Measurement of the impact of turbulence anisoplanatism on precision free-space optical time transfer
William C. Swann, Martha I. Bodine, Isaac H. Khader, Jean-Daniel Deschenes, Esther Baumann, Laura C. Sinclair, Nathan R. Newbury
Future highly precise free-space optical clock networks will require optically-based two-way time and frequency transfer links. As these networks extend over longer distances, they will include links between moving platforms, e.g. ground-to-air or ground-to-satellite. In that case, the transverse motion of the clocks coupled with the angular variations in turbulence, characterized by the isoplanatic angle, will lead to an anisoplanatic breakdown in the time- of-flight reciprocity upon which 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 across a 2-km turbulent open air path. We find only a modest degradation in the time synchronization and frequency syntonization between sites. We also find good agreement with theory. Based on this agreement, we can extrapolate this 2-km result to longer distances, finding only a modest, few-femtosecond timing noise increase for a link from ground to a mid-earth orbit satellite.