Experimental demonstration of velocimetry by actively stabilised coherent optical transfer

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.