Search

Within the framework of a simple model of the sensitivity function, the Doppler spectra are considered for different ways of generating response signals using plane wave compounding. A Doppler spectrum is obtained for coherent compounding of signals received at different steering angles of waves during their period of changing. Compared to traditional diagnostic systems, the Doppler spectrum width is increased only by limiting the duration of the signals. There is no additional increase in the spectrum width if the compound signals are formed by adding with cyclic permutation, in which signals from each new wave angle are compounded. When a Doppler signal is formed directly from Doppler signals at different steering angles, the spectral width increases both in comparison with the traditional method of sensing with stationary focused ultrasound fields and with the case of coherent signal compouding. The obtained increase in the spectral width has an intrinsic physical meaning. The increase in width is connected with a dynamic change in the Doppler angle, which increases the interval of apparent projections of the velocities of motion of inhomogeneities along the direction of transmitting of a plane wave without inclination.

Channel and delay coefficient are two essential parameters for the characterization of a multipath propagation environment. It is crucial to generate realistic channel and delay coefficient in order to study the channel characteristics that involves signals propagating through environments with severe multipath effects. While many deterministic channel models, such as ray-tracing (RT), face challenges like high computational complexity, data requirements for geometrical information, and inapplicability for space-ground links, and nongeometry-based stochastic channel models (NGSCMs) might lack spatial consistency and offer lower accuracy, we present a scalable tutorial for the channel modeling of dual mobile space-ground links in urban areas, utilizing the Quasi Deterministic Radio Channel Generator (QuaDRiGa), which adopts a geometry-based stochastic channel model (GSCM), in conjunction with an International Telecommunication Union (ITU) provided state duration model. This tutorial allows for the generation of realistic channel and delay coefficients in a multipath environment for dual mobile space-ground links. We validate the accuracy of the work by analyzing the generated channel and delay coefficient from several aspects, such as received signal power and amplitude, multipath delay distribution, delay spread and Doppler spectrum.

The Doppler spectra of the scattering fields from sinusoidal water waves illuminated by a plane wave or Gaussian beam are numerically simulated with the method of moment (MoM). When the sinusoidal water surface is illuminated by a plane EM wave, the numerical results show that the Doppler spectrum of the scattering field is distorted seriously due to the frequency leakage. For this case, not only the resonance spectral peaks corresponding to the phase velocity of the water wave but also other equally spaced harmonic peaks appear on the spectrum curve. However, when the sinusoidal water surface is illuminated by a Gaussian beam with appropriate beam width, the harmonic peaks can be effectively suppressed. To better explain this phenomenon, the theoretical model for the spectrum of the scattering field from a sinusoidal water wave is derived based on the first-order small slope approximation theory (SSA-I). And this theoretical model proposed by us could give a reasonable explanation for the above phenomenon. [ABSTRACT FROM AUTHOR]

Within the framework of a simple model of the sensitivity function, the Doppler spectra are considered for different ways of generating response signals using plane wave compounding. A Doppler spectrum is obtained for coherent compounding of signals received at different steering angles of waves during their period of changing. Compared to traditional diagnostic systems, the Doppler spectrum width is increased only by limiting the duration of the signals. There is no additional increase in the spectrum width if the compound signals are formed by adding with cyclic permutation, in which signals from each new wave angle are compounded. When a Doppler signal is formed directly from Doppler signals at different steering angles, the spectral width increases both in comparison with the traditional method of sensing with stationary focused ultrasound fields and with the case of coherent signal compouding. The obtained increase in the spectral width has an intrinsic physical meaning. The increase in width is connected with a dynamic change in the Doppler angle, which increases the interval of apparent projections of the velocities of motion of inhomogeneities along the direction of transmitting of a plane wave without inclination. [ABSTRACT FROM AUTHOR]

Abstract A method for time‐domain motion compensation of high frequency (HF) radar signals for the case of a floating transmitter and fixed receiver is proposed when the motion parameters (including the amplitude and angular frequency of the motion) are not known a priori. In this study, the floating platform is assumed to follow a single‐frequency motion model. Additionally, instead of trying to estimate platform motion parameters from the received motion‐contaminated Doppler spectrum, which is proportional to the observed radar cross‐section of the ocean surface from the floating platform, the motion parameters from the autocorrelation function of the received electric field are estimated, which is related to the received radar cross‐section by application of an inverse temporal Fourier transform. By comparing the locations of the zeros of the autocorrelation function for the fixed antenna case with that for an antenna on a floating platform and finding the zeros associated with the platform motion, the motion parameters are estimated. These parameters are matched with actual motion parameter values, from which the motion‐compensated Doppler spectrum is recovered from the Doppler spectrum of the antenna on a floating platform.

Using Deep Neural Network (DNN) modeling technology, a prediction model of Doppler spectral parameters of sea clutter based on multiple measurement conditions is established based on measured data of sea clutter from shore-based radar under different radar parameters and marine environmental parameters. The recognition of sea clutter spectral characteristics based on environmental characteristics and independent of clutter data is realized. The spectral frequency shift and broadening prediction accuracy are greater than 90%. Based on the prediction model, an analysis method of Doppler spectrum influence factors based on the parameter cycle decreasing cognition is proposed. The influence of different measurement parameters on the Doppler spectrum prediction of sea clutter is analyzed, and the change law of spectrum parameters with the main influence factors is obtained. The results are of great significance to the application of sea surface target detection based on Doppler characteristics.

