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Anna Crawford - One of the best experts on this subject based on the ideXlab platform.

  • wave Orbital Velocity skewness and linear transition ripple migration comparison with weakly nonlinear theory
    Journal of Geophysical Research, 2003
    Co-Authors: Anna Crawford
    Abstract:

    [1] Field observations of linear transition ripples during an autumn storm exhibit high correlation between cross-shore ripple migration rate and the skewness of the near-bed wave Orbital Velocity. For a bimodal spectrum of mixed sea and swell, negatively skewed near-bed Orbital velocities were accompanied by offshore ripple migration, while for unimodal swell, ripples migrated onshore under positively skewed velocities. Bispectral analysis and weakly nonlinear wave theory are used to show that the positive and negative Velocity skewness arose from higher-order interactions between frequency components of the incident wave spectrum. There is good quantitative agreement between predicted and observed Velocity skewness, indicating a connection between ripple migration and wave-wave interactions.

  • linear transition ripple migration and wave Orbital Velocity skewness observations
    Journal of Geophysical Research, 2001
    Co-Authors: Anna Crawford
    Abstract:

    Field observations were made in 3–4 m water depth of linear transition ripple geometry and migration using a high-resolution laser-video bed profiling system and acoustic scanning sensors during both the growth and decay phases of an autumn storm event. Linear transition ripples are long-crested, low-steepness bedforms of the anOrbital ripple type and were observed to occur here at relatively high wave energies just below the flatbed threshold, with wavelengths of 8.5±0.5 cm and heights of 0.3±0.1 cm. The maximum observed migration rate was 0.7 cm/min. Migration was offshore during storm growth and onshore during storm decay. The observed ripple migration velocities were highly correlated (r2 > 0.7) with nearbed wave Orbital Velocity skewness in both cross-shore directions. During storm growth the incident wave spectrum was bimodal and the Orbital Velocity skewness was negative. During storm decay the wave spectrum was unimodal and the Velocity skewness was positive. Bispectral analysis shows that the main contribution to negative Velocity skewness during storm growth was due to a difference interaction between the two principal (sea and swell) components of the bimodal Velocity spectrum. Positive skewness during storm decay was due to self-self interaction of the narrowband residual swell. The negative Velocity skewness observed during storm growth is consistent with prediction by a two-frequency second-order wave theory.

E M Meshkov - One of the best experts on this subject based on the ideXlab platform.

  • Application of Doppler Spectrum for Retrieval of Statistical Parameters of Sea Waves
    2nd European Conference on Antennas and Propagation (EuCAP 2007), 2020
    Co-Authors: V.yu. Karaev, M. B. Kanevsky, E M Meshkov
    Abstract:

    The experiment for studying of microwave backscatter by the rough water surface during the flight above the Gorky water storage basin was performed. A Doppler radar (10GHz) with a knife-like beam (1° - 16°) was installed at a helicopter and was oriented vertically downward. The analysis of the experimental data has confirmed that the width of the Doppler spectrum depends on the direction of wave propagation. The new algorithm for determination the correlation coefficient between the vertical component of the Orbital Velocity and water surface slopes, as well as the variance of Orbital Velocity was developed using the width of the Doppler spectrum of the reflected radar signal for the case of moving radar. The experimental data processing has confirmed the effectiveness of retrieval algorithm and demonstrated the possibility to determine the average phase Velocity of sea waves, the average wavelength, and the significant wave height. (6 pages)

  • The Doppler Spectrum of the Microwave Radar Signal Backscattered From the Sea Surface in Terms of the Modified Bragg Scattering Model
    IEEE Transactions on Geoscience and Remote Sensing, 2020
    Co-Authors: Vladimir Karaev, Yury Titchenko, Mariya Panfilova, E M Meshkov
    Abstract:

    Significant progress has been made in the study of electromagnetic wave scattering from the sea surface. Essential results were obtained in solving applied problems, but there are still unsolved problems in the scattering theory. The radar backscatter cross section is usually used to solve remote sensing tasks, although the Doppler spectrum (DS) of the scattered microwave signal contains more information about sea waves. The concept of a two-scale model of the scattering surface is used in this article. Correct allowance for large-scale waves, in comparison with radar wavelength, in deriving theoretical formulas enabled us to develop a modified Bragg scattering model. It is based on the previously developed scattering model that is refined here to take into account the contribution of the horizontal component of the Orbital Velocity to the DS. It is shown that allowance for large-scale waves limits the growth of the radar cross section of the Bragg component with a decrease in the incidence angle, which allows using the resonant scattering model at small incidence angles. An additional result is that the modified model of the DS can also be used in the range of middle incidence angles because it takes into account the contribution of the horizontal component of the Orbital Velocity. It is shown that if the correlation between large-scale and small-scale components of sea waves is neglected, the new formulas pass into the known formulas for the DS within the framework of the Bragg scattering model.

