Experimental Demonstration of Velocimetry by Actively Stabilized Coherent Optical Transfer

We report on the development of a system called velocimetry by actively stabilized coherent optical transfer (VASCOT) that uses active optical phase tracking to measure the in-line velocity of retroreflecting targets with narrow-band photodetection. VASCOT is experimentally demonstrated over a 584-m atmospheric link to a corner-cube retroreflector (CCR) on an airborne drone. The in-line target-velocity measurement achieves residual uncertainties below $2\phantom{\rule{0.2em}{0ex}}{\text{nm s}}^{\ensuremath{-}1}$ within 5 s of averaging. Cycle-slip-free phase tracking over the full 3-min experiment allows the relative target range to be tracked with a one-standard-deviation uncertainty of 39.5 nm. VASCOT also provides an absolute target-range measurement with a statistical error of $\ifmmode\pm\else\textpm\fi{}13.7\phantom{\rule{0.2em}{0ex}}\mathrm{mm}$, which agrees with an independent Global-Positioning-System- (GPS) derived range measurement to within the GPS uncertainty.