Replicated clock on an unmanned aerial vehicle with picosecond-level precision

Unmanned aerial vehicles (UAVs) are increasingly employed in various fields owing to their mobility and cost-effectiveness. High-precision applications, such as communications, remote sensing, formation flight, and cooperative localization, require accurate onboard frequency standards and reliable time–frequency synchronization. Conventional approaches utilizing temperature-compensated or oven-controlled crystal oscillators (TCXO or OCXO) and chip-scale atomic clocks (CSAC) are limited by insufficient precision and stability. While optical frequency synchronization achieves femtosecond-level precision, its complexity, high power requirements, and environmental constraints make it unsuitable for UAVs. Similarly, global navigation satellite system (GNSS)-based synchronization is vulnerable to interference, further limiting its effectiveness. This study proposes a novel method for replicating ground-based atomic frequency standards on UAVs using transponders and active carrier-phase compensation. The proposed approach equips UAVs with simple radio frequency (RF) transponder structures to achieve picosecond-level precision in onboard clocks. Experimental validation on a tethered UAV demonstrated frequency stability of 2.68E-12 at 1 s, 6.78E-15 at 1000 s, and a clock phase stability of 6.66 ps at 2000 s, indicating that the proposed method significantly outperforms conventional crystal oscillators by several orders of magnitude. The findings highlight the efficacy of this method in achieving high-precision frequency synchronization for UAVs, enhancing communication reliability and supporting advanced scientific and technological applications. This approach also paves the way for integrated time–frequency synchronization networks spanning air, space, and ground domains.