Research on Ranging over Near-Ground Free-Space Optical Links Based on White Rabbit Time Synchronization

White Rabbit (WR) is a technology based on fiber-optic networks that enables sub-nanosecond-level time synchronization and deterministic Ethernet data transmission among distributed devices. To investigate the transmission characteristics of WR technology over free-space optical links, this paper reports point-to-point single-link ranging experiments for WR over an approximately 1 km near-ground free-space optical link. For the 1 km fiber-optic link and the 1 km free-space optical link (scintillation index = 0.063), the intermediate precision of ranging after a power-cycle reboot is measured to be 5.829 mm (1σ) and 9.689 mm (1σ), respectively, while the ranging precision is 0.267 mm (1σ) and 1.353 mm (1σ), respectively. As the scintillation index of the free-space optical link increases from 0.063 to 0.168, the ranging precision degrades from 1.353 mm (1σ) to 2.221 mm (1σ). The results demonstrate that under a scintillation index of 0.063, both the intermediate precision of ranging after a power-cycle reboot and the ranging precision over the 1 km free-space optical link degrade compared with those of the 1 km fiber-optic link. Specifically, the intermediate precision of ranging after power-cycle reboot degrades to 1.66 times that of the fiber-optic link, and the ranging precision degrades to 5.07 times that of the fiber-optic link. Furthermore, it is also observed in this 1 km free-space optical link experiment that as the scintillation index increases from 0.063 to 0.168, the one-sigma standard deviation of ranging precision rises from 1.353 mm to 2.221 mm.