  • Measurements of the Sea Surface Parameters Using a New Modification of Underwater Sonar on a Marine Platform in the Black Sea
    OCEANS 2019 - Marseille, 2019
    Co-Authors: Yury Titchenko, Vladimir Karaev, Maria Ryabkova, E M Meshkov
    Abstract:

    This paper presents the results of experimental measurements of the sea surface parameters during the expedition to a marine platform in the Black Sea near the Katsiveli settlement in October 2018. The measurements were carried out using underwater sonar at small angles of incidence, so the scattering mechanism remains mostly quasi-specular and is well described by tangent plane approximation (Kirchhoff approximation). The method is based on the analysis of spectral and energy characteristics of the reflected acoustic waves at underwater surface waves observation. An important feature of this method is the simultaneous use of multiple antennas with different radiation patterns. The underwater sonar antenna unit consists of one radiating antenna with a symmetrical radiation pattern and three receiving antennas of various shapes: 1 - symmetric, 2 and 3 - fan beam antennas oriented perpendicular to each other. Within the framework of this expedition, the following configuration of the antenna system was used: the width of the symmetric receiving antenna, equal to the widest axis of the fan beam antennas. The main innovation of the underwater sonar is the presence of a builtin two axial inclinometer and a compass in an external unit free from magnetic interference. In this paper, we use the original approach of calibrating the backscattering cross section based on the scattering model and the wave spectrum model. As a result of this procedure, all unknown wave parameters affecting the scattering characteristics were retrieved: the vertical Orbital Velocity variance, the slope variances along and across the direction of sensing, and the correlation coefficients between slopes and Orbital Velocity. String wave gauge and wind measurements are used in the experiments to control the effectiveness of proposed algorithms.

  • The study of the influence of the surface waves formation conditions and rain on the acoustic waves backscattering by the water surface applied to remote sensing of the sea surface
    2015 1st URSI Atlantic Radio Science Conference (URSI AT-RASC), 2015
    Co-Authors: Yury Titchenko, Vladimir Karaev, E M Meshkov
    Abstract:

    The study addresses to the problem of ground calibration and verification of satellite measurements of the surface waves characteristics. For measurements we propose to use the Doppler underwater acoustic wave gauge of a special design, which can directly measure the sea wave statistical characteristics that effect on the electromagnetic wave backscatter of the same wavelength. The main features of the acoustic wave gauge are the use of Doppler ultrasound sonar and antennas with different antenna patterns. In the experiment, measurements are carried out by sonar submerged to the bottom and antennas oriented vertically upwards. Retrieval algorithms are based on the dependence of the Doppler spectrum of the reflected acoustic signal from the water surface statistical characteristics. This method allow to measure all second-order statistical parameters of sea waves, namely, the variance of the vertical Orbital Velocity component, the variance of slopes in two mutually perpendicular directions, and the cross-correlation coefficients of slopes and the vertical Orbital Velocity component. Currently, especially important is the possibility of measuring the variance of slopes of the water surface, the same as measured by radar systems.

  • measuring the variance of the vertical Orbital Velocity component by an acoustic wave gauge with a single transceiver antenna
    IEEE Transactions on Geoscience and Remote Sensing, 2015
    Co-Authors: Yury Titchenko, E M Meshkov, Yu. V. Karaev, E M Zuikova
    Abstract:

    The capabilities of a new instrument for measuring the surface wave parameters are studied in detail. An acoustic wave gauge with a single antenna is designed to measure the backscatter intensity and the Doppler spectrum of the reflected acoustic signal and to retrieve the variance of the vertical Orbital Velocity component. The new instrument is numerically simulated, and field experiments using a prototype acoustic wave gauge are carried out. The comparison of the variances of the vertical Orbital Velocity component measured by acoustic and string wave gauges under field conditions, as well as the comparison of measurements and numerical simulation results, has confirmed the high accuracy of the retrieval algorithms. The advantage of the instrument is its capability of making measurements in any water basin without the use of a fixed platform.

Yang-yang Chen - One of the best experts on this subject based on the ideXlab platform.

  • Coordinated adaptive control for coordinated path-following surface vessels with a time-invariant Orbital Velocity
    IEEE CAA Journal of Automatica Sinica, 2014
    Co-Authors: Yang-yang Chen
    Abstract:

    This article considers the problem of directing a family of fully actuated surface vessels to cooperatively follow a set of convex and closed orbits with a time-invariant reference Orbital Velocity and maintain attitude synchronization. A consensus-based adaptive control law under a bidirectional communication topology is proposed to estimate the reference Orbital Velocity so that the restriction that every vessel in the family must have access to the reference in the previous literature can be removed. The assumption of nonzero total linear speed of each vessel is removed by the use of potential function. Simulation results demonstrate the effectiveness of the proposed approach.

