Probability density of irradiance fluctuations observed over terrestrial ranges.

The irradiance fluctuations imposed on a laser beam that has propagated over horizontal terrestrial paths in the range of 2 to 24 km are compared to lognormal (LN) and gamma-gamma (GG) distributions. For the direct links reported here the irradiance fluctuations follow a LN distribution except in cases of weak turbulence, characterized by a scintillation index of less than 1 and a Fried parameter larger than the receiver aperture, in which case the GG distribution gives an improved fit. In very weak turbulence the difference between the two distributions is insignificant.