Method of precise common-view time and frequency transfer based on BDS GEO satellites

Research on time and frequency transfer between stations through Global Navigation Satellite System (GNSS) carrier-phase observation data has been a hot topic for many years. However, the estimation of ambiguity has always been a challenge. Unlike Global Positioning System (GPS), BeiDou System (BDS) is a hybrid constellation with GEOstationary (GEO) satellites included. By virtue of the stationary characteristics of BDS GEO satellites, the method of precise common-view time and frequency transfer is proposed in this paper. With this method, high-precision time and frequency transfer can be achieved by utilizing the carrier-phase observation data from BDS GEO satellites and the precise products of the International GNSS Monitoring and Assessment System (iGMAS). The advantages are as follows. 1) Unlike traditional GNSS Medium Earth Orbit (MEO) satellites, BDS GEO satellites are always visible within their coverage area. Therefore, there will only be one ambiguity for this method over long periods of 1 month or more, which can be calibrated as a systematic error; 2) By using BDS GEO satellites, the time transfer results will not be affected by the phase wind-up effect. With sites in Europe and China, experiments based this method are performed, and the results show that: 1) For zero and ultra-short baselines, the Root Mean Square (RMS) values of the proposed method are better than 0.1ns; 2) For the short baseline (33 km), the performance of the proposed method is comparable to Two Way Optical Time and Frequency Transfer; 3) For the long baseline (700 km and 1750 km), the performance of the proposed method is better than Two Way Satellite Time and Frequency Transfer; 4) For inter-continental baselines (over 7000 km), the RMS value of the residuals with respect to Precise Point Positioning time transfer is at the sub-nanosecond level.