  The establishment of quantum communication links over a global scale is
enabled by satellite nodes. We examine the influence of Earth's atmosphere on
the performance of quantum optical communication channels with emphasis on the
downlink scenario. We derive the geometrical path length between a moving low
Earth orbit satellite and an optical ground station as a function of the ground
observer's zenith angle, his geographical latitude, and the meridian
inclination angle of the satellite. We show that the signal distortions due to
regular atmospheric refraction, atmospheric absorption, and turbulence have a
strong dependence on the zenith angle. The observed saturation of transmittance
fluctuations for large zenith angles is explained. The probability distribution
of the transmittance for slant propagation paths is derived, which enables us
to perform the security analysis of decoy state protocols implemented via
satellite-mediated links.
