Research On GNSS Time Synchronization Method Based On Precise Point Positioning

High-precision timing service is an important part of the national comprehensive PNT(Positioning,Navigation,Timing)system,and plays an important role in the fields of national defense and military,mobile communication,observation astronomy,etc.Recently,the time synchronization based on fiber link and laser link can meet the synchronization requirement of sub-nanosecond level,with the drawbacks of expensive equipment and limited dynamic flexibility.In this dissertation,we propose the time synchronization method based on precise point positioning(PPP)technique,by using global navigation satellite system(GNSS)timing service.According to the PPP time transfer results,the local clock is steered,making the local time represented by the local clock synchronized with the reference time,which can achieve the precision of sub-nanosecond.Additionally,our time synchronization method has the advantages of all-weather,full-coverage,high-precision and low-cost.The dissertation focuses on the core issue of PPP time synchronization,follows the research line from post-processing mode to real-time mode,from theoretical research to engineering implementation,and completes the following work:1.The approaches of improved parameter estimation and error compensation for PPP technique.The extended Kalman filter based on a prior coordinate constraint is adopted to improve the performance of PPP clock calculation.In view of the problem that the ambiguity validation requirement of PPP timing calculation is higher than that of positioning calculation,the ambiguity resolution with integer phase clock method is studied.The quality control strategies for ambiguity resolution with hypothesis test,statistical decision-making and residual test are proposed.The error compensations for GLONASS(GLObal NAvigation Satellite System)inter-frequency biases and BDS(Bei Dou navigation satellite System)satellite-induced multipath errors are studied.The experimental results prove that,the above methods can effectively improve the performance of PPP time transfer.2.The PPP time transfer in the post-processing mode.We firstly research BDS PPP time transfer.The performance of BDS-3 PPP time transfer is analyzed.Then,the error compensation for multi-GNSS observations is studied.The multi-GNSS PPP time transfer algorithm based on GPS/GLONASS/BDS/Galileo is proposed.The clock instantaneous re-initialization(CIR)algorithm based on the refined clock model(RCM)is proposed to solve the problem of day boundary in PPP time transfer,which is applied to multi-GNSS PPP and the integer clock method.The conclusions are as follows:(1)The time transfer results based on multi-GNSS PPP and integer clock method have different degrees of day boundaries.(2)The CIR algorithm effectively eliminates the day boundary in time transfer,weakens the noise of PPP parameter estimation,and improves the PPP time transfer performance based on multi-GNSS PPP and integer phase clock method.3.The PPP time transfer and monitoring in the real-time mode.Firstly,the principle of time monitoring is introduced.Then the product quality of GPS(Global Positioning System),GLONASS,BDS and Galileo was compared and analyzed with the example of CLK93 real-time ephemerides.Based on the GPS PPP time transfer and monitoring,the multi-GNSS PPP time transfer and monitoring algorithm based on GPS/GLONASS/BDS/Galileo and the multi-GNSS with integer phase clock algorithm are proposed.The corrections of phase and pseudo-range biases in the multi-GNSS with integer phase clock algorithm are studied.Finally,the performance of time transfer and monitoring with different algorithms is analyzed and compared.The conclusions are as follows:(1)At present,the performance of time transfer and monitoring with ambiguity resolution is better than that with multi-GNSS PPP.(2)The multi-GNSS observations can enhance the reliability of PPP time transfer,and multi-GNSS with ambiguity resolution has the best performance.4.The time synchronization based on real-time PPP technique.The PPP time synchronization method is proposed to meet the demands of high-precision and flexibility for the distributed systems.Firstly,the characteristics and application scenarios of PPP time synchronization are analyzed.Then the problem of time reference for real-time ephemerides is solved.Finally,the software and hardware implementation of PPP time synchronization test system are designed.The traditional GNSS time synchronization,GPS PPP and multi-GNSS PPP time synchronization are tested.The conclusions are as follows:(1)PPP time synchronization method is suitable to solve the problem of time synchronization for the sparse distributed system in wide area space.(2)The product quality and time reference stability of CLK53 and CLK80 performed well to provide the time reference support for PPP time synchronization.(3)The root mean square error(RMSE)of traditional GNSS time synchronization experiment was about16.7 ns;the RMSE of GPS PPP and multi-GNSS PPP time synchronization was about0.41 ns and 0.33 ns,respectively.The test results demonstrate that,the performance of PPP time synchronization is better than the traditional GNSS method.In addition,and the multi-GNSS PPP time synchronization is more reliable and performs better than GPS PPP.