We report on the development of a system called velocimetry by actively stabilised coherent optical transfer (VASCOT) that is capable of overcoming these challenges. VASCOT optically tracks the phase of the returned signal to allow for narrow-band photo-detection, thus transferring the requirements on the photodetector bandwidth to the phase-tracking bandwidth. The pointing challenges are handled by an active tracking terminal capable of suppressing angular pointing errors to the moving target. VASCOT is  experimentally demonstrated over a 584 m atmospheric link to a corner-cube retro-reflector (CCR) on an airborne drone, with cycleslip-free phase tracking achieved for the three minute experiment. It was shown that the in-line target velocity measurement could achieve residual uncertainties below 2 nm/s within 5 s of averaging. VASCOT was also able to provide an absolute range measurement with a statistical error of ±13.7 mm, that agreed with an independent GPS-derived range measurement to within the uncertainty of the GPS module.

Geopotential and orthometric height differences between distant points can be measured via timescale comparisons between atomic clocks. Modern optical atomic clocks have residual instabilities on the order of 10^-18}, allowing height differences of around 1 cm to be measured. Frequency transfer via free-space optical links will be needed for measurements where linking the clocks via optical fiber is not possible, but requires line of sight between the clock locations, which is not always practical due to local terrain or over long distances. We present an active optical terminal, phase stabilization system, and phase compensation processing method robust enough to enable optical frequency transfer via a flying drone, greatly increasing the flexibility of free-space optical clock comparisons. We demonstrate a residual instability of 2.5x10^-18 after 3 s of integration, corresponding to a height difference of 2.3 cm, suitable for applications in geodesy, geology, and fundamental physics experiments.