Research On Methodology Of Multi-frequency And Multi-GNSS Precise Point Positioning Time Transfer

High-precision time plays an important role in military and people's livelihood economy.Precision time transfer method is an important factor in establishing and maintaining high-precision national standard time,is the key to a precision time system,and is the prerequisite for maintaining precision time synchronization in different places.Global Navigation Satellite System(GNSS)precise point positioning(PPP)method is one of the most important means of precision time transfer,due to the high precision,low-cost equipment,no distance restrictions,all-weather and many other features.It has become a hot research topic in the field of high-precision time application.With the rapid development of GNSS,the number of available frequency bands and satellites is also increasing rapidly;Therefore,GNSS PPP time transfer still has many key issues that need to be further studied and solved.How to make full use of the current multifrequency and multi-GNSS to achieve more high-precision time transfer has become a hot topic in the time community.This paper focuses on the core issue of multi-frequency and multi-system PPP time transfer.Our works mainly include four parts: the multi-frequency GNSS PPP method,GNSS real-time PPP time transfer method,receiver clock modeling,GNSS precision timing.To perfect the current multi-frequency and multi-system PPP time transfer theory,the main research content is as follows:1)The multi-frequency GNSS PPP time transfer method was proposed,the function and stochastic model of the triple-and quad-frequency PPP time transfer method were given.Then,the feasibility and advantage of the multi-frequency PPP time transfer model proposed was verified through experiment.2)According to the needs of different users,single-frequency PPP time transfer method was proposed,which can be applied to low-cost receivers.Three singlefrequency PPP time transfer models were given,namely,the ionospheric as parameter estimation,the ionospheric-free model,the ionospheric model correction,according to the ionospheric processing strategies.The expression of the three single-frequency PPP time transfer models was deduced.The experimental results demonstrated that the single-frequency PPP time transfer accuracy with ionospheric as a parameter was the highest,and sub-nanosecond time transfer accuracy could be achieved.3)The current PPP time transfer focuses on the post-processing mode,hence,realtime multi-GNSS PPP time transfer was presented in this work.The experimental results showed that the real-time GNSS PPP could realize the accuracy of time transfer better than 0.5 ns.The Galileo-only PPP time transfer was comparable to that of GPS-only,and better than GLONASS.In addition,the multi-GNSS PPP time transfer outperforms GPS-only.4)A clock-jump detection method was proposed,which can detect the clock jump greater than 0.5 ns in real time,due to the problem of clock jump,which has not been paid attention to by the current receiver clock model.The method can effectively detect the clock jump problem and provide effective support for the receiver clock modeling.5)Currently,receiver clock offset is estimated as white noise.It is difficult to consider the correlation of the receiver clock offsets between adjacent epochs.Hence,a receiver clock model was proposed and applied to PPP time transfer.The results indicated that,based on the clock-jump detection method,the proposed clock model could effectively improve the accuracy and frequency stability of time transfer,especially the short-term stability.6)Based on the GNSS PPP time transfer method studied in our work,the GNSS precision timing method based on IGS(International GNSS Service)real-time product was proposed due to the nanosecond magnitude for the current GNSS timing accuracy.Experiments illustrated that the timing accuracy of method proposed in this paper could achieve better than 0.5 ns.The timing method is not limited by the number of users,satellite system,user location.