# Onboard Centralized ISL-Building Planning for LEO Satellite Constellation Networks
## 6 Conclusions  
ISLs are increasingly being used in LEO constellation projects. In this context, achieving efficient and optimized satellite network ISL-building planning for ISLs is of great significance for realizing efficient LEO satellite constellation networking. For the first time, we implemented an onboard centralized network ISL-building planning method, thus decoupling ground station control and enhancing the autonomy of network management. We provided a solution to the visibility calculation process in the onboard centralized ISL-building planning scheme by using the antenna as the calculation object. The visibility calculation process takes into account the antenna installation angle, the turntable rotation threshold, and the satellite attitude. Furthermore, the visibility calculation realizes the simultaneous link-building of multiple antennas, making it closer to the engineering implementation. Then, the link-building planning process in the onboard ISL-building planning management scheme was modeled using ILP. In order to solve the problem that ILP computational complexity is too high, making it unsuitable for onboard deployment, we presented a link-building planning method based on topology stability optimization, which  
Figure 14: Topology switches of the two constellations. Figure 13: ISL duration of the two constellations. \begin{table}
\begin{tabular}{l c c} \hline \hline  & **Walker Delta 32/4/1** & **Walker Delta 500/20/1** \\ \hline Avg. Linked Ratio & 82\% & 89\% \\ \hline Avg. Delay & 2.74 hops & 11.18 hops \\ \hline Max. Delay & 4.00 hops & 20 hops \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The average linked ratio, the average delay, and maximum delay of the two constellations. significantly increased the network's usability. The simulation results demonstrated that any satellite in the satellite network can implement the onboard centralized ISL-building planning as the dynamic primary satellite. Furthermore, ISL visibility can be calculated correctly. Moreover, the link-building planning method based on topology stability optimization improves the network topology stability on the premise of reducing network delay, and the overall performance was better than MSN and LCTS in experiments. Our work will be applied to the engineering development of small- and large-scale LEO satellite networks, enabling the deployment of satellite networks with a stable and low-delay network topology. The industry will be led, by the onboard centralized ISL-building planning scheme, to move satellite network ISL-building planning from the ground station to the satellite and further advance the development of the satellite systems in the direction of networking and intelligence. Furthermore, engineering development will be supported by the visibility calculation, which considers the antenna installation angle and the turntable rotation threshold. Additionally, our link-building planning method based on topology stability offers a more practical, user-friendly, and efficient solution. The onboard centralized ISL-building planning scheme still has shortcomings, regarding its relatively low reliability and robustness. If the primary satellite is lost by accident, it may take some time to re-select the temporary primary satellite by the ground station. During this switching time, the constellation will lose control of the network topology. In the future, we will further study the onboard multi-primary centralized/distributed ISL-building planning and management scheme, with the aim of achieving a highly reliable and error-tolerant network topology. Conceptualization, L.Q., H.Y. and Y.X. ; Methodology, L.Q. and X.Z. ; Software, L.Q. and X.Z. ; Validation, H.Y. and Y.X. ; Formal analysis, L.Q., H.Y. and Y.X. ; Writing--original draft preparation, L.Q. and X.Z. ; Writing--review and editing, H.Y., L.W. and X.W. ; Supervision, L.W. and X.W. All authors have read and agreed to the published version of the manuscript.