The Research on BeiDou Time Transfer via Precise Point Positioning based on Raw Observations

The Precise Point Positioning (PPP) technique has been widely applied in the field of satellite navigation and precise positioning. In the field of time transfer, the PPP technique is able to achieve high-precision BeiDou timing thanks to precise carrier phase observations and error correction models. Moreover the PPP technique can achieve time transfer which is independent of the baseline length globally due to the fact that the positioning of single station doesn’t involve the interaction with other stations. However the dual-frequency ionosphere-free linear combination method is usually carried out for the regular PPP method, in which the integrality property of the ambiguity parameters would be ruined and the precision of the combined observations is degraded by a factor which depends on the combined frequencies. On this basis, this article proposes a BeiDou PPP time transfer model based on the raw observation approach which implements the least-squared estimation for PPP network solution using the raw pseudorange and carrier phase observations of globally distributed stations to determine the clock offset of the receivers. Due to the unification of the bias parameters (e.g. the carrier phase bias of satellites) and the VTEC parameters of ionosphere, the estimated clock offsets of multiple stations can be unified within the same bias framework, which eliminates the influence of different GNSS products, different estimation strategies and the transfer and absorption of error between bias parameters and clock offsets and maintains the estimated clock offsets within the station network self-consistent to each other on the basis of the precision improvement.