Earth’s orbits are becoming increasingly crowded by new constellations of satellites with constantly improving sensors, which drives fierce competition for space communications bandwidth. The research undertaken in this thesis addresses this growing demand for space communications bandwidth, by developing new methods for commissioning optical communications ground stations. Space communications currently rely on free-space signals in the radio-frequency part of the electromagnetic spectrum. However, radio-frequency bandwidth is not keeping pace with the growing number of satellites and advancements in Earth observation sensor technologies. Additionally, radio-frequency signal beams diverge significantly on space-ground scales, thereby leading to overlapping interference, spectrum crowding, and a reduction in power efficiency. Free-space optical signals promise three-orders-of-magnitude of bandwidth increase and three-orders-of-magnitude reduced divergence, effectively eliminating spectrum crowding and enhancing their power efficiency over radio frequency signals. Optical ground stations on Earth will provide a point of contact for spacecraft equipped with free-space optical communications terminals. New optical ground stations require methods for testing and commissioning.

The current availability of spacecraft with optical communications terminals is extremely low. Therefore, accessible spacecraft proxies are extremely valuable to accelerate testing and development of the systems comprising the optical ground station. This thesis documents methods for demonstrating and commissioning optical ground station hardware, and also shows the conditions for translating commercial-off-the-shelf fibre communications equipment to the free-space domain. The following work towards this goal, and outcomes, are described herein:

• Simultaneous measurements of phase noise, angle-of-arrival variation, and intensity scintillation were taken on a retroreflected, or ‘folded’ link, established with a corner-cube retroreflector. Analytical models were formed to relate retroreflected links to point-to-point links. Comparing the experiment with the models validates the subsequent use of folded links for free-space optical communications experiments.
• Demonstration of 100 Gb/s coherent free-space optical communications over a 10.3 km retroreflected link. This was conducted on 52 wavelength-division multiplexing channels, demonstrating the extremely high bandwidth available to coherent free-space optical communications. No atmospheric stabilisation was used, showing the absolute limits of intensity scintillation compatible with commercial-off-the-shelf fibre networking equipment.
• Demonstration of 100 Gb/s coherent free-space optical communications to a drone carrying a corner-cube retroreflector. The drone was flown across the sky, requiring a prototype of an optical ground station to track the drone at angular rates equivalent to, or exceeding, the angular tracking rates required to maintain contact with a satellite in low-Earth orbit.
• Demonstration of high photon efficiency free-space optical communications to a droneborne retroreflector. This demonstration provides a methodology for ground station operators to test their prototype systems for future lunar and deep-space missions.

These contributions advance knowledge in retroreflected free-space optical links and provide a roadmap for those who wish to test and commission their own optical communications ground stations.