Operational limits to entanglement-based satellite quantum key distribution

Space-based distribution of quantum entanglement will be essential for global quantum networking and secure communications. Modelling and analysis of the performance of satellite entanglement pair distribution is important for the architecture and design of constellations and space systems. Entanglement-based quantum key distribution, in the absence of quantum repeaters, is especially prone to finite key effects due to low coincident count rates compared to trusted node single-path links. Therefore, there is a need for a comprehensive study of finite-key effects in the context of direct dual downlink quantum key distribution taking into account the characteristics of the overpass geometries. We develop a high-fidelity model of pair distribution from a low Earth orbit satellite that captures orbital dynamics, elevation-dependent loss, background noise, and extraneous detector effects. We integrate this with a rigorous finite-key security framework for the BBM92 protocol to optimise secret key length across different overpass geometries, orbital altitudes, and optical ground station (OGS) separations. These results provide quantitative performance bounds and design guidelines for near-term SatQKD missions, enabling informed trade-offs between satellite payload complexity, ground infrastructure, and achievable secure key throughput.