The Doppler information in complement to the intensity of radar backscatter has been verified useful for ocean synthetic aperture radar (SAR) applications. An accurate estimate of the ocean surface motions requires an accurate estimate of the Doppler centroid. When we process the real SAR data, we find that different methods yield different mean values of the estimates. After a careful analysis, we find that it is caused by the underestimation of the centroids for conventional methods. The estimate bias depends on the Doppler centroid itself. To overcome this problem, we carefully study the conventional methods and the Cramér–Rao bound of the estimation. Based on the maximum-likelihood principle, a novel method is proposed. By adaptively adjusting the center and the bandwidth of the weighting function after a coarse estimation of the spectrum, the symmetry of the spectrum is maintained. The estimate bias of conventional methods is eliminated. It is a fully optimal method meeting the Cramér–Rao bound no matter whether the true Doppler centroid is high or low. Both the simulated and the real SAR data validate our novel method. In the simulation, an estimate error of tens of Hz could be avoided when the true Doppler center is hundreds of hertz. In real SAR estimation, an accuracy of 3–6 Hz can be improved. The result of the estimation is consistent with the simulation and theoretical analysis, validating the efficiency of our improved method.

The trimaran is an advanced ship with high hydrodynamic performance, but the study of its electromagnetic characteristics and Doppler spectrum is insufficient. This study presents a new procedure for evaluating the scattering echo and Doppler spectrum of a trimaran in a time-varying ocean environment. First, based on the Reynolds-averaged Navier–Stokes equations, the interpolation wave-making method is proposed for the prediction of the six degrees of freedom motion of the trimaran. The method takes into account the viscous effect of the flow field and the complexity of wave motion, which applies to the seakeeping prediction of high-speed trimaran under different sea states. Second, an accelerated geometrical optics and physical optics algorithm using the light source function (GO-PO/LSF) is proposed, which uses the light source function to realize the visibility judgment of surfaces and greatly reduces the computation time. Then, the influence of different motion conditions on the backscattering echo of the composite scene is demonstrated by the capillary wave modification facet scattering model and GO-PO/LSF. Finally, the Doppler spectrum characteristics of the trimaran are presented by the standard spectrum estimation method, which provides a reference for radar detection and identification of ships in the microwave band.

Subject and Purpose. The study of the effect that each new Solar eclipse (SE) has on radio wave characteristics is an actual scientific and technical issue. The purpose of the present work is to analyze the variations in Doppler spectra (DS), Doppler shift of frequency (DSF), and in the reflected wave amplitude (RWA) that were observed during the SE of June 21, 2020 over the People’s Republic of China. Methods and Methodology.The observations of HF radio wave characteristics were made using the Harbin Engineering University multi-frequency multipath coherent radio system. The temporal variations in DS, DSF of the main ray and RWA are analyzed further. The variations in the Doppler frequency shift (DSF) were subjected to a systematic spectral analysis that involved joint application of the windowed Fourier transform, adaptive Fourier decomposition, and the Morlet mother-function-based wavelet transformation. Results. The SE was accompanied by DS diffuseness resulting from an increase in the number of rays. The DSF temporal variations were observed to be bi-polar and asymmetrical, with extreme DSF magnitudes varying from –11 to –40 mHz and from 22 to 56 mHz. The duration of processes with negative DSF values varied from 50 to 80 min, and the duration of processes with positive DSF changed from 30 to 80 min. The multi-hop propagation (from two to five hops) took place along all propagation paths, with a 360 to 560-km one-hop range. The 4–5-min period quasi-periodic DSF variations showed 20–50 mHz amplitude, and the 8–18-min period variations exhibited 40–100 mHz amplitude. The relative amplitudes of the 4–5 min period quasi-periodic variations in the electron density were observed to be in the 0.3–6.2% range, and the amplitudes of the 8–18 min period variations were found to be in the 1.1–21.7% range. A decrease in the electron density along different propagation paths was observed to vary from –(12–16)% to –(20–26)%. Conclusions. The characteristic features of the variations in HF radio wave parameters in the ionosphere have been studied during the SE of June 21, 2020 over the People’s Republic of China.

A 1.29 GHz wind profiler was developed under a private–military–government cooperative wind profiler radar development project in the Republic of Korea. During the test operation period, radio frequency interference (RFI) contamination occurred in the spectrum. In addition to the general shape, with a continuous appearance depending on the altitude, the spectrum showed complex shapes, such as discontinuous and overlapping frequencies. The RFI characteristics in the wind profiler spectra were analyzed, and a new algorithm was developed to remove multiple RFI (MRFI). Meteorological and non-meteorological signals were separated by filtering with a spectrum width threshold of 0.1 m/s. A continuity check was performed to determine MRFI in the non-meteorological signal. The number of gates in which the same radial velocity is continuous was determined based on whether the beam was vertical or oblique; a rough continuity test was performed, considering exceptional circumstances for the meteorological signal. For overlapping MRFI, the process was repeated. Spectral contamination by MRFI was removed through filtering and iterated scans; the continuity of wind vectors calculated from the improved spectral radial velocity was verified. Good-quality wind vectors can be produced even in a bad-radio-frequency environment if proper quality control is performed. [ABSTRACT FROM AUTHOR]

A method for time‐domain motion compensation of high frequency (HF) radar signals for the case of a floating transmitter and fixed receiver is proposed when the motion parameters (including the amplitude and angular frequency of the motion) are not known a priori. In this study, the floating platform is assumed to follow a single‐frequency motion model. Additionally, instead of trying to estimate platform motion parameters from the received motion‐contaminated Doppler spectrum, which is proportional to the observed radar cross‐section of the ocean surface from the floating platform, the motion parameters from the autocorrelation function of the received electric field are estimated, which is related to the received radar cross‐section by application of an inverse temporal Fourier transform. By comparing the locations of the zeros of the autocorrelation function for the fixed antenna case with that for an antenna on a floating platform and finding the zeros associated with the platform motion, the motion parameters are estimated. These parameters are matched with actual motion parameter values, from which the motion‐compensated Doppler spectrum is recovered from the Doppler spectrum of the antenna on a floating platform. [ABSTRACT FROM AUTHOR]

