Research On Time Synchronization Technology Of Low Orbit Satellite System Based On GNSS And Inter-satellite Link

With the establishment and improvement of the national integrated PNT system,LEO navigation enhancement system,as an important part of the future integrated PNT system,is also developing rapidly.As the cornerstone of PNT system,time has an important impact on the performance of LEO navigation enhancement system.At present,the research on satellite time synchronization is mainly based on a small number of satellites in the existing navigation constellation,and there is less research on the high-precision time synchronization technology of massive LEO satellites.It is also a great challenge to realize high-precision time synchronization under the trend of massive and low-cost in the future.Based on the two scenarios of Beidou or GNSS availability and unavailability,this paper studies the high-precision time synchronization technology of LEO constellation.The main contents are as follows.In the scenario that Beidou or GNSS is available,the visibility and coverage of the Beidou constellation are analyzed.Secondly,the PPP time synchronization performance using the post precision ephemeris is analyzed based on the simulation data and the measured data of the Cosmic satellite provided on the official Cosmic website.The results show that under the condition of massive satellite constellation,the time synchronization performance of each satellite based on simulated observation data is similar,and according to the calculation results of measured data,the time synchronization accuracy of nanosecond level can be achieved under the condition of calibrating the receiver delay in advance.Firstly,the satellite orbit and clock error correction information broadcast in real time by IGS Analysis Center Using SSR information format and the forecast precision ephemeris IGU products released by IGS analysis center are introduced.The time synchronization performance of LEO satellites is analyzed and compared based on the two products,At the same time,the performance of the product is compared with that of the product based on post precision ephemeris.Finally,aiming at the interruption of SSR correction information due to network problems in practical application,a method based on cumulative standard deviation is proposed to select the calculated clock error based on IGU and prediction clock error.The results show that the maximum error between this method and the theoretical optimal selection is less than 0.5ns.In the scenario that Beidou or GNSS is unavailable,the method of autonomous time reference establishment and time synchronization of LEO satellite constellation is proposed,and the performance of time reference establishment of LEO satellite constellation under different number of atomic clock configurations is analyzed.Secondly,aiming at the configuration optimization of atomic clocks in massive constellations,a star clock configuration optimization method based on K-means clustering is proposed,and the method is improved.It is analyzed under the conditions of different constellation configurations,the number of atomic clocks configured and different link errors.The results show that the improved method can better improve the time synchronization accuracy of the whole network.