Quantum key distribution (QKD) offers future proof security based on fundamental laws of physics. Long-distance QKD spanning regions such as the United Kingdom (UK) may employ a constellation of satellites. Small satellites, CubeSats in particular, in low Earth orbit are a relatively low-cost alternative to traditional, large platforms. They allow the deployment of a large number of spacecrafts, ensuring greater coverage and mitigating some of the risk associated with availability due to cloud cover. We present our mission analysis showing how a constellation comprising 15 low-cost 6U CubeSats can be used to form a secure communication backbone for ground-based and metropolitan networks across the UK. We have estimated the monthly key rates at 43 sites across the UK, incorporating local meteorological data, atmospheric channel modelling and orbital parameters. We have optimized the constellation topology for rapid revisit and thus low-latency key distribution.

Very low Earth orbits (VLEOs) have been proposed as a beneficial space mission regime due to their propensity to increase instrument spatial resolution and reduce launch cost per unit mass. However, for visual instruments, these benefits come at the cost of a decreased instrument swath width. This reduction results in longer revisit periods for regions on Earth and longer time until global coverage is achieved. Conversely, light detection and ranging (lidar) as an active remote sensing technique, can benefit from larger swath widths at lower altitudes, due to the increased signal-to-noise ratio. Investigation of this relationship shows that lidar swath width is inversely proportional to altitude squared, and, as a result, the number of spacecraft required to provide a desired lidar coverage also decreases approximately in inverse proportion to altitude squared. Investigation of suitable propulsion systems shows that although propellant mass and number of thrusters required for orbit maintenance increases with decreasing altitude, the overall system mass, and hence launch cost, will, in general, tend to decrease with decreasing altitude due to the lower number of spacecraft required. For a given mission, spacecraft bus, and propulsion system, a VLEO altitude can be identified that will result in the minimum overall mission cost.

Communicating information at the few photon level typically requires some complexity in the transmitter or receiver in order to operate in the presence of noise. This in turn incurs expense in the necessary spatial volume and power consumption of the system. In this work, we present a self-synchronised free-space optical communications system based on simple, compact and low power consumption semiconductor devices. A temporal encoding method, implemented using a gallium nitride micro-LED source and a silicon single photon avalanche photo-detector (SPAD), demonstrates data transmission at rates up to 100 kb/s for 8.25 pW received power, corresponding to 27 photons per bit. Furthermore, the signals can be decoded in the presence of both constant and modulated background noise at levels significantly exceeding the signal power. The system’s low power consumption and modest electronics requirements are demonstrated by employing it as a communications channel between two nano-satellite simulator systems.