In this paper, a weighted arctangent function is used in conjunction with the spectral method to generate a land–sea junction composite rough surface under the spatially homogeneous and time-stationary hypotheses. The exponential correlation function and the Joint North Sea Wave Project (JONSWAP) spectrum, combined with an experiment-verified shoaling coefficient, are applied to model the land surfaces and the time-varying sea surfaces separately. The second-order small slope approximation (SSA-II) with tapered wave incidence is utilized for evaluating the electromagnetic scattering characteristics and Doppler characteristics of the generated composite rough surface. The influence of land–sea interface factors on radar cross-section (RCS) and Doppler shift of radar echoes is investigated in detail by comparing the RCS and Doppler spectra of the land–sea junction composite rough surfaces with those of finite-depth sea surfaces. It can be found that the Doppler spectra of the land–sea junction composite rough surface is narrower than that of the finite-depth sea surface under upwind directions and wider than that of the finite-depth sea surface under crosswind directions. [ABSTRACT FROM AUTHOR]

This paper describes the implementation of a Doppler spectrum measurement platform for narrowband frequency-dispersive vehicle-to-vehicle (V2V) channels. The platform is based on a continuous-wave (CW) channel sounding approach widely used for path-loss and large-scale fading measurements, but whose effectiveness to measure the Doppler spectrum of V2V channels is not equally known. This channel sounding method is implemented using general-purpose hardware in a configuration that is easy to replicate and that enables a partial characterization of frequency-dispersive V2V channels at a fraction of the cost of a dedicated channel sounder. The platform was assessed in a series of field experiments that collected empirical data of the instantaneous Doppler spectrum, the mean Doppler shift, the Doppler spread, the path-loss profile, and the large-scale fading distribution of V2V channels under realistic driving conditions. These experiments were conducted in a highway scenario near San Luis Potosí, México, at two different carrier frequencies, one at 760MHz and the other at 2,500MHz. The transmitting and receiving vehicles were moving in the same direction at varying speeds, ranging from 20 to 130km/h and dictated by the unpredictable traffic conditions. The obtained results demonstrate that the presented measurement platform enables the spectral characterization of narrowband V2V channels and the identification of their Doppler signatures in relevant road-safety scenarios, such as those involving overtaking maneuvers and rapid vehicles approaching the transmitter and receiver in the opposite direction.

The launch of a rocket requires an energy comparable to the energy of many natural processes. For large rockets, the energy release reaches 10–100 TJ, and the power of engines reaches 0.1–1 TW. The energy release per unit volume is much higher than the specific energy content and energy release of all natural processes. During the launch and flight of a large rocket, disturbances in the substratum, the atmosphere, the ionosphere, and even in the magnetosphere occur. Effects from rocket engine burns have been studied for more than 60 years. Research results have been published in hundreds of articles, handbooks, and monographs. It turns out that the effects produced exhibit diverse geophysical phenomena. The effects near the rocket trajectory, namely, the regions of depressed electron density (ionospheric holes), and the generation of infrasound and atmospheric gravity waves (density waves) are investigated better than other effects. Great attention has been paid to studying the geomagnetic effect. The following methods have been used in studies: the Doppler effect, the Faraday, incoherent scattering, ionosonde, magnetometric methods, etc. The effects accompanying the launches and flights of rockets are being actively studied even now. For many years, large-scale (1 to 10 Mm) disturbances that occur after rocket launches have been studied. Their study makes it possible to better understand the mechanisms of the propagation of disturbances from a rocket over global distances, the interaction of subsystems in the Earth–atmosphere–ionosphere–magnetosphere system, and the ecological consequences of rocket engine burns. Disturbances occurring in the atmosphere and geospace substantially depend on the state of the atmospheric–space weather, time of day, season, and phase of the solar cycle. Even with the launch of two identical rockets, disturbances in the mentioned system can be very different. It should be borne in mind that rockets differ in power, trajectories, fuel composition, and the location of cosmodromes. Therefore, studying the response of subsystems to rocket launches and flights remains an urgent problem. The purpose of this study is to describe the results of an analysis of the ionospheric effects of the Soyuz and Proton rockets launched during the 24th cycle of solar activity from the Baikonur Cosmodrome. To observe the effects in the ionosphere caused by the launch of the Soyuz and Proton rockets from the Baikonur Cosmodrome, a vertical sounding Doppler radar was used. As a rule, measurements are carried out at two fixed frequencies of 3.2 and 4.2 MHz. The smaller of them is effective when studying the dynamic processes in the E and F1 layers, and the larger one is effective when studying the F1 and F2 layers. The parameters of ionospheric disturbances that followed the launches of 81 Soyuz rockets and 53 Proton rockets from the Baikonur Cosmodrome in 2009–2021 are analyzed. It is confirmed that there are several groups of delay times for the possible reaction of the ionosphere to the launch and flight of the rockets. These delay times varied widely (from 10 to 300 min). The delay time groups correspond to several groups of apparent horizontal velocities of disturbance propagation (100–200, 390 ± 23 m/s, 0.97 ± 0.10, 1.28 ± 0.13, 1.68 ± 0.13, 2.07 ± 0.13 km/s, and approximately 8 km/s). Slow atmospheric gravity waves, atmospheric gravity waves of man-made origin, density shock waves, slow, and ordinary MHD waves have such velocities. As a rule, the generated perturbations (except for shock waves) are quasi-periodic in behavior with a period in the range from 5 to 20 min. The amplitude of the Doppler shift is 0.1–0.3 Hz. The relative amplitude of quasi-periodic variations in the electron density is typically 1‒10% and rarely reaches 20%. [ABSTRACT FROM AUTHOR]

