL and the proposed algorithms will bring a significant upgrade in the estimation of BDS-3 broadcast clock; the broadcast clock accuracy will be greatly improved, and reliance on the ground segment will also be reduced significantly.
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Data Availability
The BDS-3 ISL measurements and TSTC data are available from the corresponding author upon request. The BDS-3 precise clock products can be found at ftp://ftp.aiub.unibe.ch/CODE/. The GPS antenna file can be found at ftp://igs.gnsswhu.cn/pub/gnss/products/mgex/.
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Acknowledgements
The authors are profoundly grateful to IGS and WHU for providing GNSS products. The reviewers are very appreciated for their helpful comments and remarks that have improved the manuscript. This study was supported by the National Natural Science Foundation of China (Grant Nos. 41931076, L1924033, 41904042 and 41804030, 41874039).
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Yang, Y., Yang, Y., Hu, X. et al. BeiDou-3 broadcast clock estimation by integration of observations of regional tracking stations and inter-satellite links. GPS Solut 25, 57 (2021). https://doi.org/10.1007/s10291-020-01067-x
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DOI: https://doi.org/10.1007/s10291-020-01067-x