# Vulnerability of Satellite Quantum Key Distribution to Disruption from Ground-Based Lasers
## 2 Methods
### Satellite Optical Scattering Model  
While the QKD link is active, the QKD ground station (henceforth referred to as 'the ground station') will have its receiving optic trained at the satellite, and the satellite will have its transmitting optic trained at the ground station. The disrupting laser beam launched from the belligerent laser terminal (henceforth referred to as 'the laser terminal') will be highly off-axis compared with the path of the QKD link because the laser terminal is likely to be many kilometres away from the ground station. Because of this, it is necessary to develop a scattering model for the satellite in order to estimate the fraction of photons from the laser terminal reflected in the direction of the ground station. As noted in Table 1, _Micius_ is coated in metallized polymer blankets typically used for thermal shielding of satellites. This material is not smooth, creating a large number of facets to scatter the incident laser light. Additionally, the geometry of the satellite bus and protrusions such as solar panels and antennae will create additional scattering surfaces. We performed scattering measurements from a wrinkled gold-foil target in the laboratory in order to determine the approximate scattering profile of the satellite. A simplified schematic of the experimental setup is shown in Figure 1. Laser light was launched into free-space using a 50 mm diameter telescope with an output beam diameter of 34 mm. The laser light reflected off the gold-foil target (simulating the satellite) and into a second 50 mm telescope attached to a single photon detector (SPD) via multi-mode optical fibre. The SPD measured the rate of photons scattered off the target and coupled into the telescope. The first telescope (launch optic) simulated the laser terminal and was swept through an angle of attack, relative to the line between the target and the second telescope (receiving optic), of 5\({}^{\circ}\) to 90\({}^{\circ}\). The receiving optic simulated the QKD ground station telescope. The launch optic was kept consistently 1.00 m from the target, while the receiving optic was 2.12 m from the target. These distances were the largest possible, limited by available space in the laboratory. White card stock was also used as a target to provide a near-Lambertian reflector against which to compare the scattering profile of the gold foil. The flat face of the target was oriented towards the receiving optic, simulating the coarse bodily pointing of QKD satellites towards their ground stations. Future QKD satellites will likely incorporate quantum memories or quantum repeaters, meaning that the satellite will include a receiving optic. Photons from the laser terminal entering the satellite's receiving optic off-axis can scatter inside the telescope and couple into the QKD receiver on board the satellite. We also measured the internal scattering of our 50 mm telescopes to assess this possible disruption scenario. The launch optic was pointed at the receiving optic and swept through of 0\({}^{\circ}\) to 90\({}^{\circ}\) relative to the pointing direction of the receiving optic at a consistent distance of 1.12 m. Our 50 mm telescopes have a clear aperture of 48 mm and minimal internal optical baffles. The detected photon rates in both sets of tests were recorded using the SPD, simulating the QKD receiver, and used to build approximate scattering cross sections for each scenario in order to inform the optical loss estimates.