The non-contact vital sign estimation of an individual using radar sensors has been widely researched. However, in the case of an individual with undesirable rigid-body motion of the torso, non-contact vital sign estimation may fail owing to extreme phase fluctuations caused by the larger rigid-body motion of the torso than the micro motion of the vital sign. To address this problem, we propose a rigid-body motion compensation method using the complex echo signals of the IR-UWB radar for the vital sign estimation of an in- dividual with rigid-body motion. The proposed method extracts a complex signal vector using principal component analysis from com- plex echo signals in a radial range and pulse time domain. Next, compensation for the rigid-body motion is implemented by minimizing the entropy of the spectrum for the complex signal vector. In simulations using a point-scatterer model and experiments using IR-UWB commercial radar, we verified that our proposed method is capable of performing successful vital sign estimation of an individual with undesirable rigid-body motion. [ABSTRACT FROM AUTHOR]

Wind profiling radars are very useful instruments in meteorological forecasting for evaluating the three dimensional wind velocities. The challenging part of this job is to identify the true Doppler peaks of atmospheric origin from a noisy Doppler profile. This paper presents a novel algorithm for the Doppler peak estimation in windprofilers. The normalized Doppler spectrum of each range bin is divided into multiple clusters and low signal to noise ratio (SNR) clusters are discarded. Then, clusters having no continuity along height axis are filtered out and finally the remaining clusters are interpolated to get the wind profile. The algorithm performs very well even when the signal SNR is consistently below −3 dB over successive range bins. The proposed algorithm is found to have improved performance compared to existing algorithms with a reduced computational time. [ABSTRACT FROM AUTHOR]

The Harbin Engineering University, the People's Republic of China, multifrequency multiple‐path coherent radio system operates continuously and provides data for post analysis. The data collected during the solar eclipse of 21 June 2020 have been chosen for this study with the objectives to interpret the variations in the Doppler spectra, Doppler shift, and in the reflected radio wave amplitudes that are associated with the solar eclipse, establish the magnitude and find physical significance of these variations, determine the reduction in the electron density caused by the solar eclipse, and to estimate an increase in wave activity in the ionosphere. The eclipse was accompanied by Doppler spectrum diffuseness resulting from an increase in the number of rays, the temporal variations in the Doppler shift were observed to be bi‐polar, asymmetrical, and anomalously small, with extreme Doppler shift magnitudes varying from −11 to −40 mHz and from 22 to 56 mHz. The duration of processes with negative Doppler shifts varied from 50 to 80 min, and the duration of processes with positive Doppler shifts changed from 30 to 80 min. The multi‐hop propagation (from two to five hops) took place along all propagation paths, with a 360 to 560‐km one‐hop length, due to the anomalous radio wave propagation via the sporadic‐E layer present about 80% of the time on 21 June 2020. The Doppler shift exhibited 4–18‐min period quasi‐sinusoidal variations with 20–10‐mHz amplitudes. Plain Language Summary: In the past, the study of the influence of eclipses on the ionosphere seemed to be a rather simple subject because a solar eclipse seemed to be just mimicking night, although over a shorter time interval. Subsequently, the movement of the solar terminators has been discovered to generate atmospheric gravity waves, which is also true for eclipses. Recent advances in instrumentation and measurement techniques have facilitated the revelation of unusual and interesting phenomena that do not necessarily belong to nighttime conditions. For example, in this study, we have discovered, along with the determination of specific changes in the atmospheric and radio wave parameters, that temporal variations in the Doppler shift were bi‐polar, asymmetrical, and anomalously small. Key Points: Doppler shift variations were bi‐polar, asymmetrical, and anomalously small, with extremes varying from −11 to −40 MHz and from 22 to 56 MHzMulti‐hop propagation (from two to five hops) took place along all propagation paths, with a length of one hop varying from 360 to 560 kmAn electron density decrease was determined to vary from –(12–16)% to –(20–26)% along different propagation paths [ABSTRACT FROM AUTHOR]

Purpose: Solar eclipses pertain to high-energy sources of disturbance in the subsystems of the Sun–interplanetary-medium–magnetosphere–ionosphere–atmosphere–Earth and the Earth–atmosphere–ionosphere–magnetosphere systems. During the solar eclipse, the coupling between the subsystems in these systems activates, and the parameters of the dynamic processes become disturbed. Investigation of these processes contributes to understanding of the structure and dynamics of the subsystems. The ionospheric response to the solar eclipse depends on the season, local time, magnitude of the solar eclipse, phase of the solar cycle, the observation site, the state of space weather, etc. Therefore, the study of the effects, which each new solar eclipse has on the ionosphere remains an urgent geophysics and radio physics problem. The purpose of this paper is to describe the radio wave characteristics and ionospheric parameters, which accompanied the partial solar eclipse of 10 June 2021 over the City of Kharkiv. Design/methodology/approach: To make observations, the means of the HF Doppler measurements at vertical and oblique incidence available at the V. N. Karazin Kharkiv National University Radiophysical Observatory were employed. The data obtained at the “Lviv” Magnetic Observatory were used for making intercomparison. Findings: The radiophysical observations have been made of the dynamic processes acting in the ionosphere during the solar eclipse of 10 June 2021 and on the reference days. The temporal variations in the Doppler frequency shift observed at vertical and oblique radio paths have been found to be, as a whole, similar. Generally speaking, the Doppler spectra over these radio propagation paths were different. Over the oblique radio paths, the number of rays was greater. The solar eclipse was accompanied by wave activity enhancement in the atmosphere and ionosphere. At least three wave trains were observed. The values of the periods (about 5–12 min) and the relative amplitudes of perturbations in the electron density (δN≈0.3–0.6 %) give evidence that the wave disturbances were caused by atmospheric gravity waves. The amplitude of the 6–8-min period geomagnetic variations has been estimated to be 0.5–1 nT. Approximately the same value has been recorded in the X component of the geomagnetic field at the nearest Magnetic Observatory. The aperiodic effect of the solar eclipse has appeared to be too small (less than 0.01 Hz) to be observed confidently. The smallness of the effect was predetermined by an insignificant magnitude of the partial eclipse over the City of Kharkiv (no more than 0.11). Conclusions: The features of the solar eclipse of 10 June 2021 include an insignificant magnitude of the aperiodic effect and an enhancement in wave activity in the atmosphere and ionosphere.

