Block diagram of the 3U CubeSat. Components in brown are those used for quantum communication while those in green relate to the pointing, acquisition and tracking system. Other essential subsystems are blue. The subsystems can also be classified based on the type of components used as indicated by the grey dashed lines. All components are fixed to the CubeSat frame, communicate with the main CPU and supply/draw power to/from the CubeSat bus (all of which are not shown). TTM: Time Tagging Module, RNG: Random Number Generator.