In considering HF propagation in a random inhomogeneous ionosphere it is necessary to take into account regular and random caustics. Regular caustics connected with regular refraction of radio waves in the ionosphere form a skip zone and determine the maximum usable frequency (MUF) and the maximum of the oblique incidence backscatter sounding (OBS) signal. Random fluctuations of ionospheric radio rays “wash out” field enhancement in the vicinity of MUF and the maximum OBS signal. The presence of random caustics results in strong intensity fluctuations of ionospheric radio waves. The above mentioned problems are considered by the interference integral method. A scintillation index formula for strong fluctuations of oblique ionospheric radio waves is obtained. Average intensity and average pulse signal which form in the vicinity of MUF are investigated. The peculiarities of oblique multihop radio wave propagation, taking into account random ionospheric inhomogeneities, terrestrial surface roughness, and caustics focusing in skip distance are discussed.
This report presents results of analysis of such characteristics as the mean intensity of the short-wave signal, its angular and frequency spectra, the scintillation index depending on parameters of random and regular irregularities, as well as on the propagation range for a wave reflected from a regularly and randomly inhomogeneous layer, under conditions of strong and weak fluctuations in the vicinity of simple caustic and in the zone of a regular caustic shadow, Currently we have a considerably well developed theory €or waxe propagation in randomlyinhomogeneous media with an uniform background. Different approximate methods are extensively used for these purposes. Unfortunately, it was not until recently that such investigations on wave propagation in a randomly-inhomogeneous medium with an inhomogeneous background were made.As is known, in the theory of wave propagation in a randondy-inhomogeneous medium an important role is played by the geometrical-optics approximation. In particular, it is easy to take regular refraction into account in terms of the ray approximation, provided that the validity range of a geometrical-optics description applies €or the "background" medium and random irregularities. However, in considering short-wave propagation in a randomly-inhomogeneous medium it is necessary often to take into account regular and random caustics. Regular caustics connected with regular refraction of waves in the medium form a skip zone. Random fluctuations of rays "wash out" field enhancement in the vicinity of a regular caustic. Moreover, the presence of random caustics results in strong intensity fluctuations of radio wave fields. But it is known that geometrical-optics generalizations must be invoked in the vicinity caustics where a geometrical-optics approach is inapplicable. Therefore, with the purpose of solving problems of such a kind, using the M a s h method and the interference integral method this report dii~usses an approach to deriving an integral representation for a two-point propagator in a randomly-inhomogeneous ionosphere medium which takes into account the presence of regular and random caustics.The proposed approach uses a mixed Fourier transform of a two-point propagator with respect to both observation and transmitter coordinates, and agrees with both the geometrical optics approximation and the smooth perturbation method when they are applicable. Within such a view point this method is a generalization of the smooth perturbation method for the media with background refraction.In addition, the method agrees with phase screen results and therefore it can be considered as phase screen generalization for the oblique radiowave reflection from the randomly inhomogeneous layer.In orger to simplify the analysis of asymptotic approaches, we shall contine our subsequent treatment to a scalar two-dimensional problem. The field behaviour U(r) of a point source, located at the point r = rs, in a smoothly inhomogeneous medium is defined by the equation AU(r,r,) + k2E(r)U(r,= r,) = 6(r -ra), (1) ...
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