A 1.29 GHz wind profiler was developed under a private–military–government cooperative wind profiler radar development project in the Republic of Korea. During the test operation period, radio frequency interference (RFI) contamination occurred in the spectrum. In addition to the general shape, with a continuous appearance depending on the altitude, the spectrum showed complex shapes, such as discontinuous and overlapping frequencies. The RFI characteristics in the wind profiler spectra were analyzed, and a new algorithm was developed to remove multiple RFI (MRFI). Meteorological and non-meteorological signals were separated by filtering with a spectrum width threshold of 0.1 m/s. A continuity check was performed to determine MRFI in the non-meteorological signal. The number of gates in which the same radial velocity is continuous was determined based on whether the beam was vertical or oblique; a rough continuity test was performed, considering exceptional circumstances for the meteorological signal. For overlapping MRFI, the process was repeated. Spectral contamination by MRFI was removed through filtering and iterated scans; the continuity of wind vectors calculated from the improved spectral radial velocity was verified. Good-quality wind vectors can be produced even in a bad-radio-frequency environment if proper quality control is performed.

Two examples of low grazing angle radar sea clutter, both well-described by the compound $K$ -distribution model, are studied. Pulse Doppler processing is applied to obtain 2-D range–time textures for the intensity, centroid, and width of the Doppler spectrum. The first example exhibits a monochromatic swell pattern, allowing phase averaging to be applied to the textures. The second example has a more typical ocean wave spectrum. The intensity textures are gamma-distributed, consistent with the compound $K$ -distribution model, but the Doppler spectrum centroid and width textures are also found to be gamma-distributed. Based on this analysis, a new method for simulation of coherent radar sea clutter is proposed, where separate memoryless nonlinear transformations are applied to a simulated water surface to generate the spatially and temporally varying intensity, centroid, and width of the Doppler spectrum. The method builds on the evolving Doppler spectrum model for radar sea clutter simulation and established methods for simulation of water surfaces. [ABSTRACT FROM AUTHOR]

The sea clutter, referred to as the time-varying radar backscatter from the ocean surface, plays a significant role in marine radar development. The ocean’s complex hydrodynamics cause it to exhibit non-Gaussian and nonstationary characteristics, which brings challenges in the sea clutter modeling, especially for establishing its spatial–temporal correlated and coherent model. In this article, a data-driven method based on the Koopman mode decomposition (KMD) is proposed for modeling spatial–temporal correlated complex sea clutter. The method decomposes the coherent sea clutter dynamic behavior in terms of Koopman modes and corresponding temporal patterns. Then, these spatiotemporal patterns are used to construct the sea clutter state over time according to the approximate solution. Furthermore, this proposed state-of-the-art data-driven approach is benchmarked by the measured sea clutter data from intelligent PIXel processing radar (IPIX). It is demonstrated that the proposed approach accurately models the complex sea clutter with actual statistic characteristics, phase information, and spatial–temporal correlations. The mean absolute error (MAE) and root mean square error (RMSE) between the obtained and actual sea clutter are only 0.1817 and 0.2349, respectively. This work offers a practical approach for modeling sea clutter, especially when the spatial–temporal correlation and coherence information is needed. [ABSTRACT FROM AUTHOR]

