Atmospheric occultation of optical intersatellite links: coherence loss and related parameters

The Rytov theory is applied to find the wave structure function of a laser beam transmitted from one satellite to another and propagating through the turbulent atmosphere. The phase-screen approximation is used. Taking into account refractive-index anisotropy, outer scale, and atmospheric mean-refraction defocusing, we provide expressions of the wave structure function for a spherical wave. The width and time of coherence at the receiver are evaluated. Expression for the beam spread is found using the extended Huygens-Fresnel principle, and beam wander is assessed. Beam wander occurs only for very narrow beams. Links involving low-Earth-orbit and geosynchronous satellites are studied as examples. Finally, conditions where optical tracking is perturbed by the atmosphere are examined.