  In this paper we present a novel analysis of the feasibility of quantum key
distribution between a LEO satellite and a ground station. First of all, we
study signal propagation through a turbulent atmosphere for uplinks and
downlinks, discussing the contribution of beam spreading and beam wandering.
Then we introduce a model for the background noise of the channel during
night-time and day-time, calculating the signal-to-noise ratio for different
configurations. We also discuss the expected error-rate due to imperfect
polarization-compensation in the channel.
  Finally, we calculate the expected key generation rate of a secure key for
different configurations (uplink, downlink) and for different protocols (BB84
with and without decoy states, entanglement-based Ekert91 protocol).
