V.L. Kostina, S.I. Khomenko, I.M. Mytsenko, A.N. Roenko

One of the problems in radio wave propagation and radar technique is designing of marine navigation, communication and other radio engineering facilities that have the distance of action exceeding the limits of radiohorizon and serve to secure the safety of navigation and territorial waters. Per contra, the probability of appearance of radio wave propagation anomalies, increasing the distance of action beyond the horizon, in most regions of the World Ocean is substantially higher, than along continental and off-shore paths. Therefore the most perspective way for problem decision is to study the beyond horizon UHF radio wave propagation in tropospheric near-sea surface layer and exposure of possibilities for its effective realization. Over the time span between 1976 and 1991 the IRE (NASU) carried out experimental investigations of radio wave propagation over the Atlantic, Arctic, Indian and Pacific Ocean surfaces. Results of these researches, their theoretical analysis are the basis of this paper. The physical processes in the tropospheric near-sea surface layers substantially differ from the ones along the continental paths. Besides, reflections from the underlying surface which is near to the ideal conductor in the UHF range, lead to modulation of the antenna system directional pattern. The ship antennas can not be disposed on a large height, so the first pattern lobe angle-of-elevation value is necessary to be considered. The influence of reflections from the underlying surface in the area of line-of-sight on radio wave propagation caused by scattering on troposphere turbulence in UHF and SHF ranges is considered. This influence leads to a change of specific effective scattering cross section and general volume of the troposphere (Gordon prism), participating in creation of specific radiation. To estimate the change of the specific effective scattering cross section the Villars-Veyskopf theory is utilized. Analytical expression for attenuation factor V (r) approximate value is obtained. The calculation results show that at the identical heights of antenna systems the higher frequency range of radio waves the lower attenuation beyond the horizon. This advantage becomes negligible with distance increase and then disappears at all. The high levels of signals beyond the horizon in decimeter and meter wave ranges are the result of action of another mechanism of radio wave distant troposphere propagation, i.e. reflection from the inversion layers. Thus the attenuation factor distance dependences have a number of maximums and minimums on different distances. This fact causes an appearance of radar «visibility zones». The analytical expression for radio wave attenuation factor , being Freshnel factor of reflectivity from an infinite extensive layer with the jump of permittivity and taking into account the influence of reflections from a surface on the transmit and receive ends of propagation path, is obtained. Another feature of radio wave propagation above an ocean surface is the presence of evaporation ducts which arise up directly above the ocean surface and are caused by the rapid slump of humidity with a height. The air layer adjoining to the ocean surface is saturated with water pair (its relative humidity is 100%). Humidity decreases with a height till its value will not be attained the latter in troposphere, determined by general meteorological conditions. As a result the evaporation duct appears at a surface. The height of evaporation duct is considered as a height on which the modified index of refraction M achieves the minimum value. Evaporation duct extends from its higher bound to the ocean surface. The casual factors (refraction coefficient fluctuations, oceanic surface variability et cetera) lead to scattering of energy on these irregularities, that causes signal attenuation. Actually, if in the closed waveguide dispersion results in energy redistribution between harmonics, in the case of the opened waveguide dispersion is caused by transformation of basic wave to the strongly fading modes. Energy of these modes scatters to the upper layers of troposphere. Therefore only the first modes attenuate poorly in the evaporation ducts. Their number is determined by the duct height and deficit . Usually these quantities are such, that in the centimeter wave range the evaporation duct takes only the first mode, and energy of another ones quickly decreases with distance. In experimental part of the paper a short description of the meteorological conditions in investigated regions of the Atlantic, Indian, Arctic and Pacific Oceans is presented, the measuring techniques and equipment are described. The investigations in question were conducted aboard two ships that were outfitted with a transmitter (the first one) and a receiver (the second one). The ships have moved apart until the signal has been lost in noise. To decrease errors in attenuation factor distance dependences the signal values were measured in the interference zone and then referenced to calculated ones. The obtained experimental results allow creating a database for attenuation factor distance dependences. The database itself consists from a set of experimental data arrays, organized as a database and application program package allows operation with the data. For example, one can see the mode of the program operation, allowing selection of experimental distance dependences of attenuation factor corresponding to specific date (month, year), region of researches, path number, radiated wave-length and antennas height. Besides, it is shown the program mode which allows one to calculate the distance of action of radio engineering devices. The application programs make it possible to solve rationally the tasks of data array control, operating access to them, array selection according to the function of inquiry and their processing. The package of programs is simple and convenient in application. The use of this system does not require from the user some knowledge of programming or special experience in the field of data processing. The database can be used for calculation and development of ship radio engineering facilities, and also for verification of existent theoretical models of radio wave propagation above an ocean surface for UHF and SHF ranges.