Propagation of optical waves in the atmosphere is influenced by refractive index spatial inhomogeneities resulting from complicated dynamics of air masses. Both large-scale deviations of refractive index (refractivity) and small-scale random refractive index inhomogeneities (turbulence) can significantly impact performance of atmospheric remote sensing systems including both imaging and laser-based electro-optics systems. Typically, in analysis of atmospheric sensing systems only turbulence effects are accounted for. This simplification is justified for only operation at relatively short distances and in absence of strong refractivity layers. In this paper we discuss more general propagation scenarios for which atmospheric refraction can play an important role and could significantly alter the major laser beam and image characteristics. Atmospheric refractivity is described by a combination of the standard MUSA76 and inverse temperature layer models, and atmospheric turbulence effects are accounted for using the classical Kolmogorov turbulence framework with HV57 model for the height profile of the refractive index structure parameter. The numerical analysis demonstrated that both refractivity and turbulence could significantly impact both laser beam propagation and image formation and lead to noticeable anisotropic effects.
Using the brightness function imaging approach we analyze the impact of the desert and ocean type inverse temperature layer (ITL) on the modulation transfer function (MFT) of an incoherent imaging system. It is shown that an ITL located in vicinity of an imaging path can result in nonlinear deviation in imaging of an object spatial spectrum. Using periodical (sine) patterns of different spatial frequencies as an object, we show that presence of an ITL leads to formation of images with a broadened spatial spectrum with shifted frequency of the sine pattern image in respect to object frequency. The image spectrum shift and width depend on the ITL characteristics and its location in respect to optical wave propagation path. The observed changes in the sine pattern spectral content represent a challenge for analysis of imaging systems performance using conventional MTF based framework. We also analyzed impact of atmospheric turbulence on imaging of periodical (sine) patterns and compared this with the known theoretical results.