  • Coordinated adaptive control for formation tracking surface vessels with a time-invariant Orbital Velocity
    Proceedings of the 32nd Chinese Control Conference, 2013
    Co-Authors: Yang-yang Chen, Yu-ping Tian
    Abstract:

    This article considers the problem of directing a family of fully-actuated surface vessels to formation tracking a set of closed orbits and approach a time-invariant reference Orbital Velocity. A consensus-based adaptive control law under the bidirectional commutation topology is proposed to estimate the desired Orbital Velocity so that the restriction that every vehicle in the family has access to the reference in the literature is removed. The condition of nonzero total linear speed of each vessel is ensured by a potential function. An simulation example is presented as a proof of concept.

  • Coordinated adaptive control for formation flying vehicles with a time-varying Orbital Velocity
    2013 9th Asian Control Conference (ASCC), 2013
    Co-Authors: Yang-yang Chen, Yu-ping Tian
    Abstract:

    This article considers the problem of directing a family of nonholonomic vehicles to formation flying a set of closed convex planar orbits and approach a time-varying reference Orbital Velocity in a three-dimensional (3D) space. A novel coordinated adaptive control law based on local neighbor-to-neighbor information is proposed to estimate the desired Orbital Velocity so that the assumption that every vehicle in the family has access to the reference in the literature is removed. We show how the geometric extension design, projection tracking method and consensus-based technique can be combined together to construct the formation flying controller under the bidirectional commutation topology. Simulation results demonstrate the effectiveness of the proposed approach.

  • ASCC - Coordinated adaptive control for formation flying vehicles with a time-varying Orbital Velocity
    2013 9th Asian Control Conference (ASCC), 2013
    Co-Authors: Yang-yang Chen, Yu-ping Tian
    Abstract:

    This article considers the problem of directing a family of nonholonomic vehicles to formation flying a set of closed convex planar orbits and approach a time-varying reference Orbital Velocity in a three-dimensional (3D) space. A novel coordinated adaptive control law based on local neighbor-to-neighbor information is proposed to estimate the desired Orbital Velocity so that the assumption that every vehicle in the family has access to the reference in the literature is removed. We show how the geometric extension design, projection tracking method and consensus-based technique can be combined together to construct the formation flying controller under the bidirectional commutation topology. Simulation results demonstrate the effectiveness of the proposed approach.

  • Coordinated adaptive control for three-dimensional formation tracking with a time-varying Orbital Velocity
    IET Control Theory & Applications, 2013
    Co-Authors: Yang-yang Chen, Yu-ping Tian
    Abstract:

    This study considers the problem of directing a family of non-holonomic vehicles to formation tracking a set of closed convex planar orbits in a three-dimensional (3D) space and approach a time-varying reference Orbital Velocity. A novel coordinated adaptive control law based on local neighbour-to-neighbour information is proposed to estimate the desired Orbital Velocity, so that the assumption that every vehicle in the family has access to the reference in the literature is removed. We show how the geometric extension design, projection tracking method and consensus-based technique can be combined together to construct the formation-tracking controller under the bidirectional commutation topology. Simulation results demonstrate the effectiveness of the proposed approach.

Yu-ping Tian - One of the best experts on this subject based on the ideXlab platform.

  • Coordinated adaptive control for formation tracking surface vessels with a time-invariant Orbital Velocity
    Proceedings of the 32nd Chinese Control Conference, 2013
    Co-Authors: Yang-yang Chen, Yu-ping Tian
    Abstract:

    This article considers the problem of directing a family of fully-actuated surface vessels to formation tracking a set of closed orbits and approach a time-invariant reference Orbital Velocity. A consensus-based adaptive control law under the bidirectional commutation topology is proposed to estimate the desired Orbital Velocity so that the restriction that every vehicle in the family has access to the reference in the literature is removed. The condition of nonzero total linear speed of each vessel is ensured by a potential function. An simulation example is presented as a proof of concept.

  • Coordinated adaptive control for formation flying vehicles with a time-varying Orbital Velocity
    2013 9th Asian Control Conference (ASCC), 2013
    Co-Authors: Yang-yang Chen, Yu-ping Tian
    Abstract:

    This article considers the problem of directing a family of nonholonomic vehicles to formation flying a set of closed convex planar orbits and approach a time-varying reference Orbital Velocity in a three-dimensional (3D) space. A novel coordinated adaptive control law based on local neighbor-to-neighbor information is proposed to estimate the desired Orbital Velocity so that the assumption that every vehicle in the family has access to the reference in the literature is removed. We show how the geometric extension design, projection tracking method and consensus-based technique can be combined together to construct the formation flying controller under the bidirectional commutation topology. Simulation results demonstrate the effectiveness of the proposed approach.

