# Laser Inter-Satellite Link Visibility and Topology Optimization for Mega Constellation
## 2 Background  
Optical wireless communication in space information networks is conventionally referred to as free-space optics (FSO), which refers to optical communication systems that transmit optical signals through the atmosphere rather than through optical fibers, and is a hybrid of fiber optic and wireless communication. FSO links are used to transmit optical signals from an optical transmitter through the atmosphere or space vacuum to  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Constellation Name** & **Orbital Altitude** & **Start Year** & **Number of Satellites** & **Orbital Inclination** & **Frequency Band** \\ \hline Globalstar & 1414 km & 2007 & 48 + 8 & 52\({}^{\circ}\) & L, S, C \\ Orbcomm 2 & 825 km & 2012 & 35 & 45\({}^{\circ}\) & V \\ Iridium NEXT & 780 km & 2017 & 66 + 9 & 86.4\({}^{\circ}\) & L, Ka \\ OneWeb & 1200 km & 2019 & 720 & 87\({}^{\circ}\) & Ka, Ku, V \\ Telesat & 1000/1200 km & 2023 & 298 & 99.5\({}^{\circ}\)/37.4\({}^{\circ}\) & Ka \\ Starlink * & 340/550/1150 km & 2024 & 42,000 & 53\({}^{\circ}\) & Ka, Ku, V \\ \hline \hline \end{tabular}  
* The data of the above satellites are available in the link “[[https://www.space-track.org/](https://www.space-track.org/)]([https://www.space-track.org/](https://www.space-track.org/))” (accessed on 15 April 2020), and the data is updated in real time.  
\end{table}
Table 1: Parameters comparison of several typical international LEO satellite constellation systems.  
an optical receiver. The transmitter and receiver require a clear line of sight. A satellite laser interplanetary link communication system is illustrated in Figure 1. The system is comprised of an optical subsystem, an ATP subsystem, a communication subsystem, and an interface and control subsystem [13].  
Laser intersatellite links (LISLs) will be widely used in constellations after deployment in the coming years, such as the first phase of SpaceX's planned Starlink constellation, in which satellites can establish laser connections to other satellites in the same or different orbital planes (OPs). Satellites in this LEO layer constellation can establish laser links with other satellites in the same or different orbital planes. In comparison to conventional microwave communication, laser communication benefits from high transmission rates, wide band-width availability, strong anti-electromagnetic interference capability, high security and confidentiality, as well as the small size, light weight, and low power consumption of laser terminals, all of which contribute to LISLs gradually becoming an important means of establishing future LISLs, and one of the best ways to address the problems of high-speed space communication [14]. However, LISLs do pose several limitations. Due to the narrow beam, it is difficult to target and capture the orbit of LISLs under the high dynamic motion conditions of two satellites, which compounds the complexity of the system design [15].  
These studies are crucial for expanding routing options, enhancing satellite redundancy backup, and enhancing satellite networks' topological resilience. Additionally, in order to find reliable temporary LISLs for next-generation optical wireless satellite networks, it is necessary to develop efficient ATP systems with reasonable packet conversion and low cost. Additionally, will be necessary to equip satellites with more optional LISL terminals in order to construct a robust network topology using permanent LISLs and flexible options for a large number of variable temporary LISLs.