  Future free-space optical clock networks will require optical links for time
and frequency transfer. In many potential realizations of these networks, these
links will extend over long distances and will span moving platforms, e.g.
ground-to-air or ground-to-satellite. In these cases, the transverse platform
motion coupled with spatial variations in atmospheric optical turbulence will
lead to a breakdown in the time-of-flight reciprocity upon which optical
two-way time-frequency transfer is based. Here, we report experimental
measurements of this effect by use of comb-based optical two-way time-frequency
transfer over two spatially separated optical links. We find only a modest
degradation in the time synchronization and frequency syntonization between two
sites, in good agreement with theory. Based on this agreement, we can
extrapolate this 2-km result to longer distances, finding only a
few-femtosecond timing noise increase due to turbulence for a link from ground
to a mid-earth orbit satellite.
