| The satellite navigation system is of great significance to a country's political,economic,and military aspects.In special circumstances such as wars and unpredictable natural disasters in the country,the enemy can easily use missiles and other high-precision long-range weapons to strike the ground control system(OCS)for military strikes.In order to ensure the normal operation of satellite navigation systems in the absence of ground stations,the United States GPS proposed the concept of autonomous navigation at the beginning of its establishment.Autonomous navigation can ensure the long-term,stable and autonomous operation of the navigation system and provides services based on the measurement and communication functions of the inter-satellite link through the constellation's autonomous operation and ephemeris update when the satellite navigation system cannot obtain ground operation control support for a long time.In the autonomous navigation mode of the satellite,since there is no external clock input,the system time of the satellite navigation will deviate from the real time,which seriously affects the user's timing accuracy.Therefore,maintaining the autonomous time of the satellite navigation system is an important part of satellite autonomous navigation.In the autonomous navigation mode,how to use inter-satellite link data to achieve on-board autonomous time maintenance is an important issue in the process of Beidou construction.In recent years,there have been some studies on autonomous time maintenance in China,and some meaningful results have been obtained,but simulation data is basically used,and the measured inter-satellite link data is rarely used for autonomous time maintenance.Therefore,this article aims to study the key technology of autonomous time maintenance and apply it to the autonomous time maintenance based on the measured inter-satellite link of Beidou-3,hoping to provide technical support and reference for the establishment of the autonomous time base of our Beidou system.The main work and contributions of this article are as follows:(1)This article carries out the frequency stability analysis of the 18 MEO star clocks of the Beidou basic navigation constellation.The results showed that the stability of each satellite showed a downward trend and the overall stability of the hydrogen clock was 2.2×10-14Except for C19,the overall day stability of rubidium clock is 2.9×10-14,and the day stability of C19 satellite is 1.7×10-13.(2)This paper summarizes the basic theory of establishing a time-scale algorithm,analyzes the characteristics of different algorithms,deduces the mathematical model of autonomous time base maintenance based on the observations of inter-satellite links,and designs the Beidou autonomous time reference maintenance program based on inter-satellite links.(3)The measurement characteristics of the inter-satellite bidirectional ranging data of the Beidou-3 Ka band are analyzed.The results show that the measurement noise level of the inter-satellite bidirectional ranging data is better than 0.3ns.(4)Using the simulated inter-satellite link,orbit and clock difference data,the ALGOS algorithm,Kraw algorithm and KPW algorithm were used to establish an autonomous time system on the planet,which achieved 60 consecutive days of autonomous time maintenance.The results show that the maximum deviation of the on-plane autonomous time system with ALGOS algorithm running continuously for60 days is 16.2ns,and the sky stability is3.0×10-15.The maximum deviation of the on-board autonomous time system established by the Kalman raw algorithm and KPW algorithm from the ideal time is about 8ns,and the natural stability is 4.7×10-15and 3.7×10-15,respectively.(5)Based on the conclusion of the simulation results,using the data of the Beidou-3 inter-satellite link,and using the designed ALGOS algorithm and KPW algorithm,it has achieved 30 consecutive days of on-board autonomous time maintenance.The result shows:For the ALGOS algorithm,the maximum time deviation of using 18stars for autonomous time maintenance is 62ns,and the time base day stability is0.96×10-14;For KPW algorithm,the maximum time difference of the on-board autonomous time reference established using 18 stars is 10.4 ns,and the sky stability is1.08×10-14;The maximum time difference of the autonomous time reference on the star established by using 8 on-orbit hydrogen clocks is 10.6ns,and the day stability is1.07×10-14;Using 10 on-orbital rubidium clocks to establish an autonomous time reference time difference is up to 45ns,its day stability is 1.23×10-14. |
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