  The free-space transfer of high-fidelity optical signals between remote
locations has many applications, including both classical and quantum
communication, precision navigation, clock synchronization, etc. The physical
processes that contribute to signal fading and loss need to be carefully
analyzed in the theory of light propagation through the atmospheric turbulence.
Here we derive the probability distribution for the atmospheric transmittance
including beam wandering, beam shape deformation, and beam-broadening effects.
Our model, referred to as the elliptic-beam approximation, applies to weak,
weak-to-moderate, and strong turbulence and hence to the most important regimes
in atmospheric communication scenarios.
