## 1 Introduction  
From the Apollo era to today, deep space communication is conducted using radio-frequencies (RF), with the Deep Space Network currently able to provide speeds up to 150 Mb/s using the Ka band [1]. However, RF bandwidth presents a bottleneck to expanding deep-space communication capability. Free-space optical (FSO) communication between space and ground will enable the return of larger science data payloads, larger telemetry payloads, and streaming high-definition video to Earth. Pioneering experiments in FSO communication include NICT's 1994 space-to-ground laser downlink [2] and several space-to-space laser links in the 2000s [3; 4; 5]. In the 2013 Lunar Laser Communication Dimension, NASA demonstrated Moon-to-Earth optical communication at downlink speeds up to 622 Mb/s, using the Lunar Laser Space Transmitter (LLST) [6]. In the Artemis II Mission [7], a crew of NASA astronauts will orbit the Moon in the Orion spacecraft. This will be the first crewed Lunar orbit since Apollo 17, over 50 years ago. The Orion Artemis II Optical Communications Terminal (O2O) is the successor to the experimental LLST and will initially support space-to-ground downlink at speeds from 80 to 260 Mb/s and ground-to-space uplink at 20 Mb/s [8]. O2O will transmit information using the pulse-position modulation (PPM) format, standardised by the Consultative Committee for Space Data Systems (CCSDS) in the 141.0-B-1 Recommended Standard [9]. Earth's atmosphere remains a challenge to FSO communication because of the deleterious effects of atmospheric turbulence on optical propagation. Techniques for mitigating turbulence and correcting optical wavefronts, such as adaptive optics, have been successfully applied in optical astronomy for decades [10]. Therefore, organisations specialising in optical astronomy and space situational awareness are showing interest in establishingoptical ground station networks to support FSO communications, including several Australian institutions [11]. However, the limited accessibility of FSO communications-capable satellites in orbit remains a challenge for commissioning optical ground stations. Hardware testbeds in lieu of satellites and spacecraft are therefore useful for testing and verifying optical ground station systems and sub-systems. Retroreflected laser links to drones are one such testbed and have been used by the frequency metrology community in preparation for tests of fundamental physics over satellite laser links [12, 13]. For communications purposes, the channel statistics of retroreflected drone links have been analysed and tested for round-trip lengths up to 204 m [14]. A retroreflected communications link was demonstrated using orbital-angular momentum multiplexed light at 40 Gb/s over a round-trip distance of 100 m [15]. Extensions to this method are possible. For example, due to drone mobility, a drone may also be slewed, requiring the ground station to slew at equivalent angular rates to a low-Earth orbit satellite, therefore simulating low-Earth orbit satellite tracking [16]. However, in the context of Lunar and deep-space communication, a stationary, hovering drone is more analogous to communication with FSO payloads in cis-Lunar space, deep space or geosynchronous orbit. Furthermore, FSO communication demonstrations with stationary, long-distance drone platforms are also of interest to terrestrial network designers, as drones have been proposed as highly mobile nodes for FSO communication feeders in re-configurable networks for disaster recovery and urban centres [17, 18]. Numerical studies of drone-to-ground communication have also been conducted [19]. True drone-to-ground communication for space simulation or terrestrial communication remains difficult to accomplish due to size, weight and power requirements. One drone-to-ground demonstration was conducted with a drone-borne active optical payload linked and powered with a tether to conduct loopback measurements over the aerial link [20].