As a system of ranging and positioning based on time transfer, the timekeeping ability of a navigation satellite constellation is a key factor for accurate positioning and timing services. As the timekeeping performances depend on the frequency stability and predictability of satellite clocks, we propose a method to establish a more stable and predictable space time reference, i.e., inter-satellite link time (ISLT), uniting the satellite clocks through inter-satellite links (ISLs). The joint timekeeping framework is introduced first. Based on the weighted average timescale algorithm, the optimal weights that minimize the increment of the ISLT timescale are determined and allocated to the clock ensemble to improve the frequency stability and predictability in both the long and short term. The time deviations with respect to the system time of nine BeiDou-3 satellites through multi-satellite precise orbit determination (MPOD) are used for joint timekeeping evaluation. According to the Allan deviation, the frequency of the ISLT is more stable than the nine satellite clocks in the short term (averaging time smaller than 7000 s), and its daily stability can reach 6 × 10⁻¹⁵. Meanwhile, the short-term (two hours) and long-term (10 h) prediction accuracy of the ISLT is 0.18 and 1.05 ns, respectively, also better than each satellite clock. Furthermore, the joint timekeeping is verified to be robust against single-satellite malfunction.

A hybrid network comprising both laser and microwave inter-satellite links (ISLs) has been established within navigation satellite constellations. Leveraging the flexible construction of microwave links and the high ranging accuracy and communication efficiency of laser links, a collaborative link scheme was proposed. With Position Dilution of Precision (PDOP) and communication delay serving as performance indices, multi-objective optimization models for hybrid ISLs were established, incorporating complex constraints. Assignment algorithms based on Non-dominated Sorting Genetic Algorithm II (NSGA-II) and the standard Genetic Algorithm (GA) were proposed. The effectiveness of the collaborative link scheme and the assignment algorithms was evaluated through simulations. The results indicated that each satellite could establish 18.4 ISLs per minute on average. The average PDOP of hybrid links for geosynchronous orbit (GEO) / inclined geosynchronous orbit (IGSO) and medium Earth orbit (MEO) satellites were 1.5 and 1.1, respectively. The average communication delay from overseas to anchor satellites via pure laser links was approximately 0.15 s. When some laser links were unavailable, the remaining available laser ISLs could collaborate with microwave ISLs to provide a transmission route with a maximum delay of 3.2 s. It was demonstrated that the collaborative link scheme and assignment algorithms effectively addressed the hybrid link assignment problem, considering both inter-satellite ranging and communication.