Quantum Key Distribution for Secure LEO Satellite Constellations: A Scalable Protocol and Deployment Model

As global reliance on satellite communication networks intensifies, securing data links in Low Earth Orbit (LEO) becomes critical. Quantum Key Distribution (QKD), particularly the BB84 protocol, offers provably secure encryption through the transmission of quantum states. However, practical deployment at constellation scale poses challenges, including limited pass durations over ground stations, atmospheric attenuation, and the lack of a robust key distribution architecture in orbit.

This work introduces a hybrid QKD architecture for secure LEO constellations that combines both LEO-to-Ground Station (LEO-GS) QKD and Intersatellite Links (ISLs). We have simulated a BB84-based QKD system that incorporates orbital mechanics using real TLE data to identify viable satellite pass windows. Optical link budgets are evaluated by accounting for photon loss, beam divergence, and pointing jitter for both ground and intersatellite links.

To address the limitations of short link durations with ground stations, a key relay and pooling protocol is designed. The mechanism uses ISLs to route and buffer quantum keys across the constellation in near real-time, ensuring keys generated during short GS passes are distributed via ISLs to satellites with upcoming favorable ground contacts, forming a secure orbital mesh. We also propose a lightweight, software-defined QKD stack compatible with satellite onboard systems and explore key management strategies (e.g., key expiry, buffer thresholds, and conflict resolution) for constellation-wide scalability.

This work aims to bridge physical-layer QKD with constellation-level routing and resource scheduling, presenting a deployable framework for quantum-secure LEO communication infrastructures. Simulation results are analysed to understand the improvements in key availability, robustness against cloudy passes, and support for secure inter-node data exchanges—enabling future integration with governmental, commercial, and military space networks.