The comparison of optical reference frequency signals over free-space optical links is limited by the relative motion between local and remote sites. For ground to low-Earth-orbit comparison, the expected Doppler shift and Doppler rate typically reach $\pm 4$ GHz at 100 MHz ${\mathrm{s}}^{-1}$, which prevents the narrow-band detection required to compare optical frequencies at the highest levels of stability. As an important step to achieve these goals, we demonstrate a system capable of optical-frequency comparison in the presence of significant Doppler shifts. This system is demonstrated over a retroreflected drone link, with a maximum line-of-sight velocity of 15 m ${\mathrm{s}}^{-1}$ and Doppler shift of 19 MHz at a Doppler rate of 1 MHz ${\mathrm{s}}^{-1}$. The best fractional frequency stability obtained is $7\times {10}^{-18}$ at an integration time of 5 s. These results are an important step toward ground to low-Earth-orbit optical frequency comparison, providing a scalable terrestrial testbed.

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• Atomic, optical & lattice clocks
• Laser applications
• Optoelectronics
• Photonics
• Special relativity
• Time & frequency standards
• Wireless communication networks
• Precision measurements