Multipath dispersion for the atmosphere laser channel

Consideration is given to multipath dispersion due to atmospheric inhomogeneities for point and nonzero-area apertures under clear weather conditions. Arguments are given which show that the time dispersion is primarily due to unscattered ray paths for large apertures. Using this fact an expression is derived for the structure function of the propagation time for large receiver apertures. This expression applies for large dispersions, i.e., beyond the uninteresting case where one has coherence over the receiver aperture. The expression indicates that during medium turbulence conditions, the dispersion for a 6-m aperture receiver system is 0.07 ps for a vertical path through the whole atmosphere and 0.695 ps for a 15-km horizontal path. In contrast, when using a point receiver aperture, the dispersion is expected to be considerably less than 0.001 ps for a vertical path through the whole atmosphere. Finally, a multipath equivalent circuit model is presented for the atmospheric channel.