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  • Articles  (939)
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  • Articles  (939)
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  • 11
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    The Embrace Magnetometer Network for South America: Network Description and Its Qualification (2018)
    Wiley-Blackwell
    Details
    Publication Date: 2018-02-24
    Description: The present work is the first of a two-part paper on the Embrace Magnetometer Network. In this part, we present the new Embrace Magnetometer Network (Embrace MagNet) in South America, which is originally planned to cover most of the eastern portion of the Southern America longitudinal sector by installing and operating fluxgate magnetometer stations. We discuss the purpose and scientific goals of the network, associated with the Aeronomy and Space Weather. We provide details on the instrumentation, location of the sensors, sensitivity matching process, gain matching process, and magnetometer installation. In addition, we present and discuss details about the data storage, near-real time display and availability.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
    Published by Wiley-Blackwell on behalf of American Geophysical Union (AGU).
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  • 12
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    Power Flow, Energy Density, and Group/Energy-Transport Velocities in Spatially Dispersive Media (2018)
    Wiley-Blackwell
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    Publication Date: 2018-02-21
    Description: Electromagnetic power flow, energy density, and the energy-transport velocity that equals the group velocity of a lossless wavepacket are straightforwardly derived for the recently formulated electric and magnetic anisotropic representation of spatially dispersive materials or metamaterials. Boundary conditions in spatially dispersive media are used to confirm analytically and numerically the expressions obtained for the power flow and energy density. A long-standing alternative expression for the energy-transport velocity in spatially dispersive media is shown to be invalid. Plausible formulation-independent definitions of spatial and temporal dispersion for normal modes are proposed in terms of group and phase velocities.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
    Published by Wiley-Blackwell on behalf of American Geophysical Union (AGU).
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  • 13
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    Internal Natural Resonances Prediction through the Theory of Characteristic Modes (2018)
    Wiley-Blackwell
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    Publication Date: 2018-02-21
    Description: Some aspects in the interpretation of the resonances provided by the theory of characteristic modes for dielectric bodies are analyzed. The analysis has been performed analytically based on the Poggio-Miller-Chang-Harrington-Wu-Tsai (PMCHWT) formulation on a canonical problem, the infinite dielectric circular cylinder. Firstly, natural resonances and characteristic mode resonances are presented and compared. It has been observed that characteristic mode resonances are in general near to internal natural resonances but not to external ones. It is also demonstrated that characteristic resonances become closer to internal natural ones as the relative permittivity of the dielectric cylinder is increased.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
    Published by Wiley-Blackwell on behalf of American Geophysical Union (AGU).
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  • 14
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    Issue Information (2018)
    Wiley-Blackwell
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    Publication Date: 2018-02-21
    Description: No abstract is available for this article.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
    Published by Wiley-Blackwell on behalf of American Geophysical Union (AGU).
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  • 15
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    Status of NeQuick G after the Solar Maximum of Cycle 24 (2018)
    Wiley-Blackwell
    Details
    Publication Date: 2018-02-16
    Description: The Solar Cycle 24 will not be registered as the most intense of the last cycles. In fact, its intensity is roughly half of the previous cycle and the ionospheric effects experienced in this cycle have been far milder than originally expected, despite having several major ionospheric storms in this period, as the so-called St. Patrick's Day's ionospheric storm. On the other hand, in this same period of time the Galileo system has started the deployment phase and it started the In-Orbit-Validation campaign on 2013 with the first four full operational satellites, following the launch of a number of additional satellites allowing the declaration of Initial Services in December 2016 and targeting the Full Operational Capability (FOC) by 2020. Thus, during this period of time Galileo has been broadcasting the 3 Az coefficients needed to use the NeQuick G for correcting the ionospheric delay for single frequency users. In this work, the full analysis of the performance of the NeQuick G for the last Solar Cycle will be presented along with the detailed analysis of some of the most relevant Ionospheric storms occurred during the very same period. In general, the NeQuick G presents around 50 cm better RMS than the GPS broadcast model for all the period of study. As an internal measure of the goodness of the NeQuick G, the percentage of STEC inside of target Galileo specification will be also analyzed.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
    Published by Wiley-Blackwell on behalf of American Geophysical Union (AGU).
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  • 16
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    New results on ionospheric irregularity drift velocity estimation using multi-GNSS spaced-receiver array during high latitude phase scintillation (2018)
    Wiley-Blackwell
    Details
    Publication Date: 2018-02-10
    Description: The spaced-receiver technique using GNSS receivers offers an inexpensive approach for estimating ionospheric irregularity velocity during ionospheric scintillations. Our previous work has demonstrated that correlative studies of the GNSS carrier phase variations can be used to derive irregularity drift velocity at high latitudes. This study expanded upon our previous projects by incorporating GLONASS signals, investigation on ionospheric irregularity height assumption, and all-sky imager measurements into the methodology. A case study is presented based on GPS, Galileo, and GLONASS measurements during a geomagnetic storm event on December 20, 2015, obtained from a closely spaced receiver array at Poker Flat Research Range near Fairbanks, Alaska. The GNSS-estimated irregularity drift velocities are in general agreement with the measurements from the Poker Flat Incoherent Scatter Radar and the Poker Flat All-sky Imager. The study also shows that the irregularity altitude assumption will not lead to significant variations in the irregularity drift velocity estimates, especially for satellites with relatively high elevations. The techniques presented in this paper demonstrate that GNSS receiver arrays can be used to as a powerful means to monitor the ionospheric plasma dynamics during space weather events.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
    Published by Wiley-Blackwell on behalf of American Geophysical Union (AGU).
