| High-precision time transfer is an indispensable part of the time system maintenance,and it is also a necessary means to realize the function of the timing service.Currently,time transfer technology that based on Global Navigation Satellite System（GNSS）has become a research hotspot in the field of high-precision time transfer applications due to its full-time,full-space,high-precision,and low-cost features.The GNSS carrier phase common-view time transfer technology can eliminate the satellite clock error and hardware bias through difference,and the spatial correlation error is weakened to a certain extent,which is more conducive to the realization of real-time time transfer and the resolution of ambiguity.But this method will be affected by the number of common-view satellites.With the construction and completion of multiple GNSS systems,the number of visible satellites in the sky has increased significantly,and the common-view range between stations has also been greatly improved.Making better use of the frequency band resources of each satellite system not only avoids the waste of signal resources,but also is expected to further improve the efficiency and precision of time transfer.However,while GNSS multi-system fusion brings many advantages,it also brings new challenges to data processing,such as decoupling of multi-mode and multi-system fusion parameters,handling of receiver hardware inter-frequency/inter-system bias,data fusion processing strategy,etc.These issues still need to be further refined and resolved.In addition,due to the influence of GNSS pseudorange observation noise and the float solution of phase ambiguity,the precision,continuity,and stability of time transfer need to be further improved.Therefore,the continuity problem and ambiguity fixation in GNSS carrier phase time transfer are issues that urgently need to be further studied.In view of above issues,the thesis mainly focuses on the theory and method of BDS/GNSS carrier phase real-time common-view time transfer based on inter-station single-difference uncombined model,focusing on the fixation of GNSS single-difference ambiguity in BDS/GNSS medium-distance common view time transfer to obtain the fixed solution of receiver clock difference,as well as the model and method for long-distance real-time time transfer using multi-frequency and multi-constellation GNSS observations.Meanwhile,the real-time detection and repair of cycle slip in GNSS phase observation,the time variation characteristics of receiver differential hardware bias and its calibration,the continuity issue of time transfer,the decoupling method between parameters and the data fusion processing strategy in multi-frequency and multi-constellation fusion long-distance common view time transfer have been thoroughly and systematically studied.The main research contents and contributions of this thesis are as follow.1.A modified geometry-free phase combination（MGF）is proposed,which can efficiently detect and repair cycle slips on each frequency phase observation of low elevation satellite in real time.The traditional geometry-free（GF）phase combination can effectively detect the occurrence of cycle slips,but another pseudorange-phase combination,such as the Melbourne-Wübbena combination,is needed to determine the cycle slips.However,the pseudorange measurement noise has an adverse effect on the reliability of cycle slip determination.Aiming at the shortcomings of traditional GF phase combination that can only detect cycle slips,but cannot determine on which carrier phase the cycle slip occurring and its size,a modified geometry-free phase combination（MGF）is proposed.When the cycle slip is constrained or repaired to within±4 cycles by using phase-pseudorange observations,MGF method can directly determine the cycle slip within[-4,4]and[-3,3]cycles on BDS/GPS B1/L1 and B2/L2 carrier phases,so it is very effective for small cycle slips of low elevation satellites.The simulation experiments and statistical results show that MGF can accurately detect and successfully repair the cycle slips of satellites above 15 degrees.For cycle slips within 3 cycles of satellites whose elevation are below 15 degrees,MGF can also detect 100%accurately,and the repair success rate is better than 99.7%;Compared with the GF+MW method,the MGF method has a higher success rate in small cycle slip detection and repair of low elevation satellites,reduces the number of unrepaired cycles slips from 4005 to 141 and improves the success rate from 99.426%to 99.980%.What’s more,the computational efficiency is improved by about 144 times.2.A fast and accurate calibration method for BDS/GNSS receiver differential phase bias（DPB）and differential code-phase bias（DCPB）based on the zero/short baseline mode is proposed.The GNSS receiver hardware bias is the most important systematic error that affects the accuracy,stability,and reliability of time transfer.At present,the receiver differential phase bias is still dominated by hybrid solutions,which hinders its further application.Firstly,the time variation characteristics of the BDS/GNSS receiver differential hardware biases are analyzed and modeled,and the factors that cause the GNSS receiver hardware bias change are experimentally studied.Then,the relationship function between the change of BDS/GNSS receiver differential hardware bias and temperature is established.The random characteristics of BDS/GNSS receiver differential hardware bias is analyzed.It is recommended to use the random walk model to describe the change process of the receiver’s differential hardware bias.In summary and statistics,the process noise variance of receiver DPB is about 1×10-11～1×10-10 m2/s,and the process noise variance of receiver DCPB is about 1×10-7～1×10-6 m2/s.Finally,a method for fast calibration of receiver DPB and DCPB,which considers the prior constraints of receiver differential hardware bias and its random characteristics,is proposed.This method can fix the between-receiver single-difference ambiguity in less than 2 epochs on average,and obtain the estimation of receiver DPB with millimeter accuracy and receiver DCPB with decimeter accuracy.The experimental results show that for the type of receiver used in the experiments,the size of the receiver DPB is on the centimeter level,and the size of the receiver DCPB is on the decimeter to meter level.It provides a reference for the reasonable modeling and constraints of the receiver’s differential hardware bias in GNSS time transfer.3.Based on the rapid fixation of the single-difference ambiguity between receivers,the10-picosecond-level precision time transfer is realized for short and medium distances（≤35 km）,and the continuity of time transfer and the consistency of precision are guaranteed.Currently,the precision of the ambiguity float solution of the receiver clock difference in the BDS/GNSS carrier phase time transfer is at the sub-nanosecond level.Aiming at the fixation of between-receiver single-difference ambiguity and the problem of continuity in BDS/GNSS phase common-view time transfer,the BDS/GNSS carrier phase real-time common-view time transfer method under the medium and short distance conditions is studied.Through weighted constraints on the between-receiver differential hardware biases and the inter-station single-difference ionospheric delay,the between-receiver single-difference ambiguity of the medium-and short-baseline（≤35 km）is fixed,and the relative phase clock difference between the receivers is obtained.The fixed solution realizes the time transfer with ten picoseconds level precision,and can realize the real-time time synchronization of multiple terminal devices.Thanks to the successful fixation of between-receiver single-difference ambiguity,the discontinuity of daily boundary of time transfer is eliminated,and the stability and precision of the time transfer are ensured.4.The model and method for multi-frequency and multi-constellation fusion long-distance real-time common-view time transfer based on the inter-station single-difference,uncombined GNSS phase and pseudorange observations has been developed.In response to the requirement of multi-mode and multi-system fusion long-distance phase common-view time transfer,the BDS/GNSS multi-frequency and multi-constellation fusion long-distance phase common-view time transfer method based on the inter-station single-difference,uncombined model is studied.The time-varying receiver differential hardware biases and inter-system bias parameters are introduced into the inter-station single-difference uncombined model,and the decoupling of the receiver differential hardware bias parameters and receiver clock error parameters,inter-station single-difference ionospheric delay parameters,and between-receiver single difference ambiguity are realized through parameter reconstruction theory.It can not only obtain the time series of receiver differential hardware bias and inter-system bias,but also improve the robustness of time transfer.The experimental results show that compared with GNSS single system,GNSS multi system fusion has higher time transfer precision and better stability;The robustness of GNSS multi system fusion time transfer is improved by taking into account the influence of the variation of receiver differential hardware bias and inter system bias;Compared with the time link results of the International Bureau of Time（BIPM）,the results show that the maximum standard deviation between the two is about 0.057 ns,and the minimum is about 0.024 ns,indicating that the two have good consistency. |
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