  Quantum key distribution from satellites becomes particularly valuable when
it can be used on a large network and on-demand to provide a symmetric
encryption key to any two nodes. A constellation model is described which
enables QKD-derived encryption keys to be established between any two ground
stations with low latency. This is achieved through the use of low earth orbit,
trusted-node QKD satellites which create a buffer of keys with the ground
stations they pass over, and geostationary relay satellites to transfer secure
combinations of the keys to the ground stations. Regional and global network
models are considered and the use of inter-satellite QKD links for balancing
keys is assessed.