Purpose: The object of the radio study is to investigate dynamic processes, which occurred over the People’s Republic of China following three moderate (magnitudes 5.9-6.6) earthquakes in Japan in 2018–2019. The distances between the earthquake epicenters and the radio paths midpoints varied from approximately 1300 to 2000 km The aim of the study is to present observations of the dynamic processes in the ionosphere, which accompanied the earthquakes in Japan, and the analysis of intercomparison between the events. Design/methodology/approach: To continuously observe the ionosphere state over the ~ 100-300 -km altitude range, the multi-frequency multiple path radio system for oblique incidence soundings of the ionosphere has been designed by the specialists at the V. N. Karazin National University (Ukraine) and the Harbin Engineering University, PRC (45.78 N, 126.68 E). The basic premise upon which the system operation is based are the measurements of the Doppler shift of frequency, fD, and of the amplitude of radio waves reflected from the ionosphere. The Doppler spectra are calculated over the 20-s intervals, with the Doppler resolution of 0.02 Hz and the time resolution of 7.5 s. Findings: The seismic activity in Japan on July 7, 2018 was accompanied by an increase in a number of rays, by a significant broadening of the Doppler spectra, and by aperiodic processes in the ionosphere at distances no less than 1000–2000 km from the earthquake epicenters. Also, wave disturbances, generated by the seismic waves (speeds of ≈3 km/s), have been revealed in the 4–5-min infrasonic period range; the amplitude, δN, of the quasi-periodic variations in the electron density, N, was observed to be 4.5–9 %, and the duration of the oscillation trains to vary in the 24–55-min range. The relative amplitude δNa, of the electron concentration variations with the period ≈15-30 min caused by the propagation of atmospheric gravity wave (AGW) was estimated to be 30–55 %, the oscillation train duration was observed to be approximately 100 min, and the speed 0.3 km/s. The character of the Doppler spectrum variations, the Doppler shift of frequency over the main ray, and of the signal amplitude were found to be notably different during the September 5, 2018 earthquake and on the reference days. Two characteristic apparent speeds of 3.3 km/s and of ≈500 m/s were revealed. The former is close to that of seismic waves, and the latter to the speed of AGWs in the terrestrial ionosphere. The relative amplitudes in the infrasonic and AGW wave fields were estimated to be δN≈1.5-3 % and δNa≈6-7.5 %, respectively. The April 11, 2019 earthquake was accompanied by the Doppler spectrum broadening by 1-1.5 Hz in the 5–9.8 MHz frequency range, the generation of AGWs with 0.5-1 -km/s speeds and 8-20 -min periods, and by the generation of infrasonic waves with 2-5 -min periods and 0.3-0.4 -km/s speeds. Conclusions: Moderate earthquakes of Richter magnitudes ≈6 have been determined to give rise to dynamic processes in the ionosphere at distances no less than 1000-2000 km. The disturbances are transported by seismic waves with ≈3 km/s speeds and by acoustic and atmospheric gravity waves with 0.3–1 km/s speeds and periods varying from units to tens of minutes.

Some novel phenomena, which cannot be well explained by the traditional Bragg scattering theory, have been observed through the electromagnetic (EM) scattering from a water surface composed of two sinusoidal waves. According to the traditional Bragg scattering theory, the scattering intensity from water surface is only proportional to the spectral density of the Bragg resonant waves. However, the scattering field numerically simulated by the method of moment (MoM) method reveals that the resonant scattering field would also be affected by the amplitude of the non-resonant wave. In some special cases, despite the existence of the Bragg resonant waves, the Bragg resonant scattering field disappears when the amplitude of the non-resonant water wave is equal to some specific values. From the numerical results, another noticeable phenomenon is found that the Doppler spectrum of the scattering field is distorted seriously due to the frequency leakage. When the water surface with finite length is illuminated by a plane EM wave, not only the resonance spectral peaks corresponding to the phase velocity of the water wave but also other harmonic peaks appear on the spectrum curve. However, if a Gaussian beam is used instead of the plane EM wave, the harmonic peaks can be effectively suppressed. To better understand the phenomena, the theoretical model of the scattering field from the simple water surface is derived in the framework of the first-order small slope approximation method. And the empirical formulas for selecting the Gaussian beam width and water surface length are also proposed.

In this paper, a weighted arctangent function is used in conjunction with the spectral method to generate a land–sea junction composite rough surface under the spatially homogeneous and time-stationary hypotheses. The exponential correlation function and the Joint North Sea Wave Project (JONSWAP) spectrum, combined with an experiment-verified shoaling coefficient, are applied to model the land surfaces and the time-varying sea surfaces separately. The second-order small slope approximation (SSA-II) with tapered wave incidence is utilized for evaluating the electromagnetic scattering characteristics and Doppler characteristics of the generated composite rough surface. The influence of land–sea interface factors on radar cross-section (RCS) and Doppler shift of radar echoes is investigated in detail by comparing the RCS and Doppler spectra of the land–sea junction composite rough surfaces with those of finite-depth sea surfaces. It can be found that the Doppler spectra of the land–sea junction composite rough surface is narrower than that of the finite-depth sea surface under upwind directions and wider than that of the finite-depth sea surface under crosswind directions.

Most of the investigations on the micro-Doppler (MD) effect caused by a small unmanned aerial vehicle (UAV) have been conducted using joint time–frequency (JTF) images rather than the Doppler spectrum. On the other hand, several researchers utilized the Doppler spectrum instead of JTF images to observe the MD signature of a small UAV, and found the relationship between the spectral distribution of a small UAV and its physical specifications. However, the studies using the Doppler spectrum still lack concrete and theoretical foundations of the MD effects of a small UAV, focusing mainly on phenomena identified by measurement data. In this article, we establish the theoretical foundation connecting the MD signatures and motion dynamics of small UAVs based on the Doppler spectrum, and analyze their spectral distribution using simulations and measured data. In addition, experimental analysis is conducted using the data measured from various types of small UAVs considering the translational motion and aspect change. In contrast to already existing investigations, we completely explain and predict the changes on the Doppler spectrum relative to the physical specifications of a small UAV (e.g., blade length and rotor rotation rate). In particular, we show that the Doppler spectrum, compared to the JTF images, is a considerably simple and useful tool for analyzing the MD effects of small flying UAVs. The analysis results reveal that the MD features obtained from the measured echoes of small UAVs have considerable potential for detection and classification of small UAVs. [ABSTRACT FROM AUTHOR]

