Inter-satellite time synchronization and ranging link assignment for autonomous navigation satellite constellations

Abstract

The BeiDou Navigation Satellite System (BDS) has equipped its new-generation satellites with inter-satellite link (ISL) payloads for the autonomous operation. In the autonomous operation mode, the navigation constellation realizes the time unification and orbit determination through inter-satellite time synchronization and ranging, so as to self-maintain the time-space reference. For solving the problem of inter-satellite time synchronization and ranging link assignment for autonomous navigation constellations, assignment algorithms of time synchronization links based on satellite layers, as well as ranging links combining satellite selection and weighting, were proposed. The ISL assignment simulation of the BeiDou-3 global constellation was implemented using our algorithms, and the assignment was completed almost within 5 cycles. According to the statistical results, using 56.6% of ISL timeslots, the constellation had achieved the optimal mode of inter-satellite time synchronization with the least skips; the number of inter-satellite ranging links established per minute of each satellite was not less than 10, which met the constraint of the minimum links; the average PDOP of ranging links of the MEO, GEO and IGSO satellite was about 1, 1.6 and 1.5 respectively, close to the lower reference limit. It was demonstrated that the ISLs were reasonably assigned for the demand of inter-satellite time synchronization and ranging with constraints of the TDMA system, narrow beams, the quantity of payloads and inter-satellite visibility.