| As the on-board time reference for GNSS navigation and positioning,the performance of the atomic clock determines the service life of the satellite and directly affects the service quality of satellite navigation,positioning,and timing.Due to the influence of the external environment and the aging of the equipment,the performance of the atomic clock will change over time during the operation of the satellite.Evaluating the performance of satellite clocks in orbit is an important means to grasp the operating status of satellite clocks.It is of great significance to the integrity monitoring of the navigation system,system performance evaluation,and satellite clock error determination and prediction.This paper uses the precision clock offset products of the i GMAS WHU Analysis Center to study the long-term performance evaluation of the GNSS on-board atomic clock and the selection of the reference clock in the estimation of the real-time satellite clock offset.The main research content and research results of the thesis are as follows:(1)The article analyzes the research status of on-board atomic clock performance evaluation,introduces GNSS satellite clock performance evaluation indicators and characteristic analysis methods,and expounds the basic theory of data preprocessing methods.(2)The article evaluates the long-term performance of GNSS on-board atomic clocks.The experimental used 5 years of GNSS precision clock offset data and 2 years of BDS3 precision clock offset data to analyzes the physical characteristics of GPS,GLONASS,GALILEO and BDS satellite clocks from aspects of phase,frequency,frequency drift,fitting residuals,fitting accuracy,etc.Experimental results show that due to satellite clock switching and ground monitoring station calibration,there are multiple frequency and phase modulation operations during the long-term operation of the satellite clock.Compared with the other three systems,the continuity of BDS2satellite clock offset data is poor,and frequency and phase modulation operations are frequent.The clock offset data preprocessed by the MAD method is fitted by quadratic polynomial fitting model by day and day.It is found that the frequency drift value of GPS,GLONASS,GALILEO,and BDS3 system satellite clocks are all in the order of E-19,and the frequency drift value of the BDS2 system satellite clock is in the order of E-18.The average fitting accuracy of GPS,GLONASS,GALILEO,BDS2 and BDS3are 0.62ns,0.72ns,3.76ns,1.71ns and 0.85ns,respectively.(3)The stability of the satellite clock of the GNSS system is analyzed by using the overlapped Hadamard variance.It was found that the stability of the on-board atomic clocks of GPS,GLONASS,and BDS2 systems remained at the same order of magnitude,with their thousands-seconds stability at the level of E-13,and ten-thousand-seconds stability at the level of E-14.The thousand-second stability and ten-thousand-second stability of GALILEO and BDS3 on-board atomic clocks are both on the order of E-14.The thousand-second stability of GPS,GLONASS,GALILEO,BDS2 and BDS3 on-board atomic clocks are 2.34×10-13,3.16×10-13,3.07×10-14,1.47×10-13 and 2.92×10-14,and the ten-thousand-second stability is 8.44×10-14,11.2×10-14,1.18×10-14,5.19×10-14,and 1.10×10-14.The stability of the BDS3 on-board atomic clock is slightly better than that of the GALILEO,and the stability of the GLONASS on-board atomic clock is worse than other systems.(4)Based on the final orbit of WHU,the G24/G25 satellites with different stability are used as the reference clock,and the square root information filtering method is used to estimate the real clock difference.Through stability analysis and accuracy comparison,it is found that the thousand-second stability of the real-time clock offset calculated with G24/G25 as the reference clock is 8.31×10-13 and 1.19×10-13,respectively.Compared with the thousand-second stability of the post-precision clock offset,the stability of the real-time clock offset with the G25 satellite clock as the reference clock is better than the short-term stability of the post-precision clock offset.The accuracy of the real clock offset calculated based on the reference clocks with different stability is within 0.1ns,which meets the real-time accuracy requirements.When the ground station data is seriously missing,the satellite atomic clock can be used as a reference to estimate the real-time clock offset to meet the real-time positioning requirements. |
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