The methods for determining the height of sea waves that are relevant for maritime navigation have been investigated. Measuring the height of sea waves by the method of radar sounding with polarization-modulated signals is considered. The energy and spectral characteristics of the radar signal reflected by the sea surface at low angles of incidence are described. An algorithm for calculating the average wave height based on the results of measuring the polarization anisotropy of the specific effective scattering surface of a sea area is proposed. The relationship between the average frequency of amplitude fluctuations of the reflected radar signals and the sea waves is given. The spectral characteristics of radar signals reflected by the sea surface when it is irradiated with polarization-modulated signals are analysed. The procedure for producing an effective statistical estimate of the average wave height based on the results of spectral measurements is considered. The errors in the produced estimates of the wave height associated with the errors in measuring the polarization anisotropy of the effective scattering surface and the average frequency of the envelope of amplitude fluctuations caused by the Doppler effect are analysed. The results of experimental measurements of the spectra of amplitude fluctuations of the envelope of reflected signals at different states of the sea surface are presented. The analytical dependence of the polarization anisotropy of the effective sea scattering surface on the irradiation angle relative to the wave propagation direction is given. A comparison of the estimates of the height of sea waves obtained from the data of radar sounding and meteorological observation is carried out. [ABSTRACT FROM AUTHOR]

The presented research explores the sensing capability of the Self-Mixing Optical Feedback Interferometry (SM-OFI). The study evaluates the impact of different trade-off parameters like injection current, incident angle, target velocity, ambient temperature, the numerical aperture of collimating lens, etc. All the modes of SM signal acquisition are compared and investigate the influence of the acquisition scheme on the quality of the signal. The incident angle, wavelength, and external cavity length are also being trade-offs to increase the SNR and decrease of speckle effect. The study confirms that SNR increased linearly (R-square = 0.97) corresponds to numerical aperture. It was also analyzed that the external cavity length has a direct impact on the Doppler spectrum. The main aim of the study is to lay down the experimental narrative and to provide a simple mathematical relationship. [ABSTRACT FROM AUTHOR]

The extraction of oceanic wave spectrum information from radar data has been a challenging problem that has been the subject of a vast amount of research over the past several decades. This research has resulted in a multitude of approaches to extract ocean wave spectra from Doppler spectrum returns. One common feature of many of these methods is the reduction of the wave spectrum extraction problem from a nonlinear problem to a linear one. In this article, a new approach is introduced, which does not linearize the Fredholm integral equation relating the ocean wave spectrum to the radar Doppler spectrum, but instead maintains its nonlinear nature. Also, unlike previous nonlinear optimization solutions, the proposed method is automatic in the sense that no regularization parameters have to be manually set purely dependent on the radar data from which the ocean wave parameters are being extracted, thereby reducing the need for human intervention in the wave spectrum extraction process. In addition to describing this new method for wave spectrum extraction, this article presents results from a case study on field data from Argentia, NL, Canada, comparing the oceanographic parameters obtained with the proposed method to those recorded by in situ buoy instrumentation. The significant wave height calculated from ocean wave spectra extracted via the method are found to match with those from the buoy, whereas the values of other oceanographic parameters, such as wave period and direction extracted using the proposed method, are less accurate potentially due to the low quality of data available to test the method. [ABSTRACT FROM AUTHOR]

According to the different concentration levels of Doppler spectrum between sea clutter and target, small target in sea clutter background can be detected using Shannon entropy. However, Shannon entropy is merely a special case of Tsallis entropy and cannot reflect the multifractality of sea clutter. In this paper, the relation between Tsallis entropy and the generalized fractal dimension is first presented, and then the Doppler spectrum’s concentrative level and multifractality of sea clutter are combined; finally an algorithm for detecting small target in sea clutter background based on Tsallis entropy of Doppler spectrum rather than of Shannon entropy is proposed. By comparison via IPIX dataset, the detection’s performance of Tsallis entropy is better than that of Shannon entropy and Hurst exponent as per short observations.

Background: Arterial Doppler Ultrasound waveform (DW) analysis allows the detection and evaluation of lower extremity peripheral artery disease. The high heterogeneity of the reported description of DW is reduced by the use of classification. However, the reliability of these classifications is either unknown or low to moderate and practices of vascular caregivers regarding the use of these classifications remain unknown.Aims: This study aims to assess the inter-observer reliability of the Saint-Bonnet classification, a 13-category DW classification. The secondary objective was to determine the utilization rate of the most common classifications and the ability of these classifications to describe DW.Methods: A national survey was conducted among all vascular physicians of French society of vascular medicine. They were invited by email to describe on a website 20 DW without and with the display of the Saint-Bonnet classification. The reliability of this classification was estimated by Fleiss' Kappa expressed with [95% confidence interval]. A semantic analysis allowed us to classify the physicians' responses according to the terms used. Finally we have evaluated for each classification the rate of misuse, i.e., the addition of a complementary term to the defined categories.Results: One hundred and ten physicians participated and only 5% of these were familiar with Saint-Bonnet classification. Fifty-four percent of vascular physicians used no classification at all. Vascular physicians used the Spronk (four-category), Descotes (five-category) and Saint-Bonnet (13-category) classifications for respectively, 31, 10, and 5%. Kappa coefficient of Fleiss (κ) was 0.546 [0.544–0.547] (p < 0.001). Reliability by category ranges from κ of 0.075 to 0.864. In multivariate analysis, the use of a classification was associated with fewer years of experience and was dependent on geographic location. Misuse rate by classification was 88, 82, and 5% using Spronk, Descotes and Saint-Bonnet classifications respectively.Conclusion: The reliability of Saint-Bonnet classification is weak to moderate by vascular physicians who are not familiar with its use. However, unlike the other classifications, it seems to be sufficiently precise so that the user does not need to complete its description. There is a significant heterogeneity in the use of arterial Doppler classifications in France.