  • ASCC - Coordinated adaptive control for formation flying vehicles with a time-varying Orbital Velocity
    2013 9th Asian Control Conference (ASCC), 2013
    Co-Authors: Yang-yang Chen, Yu-ping Tian
    Abstract:

    This article considers the problem of directing a family of nonholonomic vehicles to formation flying a set of closed convex planar orbits and approach a time-varying reference Orbital Velocity in a three-dimensional (3D) space. A novel coordinated adaptive control law based on local neighbor-to-neighbor information is proposed to estimate the desired Orbital Velocity so that the assumption that every vehicle in the family has access to the reference in the literature is removed. We show how the geometric extension design, projection tracking method and consensus-based technique can be combined together to construct the formation flying controller under the bidirectional commutation topology. Simulation results demonstrate the effectiveness of the proposed approach.

  • Coordinated adaptive control for three-dimensional formation tracking with a time-varying Orbital Velocity
    IET Control Theory & Applications, 2013
    Co-Authors: Yang-yang Chen, Yu-ping Tian
    Abstract:

    This study considers the problem of directing a family of non-holonomic vehicles to formation tracking a set of closed convex planar orbits in a three-dimensional (3D) space and approach a time-varying reference Orbital Velocity. A novel coordinated adaptive control law based on local neighbour-to-neighbour information is proposed to estimate the desired Orbital Velocity, so that the assumption that every vehicle in the family has access to the reference in the literature is removed. We show how the geometric extension design, projection tracking method and consensus-based technique can be combined together to construct the formation-tracking controller under the bidirectional commutation topology. Simulation results demonstrate the effectiveness of the proposed approach.

B Aschenbach - One of the best experts on this subject based on the ideXlab platform.

  • non monotonic Orbital Velocity profiles around rapidly rotating kerr anti de sitter black holes
    Classical and Quantum Gravity, 2007
    Co-Authors: A Muller, B Aschenbach
    Abstract:

    It has been recently demonstrated that the Orbital Velocity profile around Kerr black holes in the equatorial plane as observed in the locally non-rotating frame exhibits a non-monotonic radial behaviour. We show here that this unexpected minimum–maximum feature of the Orbital Velocity remains if the Kerr vacuum is generalized to the Kerr–de Sitter or Kerr–anti-de Sitter metric. This is a new general relativity effect in Kerr spacetimes with a non-vanishing cosmological constant. Assuming that the profile of the Orbital Velocity is known, this effect constrains the spacetime parameters.

  • the anomalous Orbital Velocity effect and high frequency quasi periodic oscillations in accreting black holes
    XXIXth Spanish Relativity Meetging (ERE 2006): Einstein's Legacy: From the Theoretical Paradise to Astrophysical Observations, 2007
    Co-Authors: B Aschenbach
    Abstract:

    The recently discovered anomalous Orbital Velocity effect, which implies a reduced Velocity with decreasing radius for an orbiting test particle, is present in extreme Kerr black holes with spin > 0.9953 and radii between 1.05 and 1.78 gravitational radii. In this region a 3:1 resonance between the vertical and the radial epicyclic frequencies occurs, which may lead to quasi-peridoc light variations with two periods differing by a factor of 3. Such variations may have been observed from the Galactic Center black hole Sgr A*. At the same spin but at a larger radius a 3:2 resonance between the two epicyclic frequencies is found as well. Such 3:2 frequencies have been observed in four galactic microquasars. In either case, i.e. the 3:1 or the 3:2 resonance, the black hole mass determined from this model agrees very well with the dynamical mass for Sgr A* and each of the four microquasars. Meanwhile the microquasar GRS 1915+105 has been classified by its X-ray spectrum as an extreme Kerr black hole with a spin > 0.98 [1], which agrees quite nicely with our prediction for the spin, i.e. a = 0.996. Model and observational data are presented and discussed.

  • Mass and angular momentum of black holes: an overlooked effect of general relativity applied to the galactic center black hole Sgr A*
    Chinese Journal of Astronomy and Astrophysics, 2006
    Co-Authors: B Aschenbach
    Abstract:

    I report the discovery of a new effect of General Relativity which is important to understand very rapidly rotating(Kerr) black holes. The Orbital Velocity of a test particle is no longer a monotonic function of the orbit radius when the spin of the black hole is >0.9953, but displays a local minimum-maximum structure for radii smaller than 1.8 gravitational radii. There the rate of change of the Orbital Velocity per radius unit equals the radial epicyclic frequency and is exactly one third of the polar epicyclic frequency, suggesting a 3:1 resonant oscillatory motion of the particle. If associated with the most recently observed quasi-periods the mass of the supermassive black hole Sgr A* in the centre of our Galaxy is determined to 3.3×106 M⊙, and the spin is 0.99616.