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  • 2015-2019  (135)
  • 2017  (135)
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  • 2015-2019  (135)
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  • 1
    Publication Date: 2017-12-23
    Description: Second-order asymptotic formulas for the electromagnetic fields of a horizontal electric dipole over an imperfectly-conducting half-space are derived using the modified saddle-point method. Application examples are presented for ordinary and plasmonic media, and the accuracy of the new formulation is assessed by comparisons with two alternative state-of-the-art theories and with the rigorous results of numerical integration.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
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  • 2
    Publication Date: 2017-12-20
    Description: This paper presents a solution to the problem of specular meteor radar interferometry cast on the complex plane. This technique is applicable to all meteor radars at the output of the receiver's pulsed Doppler signal processing chain. Specular meteor trail radar echoes are modeled as exponentially decaying sinusoids impinging on a sparse interferometric antenna array. A set of 10 underdense meteor trail radar parameters are estimated using nonlinear least-squares optimization of the physical model parameters. Meteor trail spatial locations are determined by estimating all parameters through global optimization of the complex-valued interferometric angle-of-arrival equations. Statistical precision of the estimated meteor radar parameters of Doppler, diffusion coefficient, angle-of-arrival and height are subsequently characterized by calculation of the full covariance matrix. The determination of statistical uncertainties in the measured parameters is a key innovation in the field, as it lays the groundwork for calculating the statistical precision of the subsequently derived windfield.
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    Topics: Geosciences , Physics
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  • 3
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    Wiley-Blackwell
    Publication Date: 2017-12-19
    Description: No abstract is available for this article.
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  • 4
    Publication Date: 2017-12-16
    Description: Using the ionospheric total electron content (TEC) and the location of the equatorial ionization anomaly (EIA) crest derived from GPS data observed at Nanning (22.84°N, 108.33°E, dip latitude~12.14°N), China, a GPS station situated near the daytime EIA crest region, the local time, seasonal and solar cycle variations of EIA crests from 2006 to 2015 were investigated. The TEC and the magnetic latitude (MLAT) location of EIA crest experience an ascendant and descendant process with the daytime evolution of EIA crest. Also, this diurnal variation of EIA crest depends on season and solar cycle. The TEC and MLAT location of EIA crest is stronger and higher in equinoctial months than that in solistial months showing a semi-annual variation. The TEC and MLAT location of EIA crest increase with the solar activity. The local time, seasonal and solar cycle variations of EIA crest is consistent with that of the strength of equatorial electrojet (EEJ). Comparatively, the correlation of the EEJ strength with the EIA location is better than that with the TEC of EIA crest.
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  • 5
    Publication Date: 2017-12-15
    Description: We investigate time and frequency dissemination using Software Defined Radio processing of signals acquired from a Low Frequency emitter using a sound card. We use the resulting propagation time measurements for investigating some ionosphere physics and its interaction with cosmic ray flux. Rather than using the amplitude of the transmitted signal as classically considered, we here focus on a precise time of flight measurement by demodulating the spectrum spreading phase modulation added to the DCF77 amplitude modulation.
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  • 6
    Publication Date: 2017-12-10
    Description: A novel microwave sub-surface imaging scheme based on an improved electric field calibration technique is proposed in this paper. The proposed imaging scheme requires RF measurement system comprising of a uniform linear transmitter-receiver antenna array wherein the region under examination is illuminated sequentially to measure the scattering parameters of the test. An appropriate imaging frequency is selected so as to minimize the errors introduced due to coupling among the co-resident antennas. The electric field required for the imaging algorithm is obtained from the measured scattering parameters using the proposed calibration methodology in two subsequent steps. The first step of calibration removes the systematic errors inherent in the measured scattering parameters by using known standards such as a metal plate and absorber foam. The second calibration step transforms the calibrated scattering parameters of the test object into electric field values so as to use them in the required inverse reconstruction procedure. The transformation is based on comparison of the measured scattering parameters of a known specimen and test object under the same test environment. The imaging algorithm based on the electric field formulation uses the standard iterative distorted Born procedure. For determining the location of the buried object, a novel data interpretation scheme based on analysis of variance (ANOVA) applied to time domain reflection coefficients obtained from various antenna elements, is also proposed here. The applicability including performance evaluation of the proposed imaging scheme is tested by retrieving microwave image of various standard objects buried inside sand.
