## 4 Discussion and Conclusions  
This demonstration involving a drone-borne CCR provided a useful return on experience towards FSO communications developments, including potential communication with O2O on a full-sized optical ground station. This was particularly true in the domain of flight operations, not otherwise accessible without enlisting the services of professional small aircraft operators, or operators of commercial high-altitude pseudo satellites or the few FSO communication-capable satellites in orbit. The drone link demonstration required the coordination of equipment and planning of methods within the research group and was subject to scheduling with the aviation authority and local government as well as uncontrollable go/no-go conditions such as wind warnings and rain. In effect, the availability of the drone provided a scaled-back simulation of space operations. Interpreting Equation (3) implies the optical sensitivity floor represented by \(v_{o}\) is analogous to background counts in photon-counting receivers, and a comparison may be drawn between the drone link and LLCD results [25]. Downlink results from [25] reported error-free communication at 38.55 Mb/s with a link margin of 13.5 dB between signal and background photon counts. Per Table 1, the error-free link margin for the demonstration was 2.70 dB. While it is not possible for this testbed to recreate the transmission powers and link losses of a Lunar-to-ground link, the receiver may be similarly tested with the link margin at the limit of its sensitivity. This scales with detector technology, so a photon-counting detector with a lower sensitivity floor may be integrated into a future iteration of the testbed, and the receive power adjusted to recreate the link margin conditions. Figure 3 shows a main cluster of BER versus SNR results following the theoretical relationship, confirming the expected performance of the receiver. However, the demodulation algorithm implemented often failed to completely demodulate a codeword if too many early symbols were faded, or if the slot or symbols synchronisation had failed, causing'synchronised' pulses to fall across slot boundaries. These cases account for the outlying clusters at the top of the graph, stratified around BERs of 0.5 and 0.33, and indicate further margin for improvement of the offline receiver algorithm before investing engineering time into a real-time receiver. The 1.3 km retroreflected drone link presented can be extended with materials and methods previously demonstrated over shorter retroreflected links such as verification of atmospheric channel modelling [14]; demonstration of other communication formats [15]; or demonstrations of modulating retroreflectors [21]. Further extensions previously demonstrated include links to tethered drones, carrying active optical payloads [20], or drones moving in paths tangential to the ground station, thus requiring the ground station to track at angular rates equivalent to satellite tracking [16]. Furthermore, results from FSO  
Figure 3: Bit-error rate versus signal-to-noise ratio (SNR) of 720 received 15120 bit 4-PPM codewords. communication demonstrations with stationary drones will be of interest to designers and analysts planning future terrestrial and disaster-response networks [17; 18]. The drone provided a testbed to perform technical verification of the prototype communications equipment, intended for a future Lunar FSO communication link, over a real, time-varying channel. This also tests the mechanical robustness of the optical assembly and pointing systems. The effectiveness of the testbed is limited by dissimilarities to O2O in terms of modulation format and detector technology, as well as the relative differences in aperture sizes and optical power levels involved (intrinsically linked to detector technology and link distance). However, some of these limitations can be addressed with changes to the optics and launching the drone from a different location.