Doppler ultrasound technology is widespread in clinical applications and is principally used for blood flow measurements in the heart, arteries, and veins. A commonly extracted parameter is the maximum velocity envelope. However, current methods of extracting it cannot produce stable envelopes in high noise conditions. This can limit clinical and research applications using the technology. In this article, a new method of automatic envelope estimation is presented. The method can handle challenging signals with high levels of noise and variable envelope shapes. Envelopes are extracted from a Doppler spectrogram image generated directly from the Doppler audio signal, making it less device-dependent than existing image-processing methods. The method’s performance is assessed using simulated pulsatile flow, a flow phantom, and in vivo ascending aortic flow measurements and is compared with three state-of-the-art methods. The proposed method is the most accurate in noisy conditions, achieving, on average, for phantom data with signal-to-noise ratios (SNRs) below 10 dB, bias and standard deviation of 0.7% and 3.3% lower than the next-best performing method. In addition, a new method for beat segmentation is proposed. When combined, the two proposed methods exhibited the best performance using in vivo data, producing the least number of incorrectly segmented beats and 8.2% more correctly segmented beats than the next best performing method. The ability of the proposed methods to reliably extract timing indices for cardiac cycles across a range of signal quality is of particular significance for research and monitoring applications. [ABSTRACT FROM AUTHOR]

The progress of studies on ocean waves using high-frequency (HF) ocean radars is reviewed. All of the free-wave components are related to the Doppler spectrum of the HF radar. Ocean wave spectra can be estimated using HF radars by various methods, such as the semiempirical method, parameter fitting method, linear inversion method, and nonlinear inversion method . The semiempirical method is widely used for estimating wave parameters by evaluating unknown factors in the equation relating the Doppler spectrum and wave parameters. The parameter fitting method is adopted in broad beam radar systems. The linear inversion method is a well-known method to estimate a directional spectrum by approximating the nonlinear equation that relates the Doppler spectrum and wave spectrum to a linear equation. The nonlinear inversion method has been extensively studied in Japan. The accuracy of wave estimation using HF radars, and thus, coastal wave prediction, will be improved by selecting high signal-to-noise ratio Doppler spectra for analyses. [ABSTRACT FROM AUTHOR]

This article studies both experimentally and numerically the backscattering radar cross section (RCS) and Doppler spectrum characteristics from 2-D linear and nonlinear time-varying sea-like surface at UHF band under low grazing incidence. The small slope approximation and choppy wave model are applied to solve rough surface scattering and generate nonlinear sea waves, respectively. A coherent Doppler radar operating at 340 MHz was deployed at the tip of Huangqi peninsula in the southeast coast of China to measure the echo spectrum from the actual sea surfaces. At the same time, an ocean buoy was placed in the radar coverage to monitor the sea states. The RCS and Doppler spectrum are analyzed and compared comprehensively between radar measurements and numerical predictions. After compensating the influence of wind direction, the responses of radar echo power and numerical predicted RCS to wind speed are basically consistent and are more sensitive in low sea states. The correlation of Doppler spectra between radar measurements and numerical simulations exceed 0.96 during the whole experiment. The intensity of the high-order peaks increases rapidly with the wind speed, while the intensity of the Bragg peak decreases slightly. Doppler spectrum will be shifted by an amount proportional to radial surface current. [ABSTRACT FROM AUTHOR]

Most existing velocity measurement methods can only calculate the radial velocity by Doppler effect. Here, the authors study the method of measuring the lateral velocity. It is found that the lateral velocity influences the changing rate of Doppler frequency over time, which inspires them to measure the lateral velocity by analysing the Doppler spectrum or the quadratic phase of the echo. The relationship between the echoes and the lateral velocity is modelled in this study. Also the mean square error of lateral velocity measurement, including the model error and the estimation error, is also deduced. The influence of the observation time is studied according to the mean square error.

Doppler spectra of electromagnetic wave scattered from an object flying above time-varying nonlinear sea surfaces are investigated by utilizing a hybrid algorithm combining method of moments (MoM) and Kirchhoff approximation (KA). The "choppy wave" model is employed for describing nonlinear hydrodynamic interactions between ocean waves. Doppler spectra of scattered wave are simulated and analyzed for different incidence angles, target velocities, target heights, sea states as well as sea surface models, and an obvious broadening of Doppler spectrum is observed due to the coupling scattering between the flying target and rough sea surfaces. In comparison with the normalized radar cross section, Doppler spectra of scattered wave have the advantage of extracting information regarding target flying above dynamic sea surfaces. This study is of great value for the analysis of coupling scattering between flying target and rough sea surface as well as for the detection low-flying target in sea clutter environment. [ABSTRACT FROM AUTHOR]

Most existing velocity measurement methods can only calculate the radial velocity by Doppler effect. Here, the authors study the method of measuring the lateral velocity. It is found that the lateral velocity influences the changing rate of Doppler frequency over time, which inspires them to measure the lateral velocity by analysing the Doppler spectrum or the quadratic phase of the echo. The relationship between the echoes and the lateral velocity is modelled in this study. Also the mean square error of lateral velocity measurement, including the model error and the estimation error, is also deduced. The influence of the observation time is studied according to the mean square error. [ABSTRACT FROM AUTHOR]

Simulation of the strong clutter occurring from the airborne radar is essential in the efficient development and performance evaluation of the aircraft radar system. If the efficient simulation of the clutter can be successful, algorithms can be proved and analyzed and also the performance evaluation is possible in the laboratory environment. Therefore, development and implementation of the airborne radar system can be achieved very economically in the effective way. However, the clutter simulation procedure is very difficult and tedious since the clutter environment changes in numerous ways as it depends on the flight path, direction of antenna beam, reflectivity of the surface, etc.. Thus, in this paper, the general Doppler spectrum model is suggested for efficient simulation of the various clutter environment. Also, it is shown that the various type of clutter in time domain can be generated easily by changing and adjustment of parameters in the general Doppler spectrum model. [ABSTRACT FROM AUTHOR]