Radiophysics and Quantum Electronics最新文献

The propagation of Q-bursts in the Earth–ionosphere cavity with a day–night non-uniformity is numerically simulated for the first time. The vertical conductivity profiles of the middle atmosphere differ from each other in the daytime and night-time hemispheres. The model of a smooth day–night transition in the ionosphere is used. The waveguide parameters in the ambient daytime and night-time conditions are computed using the full wave method in the form of a Riccati differential equation. These parameters are included in the numerically solved 2D telegraph equation (2DTE), thus providing complex spectra of the vertical electric field component. The solution obtained in the frequency domain is verified by comparison with published data. The sought waveforms of Q-bursts are computed using the inverse Fourier transform of complex spectra calculated in the 1–1000 Hz band with a 1-Hz step. Impact of the day–night non-uniformity is estimated by comparing the pulse waveforms on different propagation paths and using instant 2D field distributions in the Earth–ionosphere cavity. Time domain models of Q-bursts in the non-uniform Earth–ionosphere cavity were obtained for the first time. The day–night non-uniformity causes a time-dependent shift of the antipode pulse maximum from the geometric antipode of the source. A modulation of the amplitude of the negative and positive half-wave of a Q-burst in the vicinity of the ionospheric irregularity is observed. The pulse shape depends on its arrival direction relative to the terminator line. The character of variations in the waveform of Q-bursts indicates that solutions for the uniform cavity can be used for evaluating the source–observer distance in a real cavity. Prospects for experimental detection of the impact of the day–night non-uniformity on the shape of Q-bursts are discussed.

We present the results of numerical simulations of VLF chorus emissions in the magnetospheric cyclotron maser within the framework of the backward wave oscillator (BWO) regime model. Specific features of the dynamic wave spectrum at various points inside the generation region in the presence of quasi-static variations of the geomagnetic field are considered. We show that discrete elements with both increasing and decreasing frequency can be generated, and the dynamic spectrum shape can change qualitatively upon the shift of the observation point inside the generation region. We discuss this dependence of the dynamic spectrum of the generated waves on the position of the observer in the generation region for various parameters of the geomagnetic field distortion. We show that the formation of elements with an increasing (decreasing) frequency is determined by the presence of the regions with local minima (maxima) of the external magnetic field.

We analyze statistical properties of resolving two signal sources with plane wavefronts via eigenvalue analysis of the sample correlation matrix of the intrinsic noise of the receiving system for a low signal-to-noise ratio. The analysis was carried out both for the signals of the same power and in the presence of a single high-power interference source. The resolution properties of two sources with different signal arrival angles are also considered. The distribution of the maximum noise eigenvalue in the presence of a weak signal source is studied numerically.

Railway crossings are sources of increased danger for railway operation and road traffic. The efficiency of detecting low-observable targets at railway crossings can be significantly improved by using forward scatter radar (FSR) systems. This paper analyzes the potential characteristics and parameters of such systems as elements of railway crossing safety equipment. We consider a target detection method that determines the architecture of the antenna array, the probing signal parameters, and the algorithms for processing of the forward scatter signal using the FSR system of the railway crossing security equipment. We present the results of calculating the detection area of a forward scatter radar for a cube target and a motor vehicle, as well as the simulation analysis for the optimal signal processing algorithm. It is shown that the simultaneous use of two frequency ranges makes it possible to detect objects approaching the railway crossing and objects that stay in the vicinity of the railway tracks (stationary objects).

We consider an automobile radar of millimeter wave lengths with phase-shift keyed (PSK) probing signals operating under the influence of active interference whose sources are radars of other vehicles. We propose a method of spatiotemporal processing which allows one to detect and estimate the parameters (range, velocity, and bearing) of the surrounding vehicles, which can simultaneously be both interference-free targets (own radar is turned off) and interference targets (own radar is turned on). We study the case where both linear frequency-modulated (LFM) and phasemodulated (PM) signals can be interference. The numerical-simulation results are presented, showing the high efficiency of the proposed method in terms of the probability of simultaneous detection of near and far interference and interference-free targets.

We show the results of estimating the accuracy of positioning a source of the ultra-wideband signals using the method for estimating the time of appearance of such signals, which are received against the background of Gaussian narrow-band interference and Gaussian white noise. We have studied the influence of the Gaussian narrow-band interference on the accuracy of positioning of the ultra-wideband signal source on the basis of the triangulation method.

Advances in well drilling technology are increasing the need for real-time geophysical data on the drilling process. It is obvious that the most promising channel of data transmission from the bottom of the well to the surface is the acoustic communication channel through structural elements of the drill string. Currently available estimates predict that the data transmission rate through the acoustic communication channel can reach several hundred bits per second. When designing an acoustic communication channel and calculating its capacity, it is generally assumed that the noise associated with the drilling process is an additive random Gaussian process. However, direct measurements of drilling noise over a wide range of frequencies show that this assumption is incorrect. There is a high probability for the occurrence of high-amplitude spikes, and the average vibration level varies greatly over short periods of time. This paper is devoted to the study of the influence of real drilling noise on the acoustic communication channel. A digital model was developed for this purpose. The model takes into account experimentally obtained data on drilling noise that has been recorded over a long period of time in natural experiments. The results of modeling the bit error probability under different approaches to noise-tolerant coding are presented and a comparison with a Gaussian channel is made. It is shown that deviations of noise that accompanies drilling from the normal random process have a fundamental effect on the quality of communication in the acoustic data transmission channel.

We study the properties of radio waves with equidistant polarization states. The characteristics of radio wave sequences with the same polarization proximity are given. The application of partially polarized radio wave sequences with a chosen value of the polarization proximity for measuring the differential reflectivity of precipitation is considered. Algorithms for measuring the polarization characteristics of radar targets using probing radio waves with equidistant polarization states are described.

We study the capabilities of nonequidistant sparse planar antenna arrays to solve the problems of detection and estimation of the source parameters. For the synthesis of nonequidistant antenna arrays with the desired characteristics, the method proposed earlier for linear arrays based on random sampling of the element positions while fixing the mean inter-element spacing is applied. This method is shown to also successfully synthesize the planar sparse arrays. The detection characteristics for the synthesized arrays are only slightly worse than those for an equidistant array. This difference can be formulated in terms of a slight deterioration of the signal-to-noise ratio (by 1–2 dB). In this case, the frequency range increases many times. This work employs the earlier proposed probabilities of correct detection and estimation, as well as the probability of an abnormal error for a quantitative description of the capabilities of the planar sparse antenna array.

We consider the one-dimensional problem of propagation of a narrow-band signal in a homogeneous dispersive medium. Within the framework of the method of moments, simple relations are obtained which permit one to use integration of smooth (non-oscillatory) functions to find the midpoint and r.m. s. duration of an arbitrary signal at an arbitrary point on the path without any additional approximations. It is shown that if the absorption dispersion is negligible, the square of the r.m. s. duration of the signal depends on the path length according to the parabolic law, i.e., it has a single minimum (“focus” of the signal). The possibility of reducing the duration (and the corresponding power increase) of linear frequency-modulated (LFM) signals during propagation in a dispersive medium is considered. For a space-limited dispersive medium, using the example of a weakly colliding plasma, estimates are obtained for the maximum possible (for a given path length) reduction of the signal duration and increase in its power, as well as for the initial signal parameters at which these capabilities are realized.