Известия ВУЗов.Радиофизика

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Estimation of the Seabed Parameters in a Nonuniform Waveguide Using the Matched-Field Method

Mon, 09/09/2019 - 00:00

We estimate geoacoustic parameters of a layered bottom in a shallow waveguide, which is nonuniform along the path, using the data of the full-scale experiment and the matched-field method. On the basis of the available a priori geological and geophysical data, a waveguide consisting of two sedimentary layers and the underlying half-space is used as the reference geoacoustic model. When seeking the geoacoustic parameters corresponding to the global minimum of the objective function, the hybrid algorithm, which is a combination of the fast simulated annealing method and the Hook–Jeeves method of direct search, is used.

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Experimental Study of Diurnal and Seasonal Variations in the Atmospheric Electric Field

Mon, 09/09/2019 - 00:00

In order to separate global and local effects of atmospheric electricity, measurements of the fair-weather electric field were performed in Nizhny Novgorod in 2013-2018. As a result of processing 139 diurnal records from four observation points spaced 6–8 km apart, diurnal variations in the fair-weather atmospheric electric field for different seasons and weekdays (working days and weekends) were studied. The curve of the local diurnal variation is shown to always have two maxima. The evening maximum of the diurnal variation (19:00–20:00 UT) coincides in time with the maximum of the Carnegie curve, which is a characteristic of the global electrical circuit. The highest values of the field amplitude are reached in the winter period. The field-intensity maximum in the first half of the day (09:00–11:00 LT) is characteristic of the urban environment and shows that local effects associated with the presence of aerosol particles in the air significantly contribute to the formation of diurnal variation, especially in summer. According to the 2013–2018 measurements, the seasonal variation in the monthly-average values of the atmospheric electric field is revealed and analyzed compared with the results of measurements of seasonal variation in other regions of the globe. The obtained results allow one to reveal the role of local effects in the formation of diurnal variation in the mid-latitude areas with temperate continental climate and provide a basis for developing a theory which can explain the physical mechanisms of local effects and suggest appropriate parametrization for finding the surface electric field in the weather and climate models.

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Compensation for the Influence of Fluctuations in the Distance to the Object During Noncontact Probing in Spectral Domain Optical Coherence Tomography

Mon, 09/09/2019 - 00:00

We propose and experimentally test a numerical method for correction of the influence of fluctuations in the distance to objects during noncontact probing in optical coherence tomography. The method is based on the analysis of phase shifts of the neighboring scans, which are due to microscale displacements, and further compensation for these displacements by using phasefrequency correction in the spectral domain. Unlike the known correlation methods, the proposed method does not distort the represented shape of the object surface. Its operability is demonstrated in model experiments in the cases of harmonic and random types of the motion of the scattering object, as well as in vivo imaging of the structures of the human middle ear.

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The Parameterization Method of Discrete VLF Chorus Emissions

Mon, 09/09/2019 - 00:00

We present a method for automatic search and parameterization of discrete elements of very low-frequency (VLF) chorus emissions. The method is based on the processing of dynamic spectrograms using a special scanning algorithm, which is intended for seeking the discrete elements of VLF chorus emissions and calculation of their parameters. We propose to create the optimal dynamic spectrograms for the scanning algorithm by using short-time Fourier transform. The paper gives general recommendations for calculation of spectrograms and their preprocessing. The scanning algorithm is based on processing of the dynamic-spectrogram images by using the methods of mathematical morphology. The developed method was tested for several cases of chorus emissions observed by the Van Allen Probes spacecraft. It is shown that the total errors related to “false positive” detection and missing the target are about 10% of the elements visible to the human eye when the optimal parameters of the scanning algorithm are used and intense discrete elements are processed. The method can be applied to both spacecraft and ground-based wave data. The results of using the method can be employed for verification of physical theories of the formation of chorus emissions and determining their statistical properties.

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Determination of the Efficient Number of Interferefnce Sources in the Problem of Adaptive Estimation of Temporal Waveforms of Narrow-Band Signals Using Antenna Arrays

Mon, 09/09/2019 - 00:00

We consider the problem of estimating temporal waveforms of the signals, which are generated by a spatially distributed set of sources, by using antenna arrays. To solve this problem, we employ the method of estimating signal parameters, which was proposed earlier and is based on maximization of the likelihood function, in which the interference correlation matrix is represented as a rank-deficient matrix. An algorithm for estimation of the efficient number of interference sources has been developed for application of this method. The results of application of the method are compared with the results obtained using the Akaike information criterion and the principle of minimum description length, as well as the earlier proposed method based on maximization of the signal-to-noise ratio.

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Dynamic Backscattering Halo of Pulsed Laser Beams in Thin Cloud Layers

Mon, 09/09/2019 - 00:00

We consider the effect of the dynamic backscattering halo of pencil beams of light in thin layers of scattering media. Numerical modeling, which immediately demonstrates this effect, is performed. A simple analytical theory that provides a numerical criterion for the effect visual detection, is proposed.

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Results of Studying the Daytime Polar Ionosphere by the Radio Occultation Method on Satellite-To-Satellite Paths

Mon, 09/09/2019 - 00:00

We analyze specific features of the radio occultation method and the possibility of using it to study the altitude profiles of electron density and the characteristics of sporadic formations in the daytime polar ionosphere of the Arctic at latitudes above 67° in the period from June 19 to July 1, 2015. Electron density at altitudes of 90 to 650 km is determined by ionospheric occultation sounding on GPS–FormoSat satellite-to-satellite paths, and its dependence on solar illumination at an altitude of 100 km is demonstrated. Data on sporadic Es formations at altitudes of 80 to 120 km are presented. Two types of Es structures attributed to the wind shear and precipitation of energetic particles from the magnetosphere are discussed.

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A Method of Separate Optimization of a Multistage Relay Mimo System

Mon, 09/09/2019 - 00:00

We propose a quasioptimal method of separate optimization in a multistage relay MIMO system, which is based on optimization of the root-mean-square error between the input and output signals successively for each transmission stage. Rigorous analytical expressions for the spatial coding and decoding matrices are obtained. Separate optimization simplifies the system construction due to a significant reduction in the number of auxiliary (service) communication links, which are necessary for transmitting the channel information, compared with the joint optimization. The simulation results show the high efficiency of the proposed method.

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