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  • Articles  (116)
  • 2015-2019
  • 2010-2014  (116)
  • 2012  (116)
  • Geosciences  (116)
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  • Articles  (116)
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  • 2015-2019
  • 2010-2014  (116)
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  • 1
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    On the retrieval of the specular reflection in GNSS carrier observations for ocean altimetry (2012)
    Wiley-Blackwell
    Details
    Publication Date: 2012-12-18
    Description: This paper presents an altimetric method for ocean monitoring by remote sensing. It uses carrier observations of reflected GNSS signals. The method is illustrated in a simulation study and applied to a long term experiment yielding an ocean tide spectrum. The altimetric concept is based on residual observations of Doppler frequency. A linear relation between Doppler residuals f0 and height departures ΔH from the surface level is derived. In contrast to existing phase-based methods which are constrained by smooth ocean conditions, the frequency-based retrieval here described holds good for rougher ocean conditions. Two retrievals of Doppler residuals are distinguished: Tracking Retrieval and Spectral Retrieval. A simulation study investigates the performance of Spectral Retrieval for a rough ocean surface with a noise-like sea level deviation ξ(t). Simulation settings were adjusted to reflection events in coastal experiments with an elevation range of [5…15] deg. In this range Tracking Retrieval tolerates a surface standard deviation σξ 〈 5 cm, whereas Spectral Retrieval tolerates σξ ≤ 30 cm. These limits correspond to significant wave heights of 20 cm for Tracking Retrieval and 1.2 m for Spectral Retrieval. The simulation results are confirmed by applying the altimetric method to the experimental data. The recovery of continuous phase tracks in experimental data is onerous and Tracking Retrieval only works for a period of smooth ocean conditions (162 events). By contrast, Spectral Retrieval yields altimetric estimates throughout the whole experiment (2607 events). The altimetric time series extends over more than 60 days and results in a tide spectrum that resolves diurnal (K1) and semidiurnal (M2, S2) constituents. The formal precision for these estimates lies in the decimeter range.
    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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  • 2
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    Second-kind integral solvers for TE and TM problems of diffraction by open arcs (2012)
    Wiley-Blackwell
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    Publication Date: 2012-12-14
    Description: We present a novel approach for the numerical solution of problems of diffraction by open arcs in two dimensional space. Our methodology relies on composition of weighted versions of the classical integral operators associated with the Dirichlet and Neumann problems (TE and TM polarizations, respectively) together with a generalization to the open-arc case of the well known closed-surface Calderón formulae. When used in conjunction with spectrally accurate discretization rules and Krylov-subspace linear algebra solvers such as GMRES, the new second-kind TE and TM formulations for open arcs produce results of high accuracy in small numbers of iterations—for low and high frequencies alike.
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    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
    Published by Wiley-Blackwell on behalf of American Geophysical Union (AGU).
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  • 3
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    Relative detection efficiency of the World Wide Lightning Location Network (2012)
    Wiley-Blackwell
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    Publication Date: 2012-12-13
    Description: Using the detected energy per strokes of the World Wide Lightning Location Network (WWLLN) we calculate the relative detection efficiency for the network as if it had a uniform detection efficiency. The model uses the energy statistics of located strokes to determine which stations are sensitive to what stroke energies. We are then able to estimate the number of strokes that may be missing from any given regions as compared to the best, most sensitive regions of the WWLLN network. Stroke density maps can be corrected with the knowledge of how sensitive various regions of the network are operating. This new model for the relative WWLLN detection efficiency compensates for the uneven global coverage of the network sensors as well as variations in very low frequency (VLF) propagation. The model gives a way to represent the global distribution of strokes as if observed by a globally uniform network. The model results are analyzed in spatial and temporal regimes, and the effects of a single VLF detector going offline are investigated in areas of sparse and dense detector coverage. The results are also used to show spatial, temporal and energy distributions as seen by the detection efficiency corrected WWLLN.
