Composite scanning approach for beaconless spatial acquisition and optimized parameters under satellite platform vibrations

Mega Low Earth Orbit (LEO) satellite constellations can provide pervasive intelligent services in the forthcoming Six-Generation (6G) network via the Free-Space Optical (FSO) Inter-Satellite Link (ISL). However, the challenges posed by the mega LEO satellite constellations, such as limited onboard resources, high-speed movement and the vibration of satellite platforms, present significant obstacles for the existing Pointing, Acquisition and Tracking (PAT) schemes of FSO-ISL. To address these challenges, we propose a beaconless PAT scheme under satellite platform vibrations, employing a composite scanning approach combining an inner Archimedean spiral scan with an outer regular hexagon step scan. The proposed composite scanning approach covers a wide range of the Field of Uncertainty (FOU) and reduces the required scans by actuator, which can ensure a high Acquisition Probability (AP) while reducing the Average Acquisition Time (AAT) for the inner scan. Specifically, we model and analyze the effect of satellite platform vibrations on the acquisition performance of our beaconless PAT scheme, and derive closed-form expressions for both AP and AAT by utilizing a 2-track model where the acquisition happens on two adjacent spiral scan tracks. By utilizing the theoretical derivations, we can achieve a minimum AAT under diverse APs by selecting appropriate values of overlapping region and scanning range. Simulation results validate that our optimized composite scanning approach for beaconless PAT scheme outperforms the existing schemes.