Objectives Inter-satellite link of BeiDou navigation constellation has better application value in remote time synchronization among space crafts because it can eliminate a part of channel error by dual-one way ranging method and it has a flexible link-building way. But current inter-satellite link system time synchronization method is highly depending on ephemeris parameters, and when one user has high orbital velocity and incomplete space position information, the conventional time synchronization algorithms are limited.
Methods This study is carried out mainly aiming at the application requirements of expanding users in navigation constellation inter-satellite link technique, and an improved dynamic delay correction method based on single-point pseudorange epoch conversion is proposed. The method only needs to synchronize the movement speed of one of the two sides and the position of both sides to correct the motion delay error. The advantage of this method is that the two-way time synchronization accuracy is not affected by one user's speed error with poor speed accuracy. In this paper, the basic theory and implementation method of the algorithm is given, and requirements to positional accuracy of spacecraft are analyzed. The algorithm is verified by simulation and test data.
Results From the simulation and test data analysis, this work could be classified some major clusters apparently: (1) By analyzing the requirement to the position accuracy of spacecraft in the course of time synchronization, the time synchronization accuracy between BeiDou-3 MEO (medium earth orbit) and spacecraft can arrive to 0.01 ns when the position error of spacecraft is less than 230 m, and the time synchronization accuracy between BeiDou-3 IGSO/GEO (inclined geosynchronous orbit/geostationary orbit) and spacecraft can arrive to 0.01 ns when the position error of spacecraft is less than 290 m. (2) By using single-point pseudorange epoch conversion method, the time synchronization accuracy between Beidou-3 M1 and spacecraft can arrive to 0.003 ns when the position error of spacecraft is 200 m and it has no velocity message. (3) By analyzing the inter-satellite link measuring data between BeiDou-3 M7 and Xi'an Ka ground station, the time synchronization accuracy between the satellite-ground is about 0.07 ns.
Conclusions This algorithm based on single-point pseudorange epoch conversion can solve the time synchronization with incomplete ephemeris parameters, and its time synchronization accuracy is better than 0.1 ns. This method also adapts to the precise time transferring among the low-orbit spaces and deep-space detectors.
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