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  • 7
    Publication Date: 2017-12-10
    Description: Ionospheric refraction is one of the principal error sources for limiting the accuracy of radar systems for space target detection. High accuracy measurement of the ionospheric electron density along the propagation path of radar wave is the most important procedure for the ionospheric refraction correction. Traditionally, the ionospheric model and the ionospheric detection instruments, like ionosonde or GPS receivers, are employed for obtaining the electron density. However, both methods are not capable of satisfying the requirements of correction accuracy for the advanced space target radar system. In this study, we propose a novel technique for ionospheric refraction correction based on radar dual-frequency detection. Radar target range measurements at two adjacent frequencies are utilized for calculating the electron density integral exactly along the propagation path of the radar wave, which can generate accurate ionospheric range correction. The implementation of radar dual-frequency detection is validated by a P-band radar located in mid-latitude China. The experimental results present that the accuracy of this novel technique is more accurate than the traditional ionospheric model correction. The technique proposed in this study is very promising for the high-accuracy radar detection and tracking of objects in geospace.
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  • 8
    Publication Date: 2017-12-10
    Description: In this study we calibrate and validate in situ ionospheric electron density ( N e ) and temperature ( T e ) measured with Langmuir probes (LP) on the three Swarm satellites orbiting the Earth in circular, nearly-polar orbits at ~500 km altitude. We assess the accuracy and reliability of the LP data (Dec/2013-Jun/2016) by using nearly-coincident measurements from low- and middle-latitude Incoherent Scatter Radars (ISRs), low-latitude ionosondes, and Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites, covering all latitudes. The comparison results for plasma frequency ( ) for each Swarm satellite are consistent across these three, principally different measurement techniques. It shows that the Swarm LPs systematically underestimate plasma frequency by about 10 % (0.5-0.6 MHz). The correlation coefficients are high (≥0.97), indicating accurate relative variation in the Swarm LP densities. The comparison of T e from high-gain LPs and those from ISRs reveals that all three satellites overestimate it by 300-400 K, but exhibit high correlations (0.92-0.97) against the validation data. The low-gain LP T e data show larger overestimation (~700 K) and lower correlation (0.86-0.90). The adjustment of the Swarm LP data based on Swarm-ISR comparison results removes the systematic biases in the Swarm data and gives plasma frequencies and high- and low-gain electron temperatures that are precise within about 0.4 MHz (8 %), 150-230 K and 260-360 K, respectively. We demonstrate that the applied correction significantly improves the agreement between (1) the plasma densities from Swarm, and from ionosondes and COSMIC, and (2) the T e from Swarm LPs and International Reference Ionosphere 2016.
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  • 9
    Publication Date: 2017-12-09
    Description: In this work, we developed an electromagnetic scattering model to analyze radar scattering from tall-grass-covered lands such as wetlands and marshes. The model adopts the Generalized Iterative Extended Boundary Condition Method (GIEBCM) algorithm, previously developed for buried cylindrical media such as vegetation roots, to simulate the scattering from the grass layer. The major challenge of applying GIEBCM to tall grass is the extremely time-consuming iteration among the large number of short sub-cylinders building up the grass. To overcome this issue, we extended the GIEBCM to Multilevel GIEBCM, or M-GIEBCM, in which we first use GIEBCM to calculate a T-matrix (Transition matrix) database of ‘straws’ with various lengths, thicknesses, orientations, curvatures, as well as dielectric properties; we then construct the grass with a group of ‘straws’ from the database and apply GIEBCM again to calculate the T-matrix of the overall grass scene. The grass T-matrix is transferred to S-matrix (Scattering matrix) and combined with the ground S-matrix, which is computed using the Stabilized Extended Boundary Condition Method (SEBCM), to obtain the total scattering. In this article, we will demonstrate the capability of the model by simulating scattering from scenes with different grass densities, different grass structures, different grass water contents, and different ground moisture contents. This model will help with radar experiment design and image interpretation for marshland and wetland observations.
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  • 10
    Publication Date: 2017-12-02
    Description: In this paper, we present a comprehensive study of occurrence of L-band scintillation in association with the appearance of sporadic E (Es) along the magnetic dip equator during daytime in 2013. The presence of L-band scintillation was determined from signals collected with GNSS (Global Navigation Satellite System) ground-based SCINDA (Scintillation Network Decision Aid) receivers from five stations situated at the magnetic dip equator. The detection and analysis of Es layers was obtained from GNSS FORMOSAT-3/COSMIC (F3/C) radio occultation (RO) data. Combining ground-based data with the limb-viewing geometry from space provides a unique opportunity to retrieve complementary information about scintillation and association with equatorial E region irregularities (i.e. Es) during daytime. Results for the first time show that daytime scintillation does occur at the magnetic dip equator and the occurrence is associated with the appearance of Es observed using GNSS F3/C RO data.
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