A cryptographic key is shared between a satellite 4 and a ground-based satellite access station 2 using QKD. The key is used to encrypt commands or/and data sent over a telemetry, tracking and telecommand (TT&C) link 9 between the access station and the satellite. The cryptographic key may be sent over an optical QKD link 8, and the TT&C link 9 may be a classical RF link. The key may be shared using a “prepare and measure” QKD protocol such as BB84 or an entanglement protocol such as E91. The key may be output either by the satellite or the access station. The cryptographic key may be periodically updated and may replace an initial key stored before launch. The cryptographic key may be partially set-up using QKD before launch and subsequently resolved using the QKD link. If the satellite includes a hosted payload a further QKD link may be set up for use by the owner of the hosted payload.
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