Cooperative time synchronization via laser/radio inter-satellite links in navigation constellations

Time synchronization is vital in positioning, navigation, and time services in satellite navigation systems. Under the autonomous operation mode, time synchronization of the entire constellation is realized via inter-satellite links to support normal navigation and positioning services. Compared with radio inter-satellite links, continuous laser links are able to provide clock deviation measurements with the same time tag and higher accuracy. In this paper, cooperative time synchronization via laser/radio inter-satellite links is proposed. First, a time comparison network based on the continuous laser inter-satellite link is established, and an inter-satellite time reference is generated using the weighted average algorithm. Then, through radio inter-satellite links, clocks of satellites outside the laser inter-satellite link network are traced to the time reference by using the distributed Kalman filter algorithm, thereby realizing time synchronization of the entire constellation. Finally, a simulation based on the new generation of BeiDou constellation is performed. Results show that the stability in the short and medium terms of the time reference generated via the cooperative time synchronization method is significantly improved compared with that generated via radio inter-satellite links. The standard deviation of the time synchronization error in the constellation is better than 0.05 ns.