OPTICAL SCINTILLATIONS AND FADE STATISTICS FOR A SATELLITE-COMMUNICATION SYSTEM

Estimates of the scintillation index, fractional fade time, expected n umber of fades, and mean duration of fade time associated with a propa gating Gaussian-beam wave are developed for uplink and downlink laser satellite-communication channels. Estimates for the spot size of the b eam at the satellite or the ground or airborne receiver are also provi ded. Weak-fluctuation theory based on the log-normal model is applicab le for intensity fluctuations near the optical axis of the beam provid ed that the zenith angle is not too large, generally not exceeding 60 degrees. However, there is an increase in scintillations that occurs w ith increasing pointing error at any zenith angle, particularly for up link channels. Large off-axis scintillations are of particular signifi cance because they imply that small pointing errors can cause serious degradation in the communication-channel reliability. Off-axis scintil lations increase more rapidly for larger-diameter beams and, in some c ases, can lead to a radial saturation effect for pointing errors less than 1 mu rad off the optical beam axis.