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    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
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  • 4
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    Equatorial range spread F echoes from coherent backscatter, and irregularity growth processes, from conjugate point digital ionograms (2012)
    Wiley-Blackwell
    Details
    Publication Date: 2012-12-12
    Description: Radio wave returns from spread F plasma structures as received by ionosondes can originate from total/specular reflection, partial reflection or coherent backscattering. The dominant mechanism to account for the spread F traces in equatorial ionograms is still an open question. Depending upon the precise mechanism, ionosondes are sensitive to irregularity scale sizes of tens of meters to several tens/hundreds of kilometers. In this paper we analyze signatures of range spreading F layer traces in Digisonde ionograms, taken at Fortaleza (3.9°S, 38.45°W, dip: −9°) and Sao Luis (2.33 S, 44.2W, dip angle: −0.5°), and at a dip equatorial site Cachimbo (9.5°S, 54.8°W, dip: −4.2°) and at its two conjugate sites Bova Vista (02.8°N, 60.7°W, dip: 22.0°) and Campo Grande (20.5 S, 54.7 W, dip −22.3°) in Brazil, to determine the dominant process/mechanism of echo returns from the irregularity structures. A significant component of the ESF trace structures is found to be consistent with the echoes originating from coherent backscattering at field line perpendicular directions. The degree of range spreading of the echoes is found to increase linearly with the top frequency of the echo trace, which is shown to be a more precise indicator of the irregularity strength. Further, the irregularity strength exhibits a significant increase from the equator toward the EIA crests, as well as a strong asymmetry between the conjugate sites.
    Print ISSN: 0048-6604
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    Topics: Geosciences , Physics
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  • 5
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    An iterative semi-analytical technique solving the boundary value problem of transmission through an anisotropic wedge (2012)
    Wiley-Blackwell
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    Publication Date: 2012-12-12
    Description: An iterative technique imitating the physical mechanism of transmission through an anisotropic prism is introduced, analyzed and tested. The solution is obtained by treating the boundary value problem along one boundary of the device at a time, as if the other is extended to infinite. This approximation makes the proposed method more suitable for small wedge angles. However, comprehensive numerical checks concerning the accuracy and the convergence speed of the method are performed with satisfying results. Several geometrical configurations with various constituent materials have been considered and certain conclusions for the operational features of the corresponding devices have been obtained.
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    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
    Published by Wiley-Blackwell on behalf of American Geophysical Union (AGU).
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  • 6
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    Whistler mode resonance-cone transmissions at 100 kHz in the OEDIPUS-C experiment (2012)
    Wiley-Blackwell
    Details
    Publication Date: 2012-11-22
    Description: A radio transmitter was operated at one end of the tethered sounding rocket double payload OEDIPUS C, and a synchronized receiver at the other end. Both the transmitter and the receiver were connected to “double-V” dipoles. On the flight downleg after the tether had been cut, direct bistatic propagation experiments were carried out successfully with the transmitter-receiver pair. This paper addresses the transmission of 300-μs pulses at a carrier frequency of 100 kHz between the dipoles over distances of about 1200 m. The waves of interest propagate in the whistler mode close to its resonance cone, where the transmitter is situated in the cone apex. The radiated field under these conditions is computed as well as the resonance response of the receiving antenna, i.e., its effective length. In the whistler mode, the influence of the plasma is important and it results in qualitative changes in the structure of the radiated field and in the value of the receiving antenna effective length as compared to the free space case. Our main concern is the excitation and reception of a pulsed signal when time and space dispersion play important roles in both the delay and spreading of such a signal.
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    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
    Published by Wiley-Blackwell on behalf of American Geophysical Union (AGU).
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  • 7
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    Second order statistics of selection combining receiver over κ-μ fading channels subject to co-channel interferences (2012)
    Wiley-Blackwell
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    Publication Date: 2012-11-13
    Description: Radio propagation performances in interference-limited faded environment are studied in this paper. Selection combining (SC) based on signal-to-interference ratio (SIR) over κ-μ fading channels is performed. Probability density function (PDF) and cumulative distribution function (CDF) of the received SIR are determined. Based on the results obtained for PDF and CDF, infinite-series expressions are derived for the output level crossing rate (LCR) and average fade duration (AFD). These second order statistical measures are regarded as necessary for supporting technical documentation in every radio communication link design. Influences of various system parameters such as fading severity and the number of co-channel interferences affecting these measures are graphically presented and discussed.
