## 2 Materials and Methods
### Optical Systems  
The optical terminal is a precursor to a field-deployable mobile optical ground station. Coarse pointing with the terminal mount (L-350, Planewave Instruments, Adrian, MI, USA) is achieved with a machine vision (MV) system, comprising a visible-light camera, lens and single board computer, used to image the drone and payload. The drone-borne optical payload includes a square array of visible wavelength (green) beacon LEDs, arranged uniformly around the centre of the CCR. This beacon-camera arrangement feeds back to the MV system to automatically servo the mount when engaged, keeping the beam within  
Figure 1: (**a**) Optical terminal. The upper-right box contains the free-space optics and machine vision system. The lower-left box is a tip-tilt mirror high voltage stage. The upper-left telescope is from an unrelated system. (**b**) Drone carrying the optical payload. Inset: close-up of the optical payload, consisting of LED array, pilot camera and corner-cube retroreflector.  
Figure 2: Schematic of the free-space optical pulse-position modulation demonstration. CCR: corner-cube retroreflector; FFC: fibre-to-free-space collimator; FPGA: field programmable gate array; GBE: Galilean beam expander; PD: photodetector; PID: proportional, integral, derivative; SWIR: short-wave infrared; VOA: variable optical attenuator.  
the field of view of the optical terminal's laser path. The field-of-view of the machine vision system is \(17.5\times 13.1\) mrad, with an angular resolution of 9 \(\upmu\)rad.  
The optical terminal also houses the FSO components for directing the laser beam. Optical signals from the transmitter are fed to the terminal using fibre. At the fibre-to-free-space collimator (FFC), the signal is launched into free space. This beam passes through a 50:50 beam splitter to a Galilean beam expander with a clear aperture of 43.5 mm. The beam splitter is required to image the returning beam, so half of the outgoing power must be directed to a beam trap and is lost. The outgoing beam is then steered by the tip-tilt (TT) mirror and stationary fold mirror, out of the terminal. The incoming retroreflected beam follows the same path, but this time half of the power is coupled into the fibre via the FFC, and then to the optical receiver. At the link distance, no clipping due to beam divergence is caused by the aperture of the CCR or the Galilean beam expander. Half of the incoming beam is directed by the beam splitter to the imaging arm.  
In the terminal's imaging arm, a lens focuses the incoming beam onto a 3 mm quadrant photodetector (QPD) with a field-of-view of \(2.9\times 2.9\) mrad. Two position signals from the QPD are used to estimate angle-of-arrival variations and are input into a proportional, integral, derivative (PID) controller (one for each of the pitch and yaw mirror axes) and output to the TT mirror piezo actuators. The mirror has an actuation range of 4 mrad, covering the QPD field of view. Co-alignment between the QPD and FFC means the TT mirror provides high-frequency, narrow field-of-view pointing for fibre coupling.  
Finally, the drone-borne optical payload also carries a camera to assist the drone pilot in pointing the payload towards the optical terminal. The optical payload chassis is a 3D-printed enclosure and mounts to the COTS gimbal controlled using the drone's native software and controller.