The Research On PPP Time Comparison Technology With BeiDou-3

High precision time comparison by using satellite navigation system plays an important role in time keeping,time synchronization and traceability.The time keeping laboratories participate in the calculation of Coordinated Universal Time（UTC）by different time comparison links（include common view,all-in-view and carrier phase time comparison technologies based on Global Navigation Satellite System,and two way satellite time and frequency transfer technology based on communications satellite）.Now,the Circular-T bulletin which is published by Bureau International des Poids et Mesures（BIPM）every month show that GPS code pseudorange and carrier phase time comparison technologies are adopted by more than 80%time keeping laboratories to steere their UTC（k）to UTC.With the development and application of multi-technology time comparison methods,the multi-technology time comparison methods are encouraged in the calculation of UTC to ensure the continuity,stability and reliability of international time comparison links.At the 21^th Consultative Committee for Time and Frequency（CCTF）working group meeting,the BIPM experts recommended that the BDS time comparison technology can be taken as the backup time comparison link in UTC calculation with other navigation satellite systems.Therefore,the research on PPP time comparison technology with Bei Dou-3 is an important work for the calculation of UTC,forthemore the BDS-3 PPP is officially adopted by BIPM for international time comparison in future.The research is also great significance to promote the the international application of BDS.The BDS-3 PPP time comparison technology is studied in this paper based on China’s national standard time generating system which output the standard time signal UTC（NTSC）,the data of undifferenced ionosphere-free combination and international time comparison network.The key technologies of the BDS time comparison link have been improved and upgraded,the application in time scale calculation is realized,and the results are evaluated.The main research work and related conclusions are as follows.（1）The high precision time comparison technologies of BDS-3 common view,all-in-view and PPP were researched and analyzed,and the error correction algorithm for BDS-3 high precision time comparison was described.Based on the measurement data,the error related with satellites,signal transmission paths and station were analyzed and evaluated.The GNSS simulator was used,and the time delays at different frequency channel of BDS-3 receiver were measured.The results show that the standard deviation of delay at B1C,B2a,B1I and B3I are 0.0334ns,0.0320ns,0.0571ns and 0.0361ns respectively,and the B2a is the smallest.When the three time comparison technologies were used,and the high precision time comparison between different time keeping laboratories was achieved.The results show that the noise of time comparison in long baselines（baseline length≥1000km）or short baselines（baseline length≤10m）by using ionosphere-free combination of B1C and B2a is smaller than combination of B1I and B2I,B1I and B3I in BDS-3 common view and all in view.When the BDS time comparison technology is applied to common clock short-baseline time comparison,the standard deviations of the B1C and B2a ionosphere-free combination relative to BDS-2 frequency is improved by 30%.For long baseline time comparison,the frequency stability of time difference between UTC（NTSC）and UTC（TP）by using the B1C and B2a combination is significantly improved relative to BDS-2,and the short-term stability is improved by more than35%.In the BDS-3 PPP time comparison,the result of short baseline time comparison by using BDS-3 PPP and GPS PPP are better than 0.1ns,and the standard deviations of BDS-3 PPP is slightly better than GPS.When the BDS-3 PPP is applied to the long baseline time,the time difference between UTC（NTSC）and UTC（TP）have the same trend as the results released by BIPM,the residual of result relative to BIPM is within±0.5ns.（2）The prior information of precise zenith tropospheric delay is used which is released by IGS or local station monitoring,and the application of considering prior information in BDS-3 PPP time comparison is proposed.The error terms are corrected in the residual matrix by using the prior zenith tropospheric delay,station coordinates and phase deviation information,and the accuracy of receiver clock error estimation is improved.The results show that the standard deviation of common clock time different between two receivers in short baseline with prior information is improved by more than 20%than that without the prior information.When the priori zenith tropospheric delay and phase deviation information are applied to the different time keeping laboratories for long baseline time comparison,the results show than the frequency stability and time variance are improved relative to uncorrected,and the improvement was pronounced at an average time of 60 seconds.When the priori antenna coordinates information is applied to the time comparison,the results show that the frequency stability and time variance is improved relative to uncorrected,and the improvement is reduced in the later period.（3）In the BDS-3 PPP time comparison,the state covariance matrix in the Kalman estimator is used as a weight factor,and the research on BDS-3 PPP with weighted state covariance matrix is proposed.The weighted state covariance matrix is used to reduce the divergence and slow convergence at the beginning of data,and the accuracy of BDS-3 PPP time comparison is improved.When the weighted state covariance matrix is used for short baseline time comparison,the standard deviation of result with weighted filtering is 0.0517ns,and the one-way filtering is 0.1243ns,the weighted filtering has increased by 58.41%relative to one-way.For long baseline time comparison,the frequency stability and time variance of time comparison difference with weighted state covariance matrix between different laboratories is better than one-way filtering.（4）The data of BDS-3 PPP,GPS PPP and TWSTFT which are operated from major time keeping laboratories in the world,and the BDS-3 PPP and GPS PPP,BDS-3 PPP and TWSTFT fusion time comparison technology are carried out.The results show that the short term stability of time difference is improved significantly by BDS-3 PPP and other time comparison link fusion.When the BDS-3 PPP and TWSTFT fusion time comparison is used in the time difference between two stations,the residual of results relative to GPS and TWSTFT fusion is within±0.2ns,and the short-term stability of TWSTFT is improved.The standard deviation of fusion results was increased by 35.43%and 45.91%relative to TWSTFT and BDS-3 PPP respectively.Meanwhile,the data of BDS-3 time comparison network and atomic clock from international time keeping laboratories are used,and the joint atomic time calculation based on BDS-3 time comparison link is carried out.The algorithm of Kalman filtering are adopted,the joint atomic time calculation were achieved and studied with atomic clock,BDS-3 PPP and GPS PPP time link between different time keeping laboratories.The results show that when the Kalman filter algorithm is used in atomic time calculation,the equation of state is estimated by improving noise variation of time link and atomic clock in real time.The stability of time scale calculation is guaranteed,and the stability of the atomic time calculation with BDS-3time link is slightly better than GPS,the stability of atomic time calcuated with cesium clocks and hydrogen clocks at E-15/day and E-16/day level respectively.