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    Topics: Geosciences , Physics
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  • 8
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    Media responsible for Faraday rotation: A review (2012)
    Wiley-Blackwell
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    Publication Date: 2012-11-07
    Description: Recent technological advances have led to a resurgence of interest in low frequency radio astronomy. Ionospheric distortion of cosmic radiation has, however, been a challenge for high fidelity and high sensitivity measurements at these long wavelengths. Several new and innovative low radio frequency interferometers are currently in varying stages of development, construction and commissioning across the globe. They will pursue a broad range of scientific objectives, and precise ionospheric calibration over the wide field-of-view of these new generation instruments will be a prerequisite for achieving these science goals. The task of calibration is made more difficult by the Faraday rotation (FR) of polarized flux as it passes through the magnetized plasma of the ionosphere, the plasmasphere, the magnetosphere, and the heliosphere. To quantify these effects, we present a survey of the order of magnitude of FR associated with these media and their spatial and temporal variations.
    Print ISSN: 0048-6604
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    Topics: Geosciences , Physics
    Published by Wiley-Blackwell on behalf of American Geophysical Union (AGU).
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  • 9
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    Initial GPS scintillation results from CASES receiver at South Pole, Antarctica (2012)
    Wiley-Blackwell
    Details
    Publication Date: 2012-10-31
    Description: Connected Autonomous Space Environment Sensor (CASES) Global Positioning System (GPS) software-defined receivers developed for ionospheric scintillation studies have been deployed on Autonomous Adaptive Low-Power Instrument Platforms (AAL-PIP) at South Pole, Antarctica. In this paper, we describe the AAL-PIP experimental setup focusing on CASES. We explain in detail the method developed for analyzing CASES data, and report initial AAL-PIP CASES results. Furthermore, we compare the CASES measurements with those from a modified Novatel GSV4004 GPS Ionospheric Scintillations and TEC Monitor (GISTM) receiver at the South Pole. CASES receivers have been successfully deployed and reliably operated in equatorial and midlatitude regions. Four of these GPS receivers, for the first time, are deployed in high-latitude regions as a part of the National Science Foundation (NSF) funded project of deploying space science instrument platforms, AAL-PIPs, in Antarctica since December 2010–2011. We present initial scintillation results recorded by a CASES receiver at South Pole during the storm on 24 January 2012 along with AAL-PIP magnetometer observations. We have deduced that the CASES receiver scintillation observations agree with those from the Novatel GPS scintillation receiver. Since this is the first time a CASES receiver has been deployed to operate in a high latitude, low temperature, and low humidity environment, we consider this comparison a demonstration of its reliable operation as a science-grade scintillation receiver in such conditions. We plan to study high latitude ionospheric irregularities by using observations from CASES and other ancillary instruments from Antarctica coupled with physical parameters derived from models.
    Print ISSN: 0048-6604
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    Topics: Geosciences , Physics
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  • 10
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    Time-reversal techniques for MISO and MIMO wireless communication systems (2012)
    Wiley-Blackwell
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    Publication Date: 2012-10-20
    Description: We consider the application of different time-reversal (TR) signal processing and beamforming techniques to multiple-input single-output (MISO) and multiple-input multiple-output (MIMO) wireless communication systems. Conventional TR beamforming provides spatial focusing at the intended receiver; however, it does not yield perfect channel equalization. Time-reversed pilot can be normalized to provide perfect equalization at the expense of power level. This equalization is particularly important for high data rates where the bit error rate performance is dominated by internal noise due to intersymbol interference. To increase physical layer covertness, TR beamforming is combined with the multiple-signal-classification (MUSIC) technique to produce null fields at eavesdroppers. This technique is also applied to MIMO setups to eliminate interuser interference and hence increase system capacity. Differential TR is used to obtain and update pilot signals for passive moving receivers, i.e., those that cannot (or do not) transmit pilot signals. Time-reversed differential backscattered signal is able to provide satisfactory spatial and temporal focusing at the moving receiver.
    Print ISSN: 0048-6604
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    Topics: Geosciences , Physics
    Published by Wiley-Blackwell on behalf of American Geophysical Union (AGU).
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