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  • Articles  (148)
  • Wiley  (148)
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  • Articles  (148)
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  • Wiley  (148)
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  • 1
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    Efficient adaptive numerical integration algorithms for the evaluation of surface radiation integrals in the high-frequency regime (2011)
    Wiley
    Details
    Publication Date: 2011-06-15
    Description: The possibility of reducing the sampling point density in the numerical evaluation of radiation integrals is discussed by resorting to asymptotic high-frequency technique concepts. It is shown that the numerical evaluation of the radiation integrals becomes computationally more efficient by introducing an adaptive sampling. Using this approach, the number of sampling points is found to be drastically smaller than that resulting from a standard Nyquist sampling rate.
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    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
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  • 2
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    Effects of diffraction by ionospheric electron density irregularities on the range error in GNSS dual-frequency positioning and phase decorrelation (2011)
    Wiley
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    Publication Date: 2011-06-23
    Description: It can be important to determine the correlation of different frequency signals in L band that have followed transionospheric paths. In the future, both GPS and the new Galileo satellite system will broadcast three frequencies enabling more advanced three frequency correction schemes so that knowledge of correlations of different frequency pairs for scintillation conditions is desirable. Even at present, it would be helpful to know how dual-frequency Global Navigation Satellite Systems positioning can be affected by lack of correlation between the L1 and L2 signals. To treat this problem of signal correlation for the case of strong scintillation, a previously constructed simulator program, based on the hybrid method, has been further modified to simulate the fields for both frequencies on the ground, taking account of their cross correlation. Then, the errors in the two-frequency range finding method caused by scintillation have been estimated for particular ionospheric conditions and for a realistic fully three-dimensional model of the ionospheric turbulence. The results which are presented for five different frequency pairs (L1/L2, L1/L3, L1/L5, L2/L3, and L2/L5) show the dependence of diffractional errors on the scintillation index S4 and that the errors diverge from a linear relationship, the stronger are scintillation effects, and may reach up to ten centimeters, or more. The correlation of the phases at spaced frequencies has also been studied and found that the correlation coefficients for different pairs of frequencies depend on the procedure of phase retrieval, and reduce slowly as both the variance of the electron density fluctuations and cycle slips increase.
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  • 3
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    Discontinuous Galerkin methods with transient hp adaptation (2011)
    Wiley
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    Publication Date: 2011-06-08
    Description: A discontinuous Galerkin method (DGM) for Maxwell’s equations in time domain and dedicated techniques for adaptive mesh refinement are presented. Since the DGM is a finite element–type method, it offers two refinement mechanisms: the manipulation of the local mesh step size (h adaptation) and the adaptation of the local approximation order (p adaptation). For both cases, a new approximation is obtained by means of projections between finite element spaces. The projection operators introduced are optimal with respect to the projection error. A reliable estimator for the local smoothness of the solution is presented, which forms the basis for the hp decision, i.e., the choice of the type of adaptation to be performed. The stability and efficiency of the adaptive method are demonstrated, allowing for performing transient mesh refinement, i.e., the continuous adaptation of the mesh according to the current situation.
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  • 4
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    Bipolar climatology of GPS ionospheric scintillation at solar minimum (2011)
    Wiley
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    Publication Date: 2011-06-24
    Description: High-rate sampling data of Global Navigation Satellite Systems ionospheric scintillation acquired by a network of GPS Ionospheric Scintillation and TEC Monitor receivers located in the Svalbard Islands, in Norway and in Antarctica have been analyzed. The aim is to describe the “scintillation climatology” of the high-latitude ionosphere over both the poles under quiet conditions of the near-Earth environment. For climatology we mean to assess the general recurrent features of the ionospheric irregularities dynamics and temporal evolution on long data series, trying to catch eventual correspondences with scintillation occurrence. In spite of the fact that the sites are not geomagnetically conjugate, long series of data recorded by the same kind of receivers provide a rare opportunity to draw a picture of the ionospheric features characterizing the scintillation conditions over high latitudes. The method adopted is the Ground Based Scintillation Climatology, which produces maps of scintillation occurrence and of total electron content relative variation to investigate ionospheric scintillations scenario in terms of geomagnetic and geographic coordinates, interplanetary magnetic field conditions and seasonal variability. By means of such a novel and original description of the ionospheric irregularities, our work provides insights to speculate on the cause-effect mechanisms producing scintillations, suggesting the roles of the high-latitude ionospheric trough, of the auroral boundaries and of the polar cap ionosphere in hosting those irregularities causing scintillations over both the hemispheres at high latitude. The method can constitute a first step toward the development of new algorithms to forecast the scintillations during space weather events.
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  • 5
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    Comparison of FORMOSAT-3/COSMIC radio occultation measurements with radio tomography (2011)
    Wiley
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    Publication Date: 2011-05-26
    Description: The paper reports a comparison between the continuous tomographic observations of the ionosphere during July–August 2008 and radio occultation (RO) measurements made by the Formosa Satellite 3/Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT-3/COSMIC or F3/C). The two sets of observations agree well qualitatively and bring out the main features of the low-latitude to midlatitude ionosphere: (1) The electron density enhancements observed at ∼26°N–31°N geographic (∼16°N–21°N magnetic) latitudes during daytime are related to the equatorial ionization anomaly (EIA), and (2) the enhancements observed at night at latitudes poleward of ∼35°N geographic are related to the midlatitude summer nighttime anomaly. The comparison shows that the EIA crest densities are highly underestimated in the RO inversions that use F3/C data. It is also seen that the agreement is better in the midlatitude region, with overall features represented quite well in the RO data, highlighting its usefulness as the two techniques capture similar electron density structure, although there are some magnitude differences.
