# Onboard Centralized ISL-Building Planning for LEO Satellite Constellation Networks
## 1 Introduction
### Literature Review  
To date, onboard network routing algorithms have been presented in some studies [7]; however, in most research on ISL-building planning, the topology is planned in advance at the ground station and then uploaded to the satellite in a timely manner [8]. There is currently no published research focused on realizing centralized network ISL-building planning onboard the satellite. The onboard implementation of the entire centralized network ISL-building planning process ensures greater ground station control decoupling and lower management latency. However, it also brings issues, such as the difficult collection of planning input parameters and limited computing resources. When a satellite performs tasks such as remote sensing and reconnaissance, its orbit and attitude change adaptively, which affect the visibility of ISLs, leading to different topology tables in each period. As such, the topology table in the previous period cannot be simply used in the next period. Moreover, during the satellite's onboard operation, the satellites may be damaged and the ISL terminals may suddenly lose efficacy, which requires the primary satellite in the constellation to plan a new network topology in real time. These are necessary considerations in onboard ISL-building planning. [PERSON] et al. [9] have proposed an ISL-building method suitable for distributed topology planning, which is a new idea for ISL-building planning. However, they did not describe the whole process of onboard distributed planning, nor did they explain the key processes such as the acquisition and synchronization of visibility calculation input information or the comparison and confirmation of planning results. From our perspective, there are still many problems to be solved in the field of distributed planning. Several researchers have attempted to overcome the challenge of satellite network ISL-building planning by completing the computation process at the ground station. Most of these studies regard the satellite as a particle with an infinite visible angle [10], or assume that one satellite carries only one antenna [9; 11], when calculating the visibility between ISL terminals, or that all of the installed antennas have the same installation direction and rotation threshold [2; 12; 13]. Furthermore, the satellite attitude in these studies is always regarded as a zero attitude. Some researchers simply assumed that the antennas used for the inter-plane ISLs are located at both sides of the pitch axis [13; 14]. However, considering the shading of solar wings, cameras, antennas, and so on, the antennas used for the inter-plane ISLs may be installed at sides of the roll or yaw axes. Although these factors can substantially reduce the model complexity, they ignore the fact that, in future large-scale constellations, satellites need to carry more than one ISL terminal with different installation directions and construct several ISLs at the same time. Furthermore, the terminal installation direction and the turntable rotation angle thresholds significantly impact the ISL visibility. To sum up, the state-of-the-art articles did not take all the parameters of visibility calculation, such as the antenna installation direction, rotation threshold, and attitude, into consideration. Thus, it is necessary to study the visibility calculation method that can adapt to various satellite platforms. Compared with the Geostationary Earth Orbit (GEO) constellations, the ISL-building planning problem is more challenging to solve when considering LEO constellations. This is because the relative position relationship among the satellites fluctuates regularly, resulting in more frequent ISL switches. Many works have used finite state automation (FSA) to solve this problem [11; 15; 16]; that is, the satellite network operation period is divided into fixed equal-length time segments, transforming the dynamic state into fixed states.