Analysis of Beam Misalignment Effect in Inter-Satellite FSO Links

Abstract

Free-space optical (FSO) communication has emerged as a promising technology for inter-satellite links (ISLs) due to its high data rate, low power consumption, and reduced interference. In inter-satellite FSO systems, pointing error is the dominant channel impairment, arising from stochastic jitter and deterministic misalignment induced by relative satellite motion. While prior studies have characterized pointing error through statistical distributions, the misalignment displacement has typically been treated as a fixed parameter, without directly connecting to the underlying orbital dynamics. This paper addresses the gap by deriving the misalignment displacement from satellite orbital kinematics and incorporating it into a unified pointing error model that jointly captures jitter and motion-induced misalignment. Based on the model, a closed-form expression for the cumulative distribution function (CDF) of the FSO channel gain is derived, and a truncated CDF formulation with a bisection-based index search is introduced for efficient outage probability evaluation. The displacement is computed for intra-orbital-plane and inter-orbital-plane link configurations using orbital data from Iridium and Starlink constellations, enabling a systematic assessment of how satellite geometry affects link performance. Numerical results verify that the analytical expressions closely match Monte Carlo simulations, and demonstrate that inter-orbital-plane links are more susceptible to misalignment-induced degradation, with optimizing the beam waist and increasing the number of orbital planes being effective mitigation strategies.