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  • 17
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    Computationally Efficient Radio Frequency Source Localisation for Radio Interferometric Arrays (2018)
    Wiley-Blackwell
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    Publication Date: 2018-02-09
    Description: Radio frequency interference (RFI) is an ever-increasing problem for remote sensing and radio astronomy, with radio telescope arrays especially vulnerable to RFI. Localising the RFI source is the first step to dealing with the culprit system. In this paper, a new localisation algorithm for interferometric arrays with low array beam sidelobes is presented. The algorithm has been adapted to work both in the near-field and far-field (only the direction of arrival can be recovered when the source is in the far-field). In the near-field the computational complexity of the algorithm is linear with search grid size compared to cubic scaling of the state-of-the-art 3D MUSIC method. The new method is as accurate as 3D MUSIC. The trade-off is that the proposed algorithm requires a once-off a priori calculation and storing of weighting matrices. The accuracy of the algorithm is validated using data generated by LOFAR (Low Frequency Array) while a hexacopter was flying around it and broadcasting a continuous-wave signal. For the flight, the mean distance between the differential GPS positions and the corresponding estimated positions of the hexacopter is 2 m at a wavelength of 6.7 m.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
    Published by Wiley-Blackwell on behalf of American Geophysical Union (AGU).
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  • 18
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    Automatic Detection and Classification of Buried Objects using Ground Penetrating Radar for Counter-Improvised Explosive Devices (2018)
    Wiley-Blackwell
    Details
    Publication Date: 2018-02-08
    Description: In this paper, a technique that can automatically detect and classify objects buried under the ground is proposed. The technique employs a ground penetrating radar (GPR) which transmits electromagnetic (EM) waves in order to strike the objects and then receives the backscattering EM wave to perform signal processing. This signal processing is divided into four main steps as follows. First, preprocessing is used to reduce the clutter due to the effect of the media layer interface. Second, the late time of the scattering signal is estimated using a simple cross correlation. Third, a few successive poles are extracted from the scattering response at the estimated late time by using the short-time matrix pencil method (STMPM). Finally, the extracted poles are fed for object classification with different constitutions and/or shapes using a support vector machine (SVM). Simulations according to the practical situation in three southern provinces of Thailand to counter the improvised explosive devices (IEDs) were set up. The performance of the proposed technique was evaluated. The simulation results showed that the proposed technique can efficiently detect and classify buried objects for counter-IED operations in the military.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
    Published by Wiley-Blackwell on behalf of American Geophysical Union (AGU).
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  • 19
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    Incoherent scatter spectra based on Monte-Carlo simulations of ion velocity distributions under strong ion frictional heating (2018)
    Wiley-Blackwell
    Details
    Publication Date: 2018-02-07
    Description: Under strong electric field conditions often found at high latitudes, the ion velocity distribution of the weakly ionized F region plasma can differ enough from a Maxwellian shape to substantially change Incoherent Scatter (IS) spectra and thus the analysis of those spectra. With the goal to provide a quantitative and reliable description of the IS spectra, this study directly uses for the first time an advanced Monte-Carlo calculation of the ion velocity distribution to derive IS spectra for a range of electric fields and aspect angles. For most cases the spectra associated with NO + maintains a shape that closely resembles that of a spectrum derived from a Maxwellian distribution with the same line-of-sight ion temperature as the equivalent Monte-Carlo simulated distribution. This study also fully characterizes the spectral shape as well as the ion temperature and its anisotropy for two different models of the resonant charge exchange between O + and O. It confirms that the distortions from the Maxwellian shape can be substantial for this particular interaction. The distortions are also such that along the magnetic field direction, the extracted apparent electron temperature is always greater than the real temperature. This work also includes a determination of the stability of the plasma against magnetic field-aligned electrostatic instabilities. It is found that the NO + distribution is always stable, whereas the O + distribution may or may not be stable, depending on the choice of the model chosen for the resonant charge exchange cross-section in collisions with the background atomic oxygen gas.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
    Published by Wiley-Blackwell on behalf of American Geophysical Union (AGU).
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  • 20
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    Cramér-Rao Lower Bounds for Joint Target Parameter Estimation in FM-Based Distributed Passive Radar Network with Antenna Arrays (2018)
    Wiley-Blackwell
    Details
    Publication Date: 2018-02-02
    Description: To avoid the disadvantages of the active radar which utilizes its own transmitter to emit electromagnetic radiations, passive radars use the signals readily available in the environment and can provide superior capabilities of stealth target detection, low probability of intercept (LPI), low cost and robustness. This paper investigates the joint target parameter (delay and Doppler) estimation performance for a frequency modulation (FM)-based distributed passive radar network (DPRN) system with antenna arrays. The DPRN system consists of multiple FM-based illuminators of opportunity and multiple radar receivers, which are placed on moving platforms. First, we consider the scenario where the target state parameters are unknown, the maximum likelihood (ML) estimator is developed and the log-likelihood ratio of the received signal for a complex Gaussian extended target is derived. Then, the Cramér-Rao lower bounds (CRLBs) on the Cartesian coordinates of target position and velocity are computed for a DPRN system with M T FM-based transmitters of Q antenna elements and M R multichannel receivers of P antenna elements. Finally, numerical examples demonstrate that grouping the receiving antenna elements into properly sized arrays can reduce estimation errors. It is also shown that the joint CRLB is a function of signal-to-noise ratio (SNR), the number of receiving antenna elements, the properties of the transmitted FM waveform, as well as the relative geometry between the target and the DPRN architecture. The analytically closed-form expressions for CRLB are an important performance metric in that they enable the optimal placement of radar receivers to improve the target parameter estimation performance.
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    Topics: Geosciences , Physics
    Published by Wiley-Blackwell on behalf of American Geophysical Union (AGU).
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