  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 4 GHz at 100 MHz/s, which prevents the narrow-band
detection required to compare optical frequencies at the highest levels of
stability. We demonstrate a system capable of optical frequency comparison in
the presence of these significant Doppler shifts, using an electro-optic phase
modulator with an actuation bandwidth of 10 GHz, which will enable
ground-to-space frequency comparison. This system was demonstrated over a
retro-reflected drone link, with a maximum line-of-sight velocity of 15 m/s and
Doppler shift of 19 MHz at a Doppler rate of 1 MHz/s. The best fractional
frequency stability obtained was 7E-18 at an integration time of 5s. These
results are an important step toward ground to low earth orbit optical
frequency comparison, providing a scalable terrestrial test bed.