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  • 6
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    The characteristics and simulation of close leader/return stroke field change waveforms (2011)
    Wiley
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    Publication Date: 2011-02-24
    Description: In order to reproduce the V-shape structure characteristics of the close dart leader/return stroke field change, we employ two existing models, one for the “source charge” leader model and the other for MTLL return stroke model, both based on the assumption of uniform leader charge distribution along the channel and the charges deposited by the dart leader are completely neutralized by the following return stroke process. The simulated results show that the return stroke electric field is inversely related with the return stroke speed at early times (within few tens of microseconds of the beginning of the return stroke), while at later times (after 100 μs or so) the field is dominated by the deposited charge density component, the close electric field is independent of speed and the ratio of the leader field to the corresponding return stroke field tends to be equal to −1. Therefore, at early times (within few tens of microseconds) there is often some uncertainty regarding whether the charges deposited by the dart leader are completely neutralized by the following return stroke process based on the difference between the return stroke and the leader field on the ground. However, although several other return strokes models (TL, MTLE, TCS, BG and DU) exist, the results shown in the paper are strictly valid only for the used MTLL model.
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  • 7
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    Real-time beamforming using high-speed FPGAs at the Allen Telescope Array (2011)
    Wiley
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    Publication Date: 2011-02-24
    Description: The Allen Telescope Array (ATA) at the Hat Creek Radio Observatory (HCRO) is a wide-field panchromatic radio telescope currently consisting of 42 offset-Gregorian antennas each with a 6 m aperture, with plans to expand the array to 350 antennas. Through unique back-end hardware, the ATA performs real-time wideband beamforming with independent subarray capabilities and customizable beam shaping. The beamformers enable science observations requiring the full gain of the array, time domain (nonintegrated) output, and interference excision or orthogonal beamsets. In this paper we report on the design of this beamformer, including architecture and experimental results. Furthermore, we address some practical considerations in large-N wideband beamformers implemented on field programmable gate array platforms, including device utilization, methods of calibration and control, and interchip synchronization.
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  • 8
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    Calibratability and its impact on configuration design for the LOFAR and SKA phased array radio telescopes (2011)
    Wiley
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    Publication Date: 2011-11-08
    Description: The Low Frequency Array (LOFAR) is a synthesis radio telescope covering the 10–240 MHz range. LOFAR is the first operational pathfinder to the Square Kilometre Array (SKA), a future radio telescope envisaged to be at least an order of magnitude more sensitive than current instruments. LOFAR exploits the same hierarchical beamforming structure as envisaged for the SKA phased array systems. In this paper, we describe the system requirements imposed by calibratability, i.e., the ability to perform a proper self-calibration of the instrument, for the low frequency regime. We derive requirements on station size, aperture efficiency and side lobe level. We also discuss the impact of the polarimetric response of the stations. We discuss the LOFAR design choices made to satisfy these requirements and indicate their implications for SKA phased array systems. This demonstrates that calibratability imposes requirements complementary to those based on imaging requirements and that self-calibratability has a significant impact on configuration design considerations.
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  • 9
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    The effects of earthward directed interplanetary coronal mass ejections on near-Earth S band signal links (2011)
    Wiley
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    Publication Date: 2011-11-11
    Description: Human space exploration is expected to enter its next phase in the coming decades as the United States prepares to return to the Moon or perhaps venture even further with a crewed mission to a near-Earth asteroid. Both mission classes are viewed by NASA as precursors of eventual crewed missions to Mars. In anticipation of extensive robotic and human presence in the space environment beyond the protection of the Earth's magnetosphere, it is important to better quantify and bound effects of earthward directed solar storms not just on the human body but also on engineering signals. In this paper, we study the effects of solar storms on S band (∼2.3 GHz) radio links in the near-Earth environment, primarily for application to navigation. In particular, we are concerned with induced long-period signatures on Doppler tracking data that could be confused with the Earth's gravity signature, resulting in perturbed trajectory solutions of returning spacecraft during Earth entry targeting. We have quantified “worst-case” levels of such induced signatures on S band signal phase using model predictions based on measured in situ charged particle content from satellites and have compared these results with signatures seen in actual tracking data during periods of interplanetary coronal mass ejections (ICME) and related geomagnetic storms. We show that induced Doppler can mask Earth gravity field effects in navigation trajectory solutions at S band, a commonly used frequency for near-Earth communications and navigation. Finally, we suggest a few ways that such effects can be identified, alleviated or eliminated in near real-time.
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  • 10
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    Design and analysis of low frequency strut-straddling feed arrays for EVLA reflector antennas (2011)
    Wiley
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    Publication Date: 2011-10-18
    Description: A new feed system is designed for operation below 100 MHz. The only existing system on the EVLA operating below 100 MHz is the “4 m” (74 MHz) system which uses crossed half-wave dipoles located in front of the Cassegrain subreflector as the feed. However, the dipole feeds of this system introduce blockage, and a reduction in system sensitivity (estimated to be ∼6% at 1.4 GHz) is observed at higher frequency bands; hence the dipoles are removed most of the time. An alternative feed concept is therefore proposed in this paper. The proposed system appears to reduce sensitivity degradation at 1.4 GHz by 3% and thus might be permanently mounted. Moreover, the new system has sensitivity comparable to the existing system at frequencies below 100 MHz. The feed for this system consists of dipoles mounted between the adjacent struts of the reflector and is thus referred to as a strut-straddling feed array. This design and the analysis methodology used in this paper should be applicable in meeting the contiguous frequency coverage requirement (50–470 MHz) of the new low frequency system proposed for the EVLA. Also, it may be applied in the modification of other existing large reflector antennas for low frequency operation.
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