Refraction and scintillation in the atmospheric surface layer

An infrared or optical signal propagating along a line-of-sight horizontal path near the earth's surface can encounter substantial perturbations. There are two prominent factors which can generate fluctuations: first, refractive distortions are low-frequency modulations which can amplify or reduce a signal, and second, scintillation is a higher frequency fluctuation in signal intensity. We will discuss models developed to predict these effects, and associated field test efforts to corroborate and correct the model predictions. The field efforts include tests along horizontal near-surface paths over land and over the coastal ocean surface. Previous transmission field tests have revealed that slow-scale refractive effects can create very pronounced changes in the recorded one-minute average intensity of a source. We will show the results of an analysis of this signal based upon wavelet transforms and filtering. We focus on the detection of a detectable signature frequency of the variations in signal intensity that is based upon the Fresnel zone size. This Fresnel frequency is correlated with the location of the Kolmogorov power law scaling.