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Enhancing multi-GNSS time and frequency transfer using a refined stochastic model of a receiver clock

The precise point positioning (PPP) technique has been widely used for remote time and frequency transfer. With the development of the multiple global navigation satellite system (multi-GNSS), the PPP technique with multi-GNSS has been studied in order to improve the performance of time and frequency transfer. However, the day boundary problem, model correction residuals, and noise may affect the clock estimation in multi-GNSS PPP. In this study, we refine the receiver clock parameter using a stochastic model of an atomic clock to solve these problems. First, the receiver clock modeling method is proposed to reduce the additional noise in the clock estimation. Second, we present the clock instantaneous reinitialization method to solve the day boundary problem. Finally, an enhanced multi-GNSS time transfer method can be obtained. We combine the observations of the global positioning system, the global navigation satellite system, the BeiDou satellite navigation system, and Galileo for the experiment. Standard deviation (STD) and modified Allan deviation (MDEV) are employed to evaluate our approach. The experimental results showed that after using the enhanced transfer method, the STD values of the time transfer results with respect to the International GNSS Service final products were reduced, with an average reduction ratio of 18.3% for all time links. Meanwhile, the stability of frequency transfer results was effectively improved, especially in the short term. The average reduction ratios of MDEV at 30, 60, and 120 s were up to 55.5%, 39.1%, and 23.2%, respectively. Our approach can effectively solve the problem of discontinuities, model correction residuals, and noise in clock estimation, and significantly improve the performance of multi-GNSS time and frequency transfer.