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  • Articles  (109)
  • 2015-2019  (109)
  • 2015  (109)
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  • Articles  (109)
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  • 2015-2019  (109)
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  • 1
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    Diurnal variation of ducts observed over a tropical station, Gadanki using high-resolution GPS radiosonde observations (2015)
    Wiley-Blackwell
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    Publication Date: 2015-12-30
    Description: A comprehensive study on diurnal variation of ducting using high vertical and temporal resolution radiosonde measurements over the Indian tropical region, Gadanki (13.5 o N, 79.2 o E) was presented. The diurnal variation of ducts was examined statistically based on both the refractivity and modified refractivity using 3 hourly radiosonde soundings obtained during October 2010 to March 2014 as a part of tropical tropopause dynamics (TTD) campaigns conducted under CAWSES India Phase II program. Strong diurnal variation in the altitude of occurrence of the duct has been found and is at maximum altitude (~2.5 km) during 11-17 LT. Interestingly, it is found that the occurrence of duct altitude closely follows the boundary layer altitude. Diurnal variation of duct altitude is maximum during post-monsoon followed by winter, monsoon and minimum in pre-monsoon. However, duct strength is maximum during winter followed by pre-monsoon, post-monsoon and minimum in monsoon. Duct thickness is found to be varying between 0.4 km and 1 km diurnally with the highest thickness during the winter season. Strong diurnal and seasonal variation in the percentage occurrence of the ducts was found with the highest percentage of ducts observed during winter (77%) followed by post-monsoon (51%), pre-monsoon (44%) and monsoon (10%). All the characteristics of ducts during all the seasons are maximum at 14 LT due to the high solar irradiance over Gadanki from 11-17 LT. The minimum frequency being trapped has been investigated and found that wave trapping occurs for the radars with frequencies 56-438 MHz over this station.
    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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    Using GNSS Signals as a Proxy for SAR Signals: Correcting Ionospheric Defocussing (2015)
    Wiley-Blackwell
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    Publication Date: 2015-12-29
    Description: Ultra-high frequency (UHF) space-based (SB) synthetic aperture radar (SAR) can suffer from the degrading effects of a scintillating ionosphere which modulates both the phase and the amplitude of the radar signal. In this paper, we use Global Navigation Satellite System (GNSS) signals to synthesize an L-band SAR point spread function (PSF). The process of transforming the GNSS signal to the equivalent SAR PSF is described. The synthesized PSF is used to explore the possibility of using a phase correction determined from a point target in a SAR image to correct the ionospheric degradation. GNSS data recorded on equatorial Ascension Island during scintillation events are used to test the feasibility of this approach by applying a phase correction to one GNSS receiver from another located along a magnetic east–west baseline. The peak-to-sidelobe-ratios of the synthesized L-band SAR point spread functions before and after the correction are compared and it is shown that this approach improves the L-band PSF over distances of ~3000 m in the ionosphere, corresponding to 6000 m on the ground.
    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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  • 3
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    Issue Information (2015)
    Wiley-Blackwell
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    Publication Date: 2015-12-19
    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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  • 4
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    A directional HF noise model: Calibration and validation in the Australian region (2015)
    Wiley-Blackwell
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    Publication Date: 2015-12-16
    Description: The performance of systems using HF (high frequency) radio waves, such as over-the-horizon radars, is strongly dependent on the external noise environment. However, this environment has complex behaviour and is known to vary with location, time, season, sunspot number and radio frequency. It is also highly anisotropic, with the directional variation of noise being very important for the design and development of next-generation OTHR. By combining global maps of lightning occurrence, raytracing propagation, a model background ionosphere and ionospheric absorption, the behaviour of noise at HF may be modeled. This article outlines the principles, techniques and current progress of the model and calibrates it against a 5 year data set of background noise measurements. The calibrated model is then compared with data at a second site.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
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  • 5
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    Accurate Measurements of the Dielectric Constant of Seawater at L Band (2015)
    Wiley-Blackwell
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    Publication Date: 2015-12-03
    Description: This paper describes measurements of the dielectric constant of seawater at a frequency of 1.413 GHz, the center of the protected band (i.e. passive use only) used in the measurement of sea surface salinity from space. The objective of the measurements is to accurately determine the complex dielectric constant of seawater as a function of salinity and temperature. A resonant cylindrical microwave cavity in transmission mode has been employed to make the measurements. The measurements are made using standard seawater at salinities of 30, 33, 35 and 38 psu over a range of temperatures from 0° C to 35° C in 5° C intervals. Repeated measurements have been made at each temperature and salinity. Mean values and standard deviations are then computed. The total error budget indicates that the real and imaginary parts of the dielectric constant have a combined standard uncertainty of about 0.3 over the range of salinities and temperatures considered. The measurements are compared with the dielectric constant obtained from the model functions of Klein and Swift (KS) and those of Meissner and Wentz (MW). The biggest differences occur at low and high temperatures.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
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  • 6
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    Automatically Determining the Origin Direction and Propagation Mode of High-Frequency Radar Backscatter (2015)
    Wiley-Blackwell
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    Publication Date: 2015-11-19
    Description: Elevation angles of returned backscatter are calculated at SuperDARN radars using interferometric techniques. These elevation angles allow the altitude of the reflection point to be estimated, an essential piece of information for many ionospheric studies. The elevation angle calculation requires knowledge of the azimuthal return angle. This directional angle is usually assumed to lie along a narrow beam from the front of the radar, even though the signals are known to return from both in front of and behind the radar. If the wrong direction of return is assumed, large uncertainties will be introduced through the azimuthal return angle. This paper introduces a means of automatically determining the correct direction of arrival and the propagation mode of backscatter. The application of this method will improve the accuracy of backscatter elevation angle data and aid in the interpretation of both ionospheric and ground backscatter observations.
    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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  • 7
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    Issue Information (2015)
    Wiley-Blackwell
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    Publication Date: 2015-11-18
    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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  • 8
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    Characteristics of post-midnight L-band scintillation in the transition region from the equatorial to mid-latitudes over the Indian longitude sector using COSMIC, C/NOFS and GPS measurements (2015)
    Wiley-Blackwell
    Details
    Publication Date: 2015-11-18
    Description: Occurrence of L-band scintillations around midnight and post-midnight hours have not been well studied and reported from the higher equatorial latitudes in the transition region from the equatorial to mid-latitudes over the Indian longitude sector. The present paper reports cases of post-midnight L-band scintillation observations by COSMIC during March 2014 over the Indian longitude sector. GPS S4 measurements from the IGS station at Lucknow (26.91°N 80.96°E geographic; magnetic dip: 39.75°N) corroborates occurrence of post-midnight scintillations. The F-region vertical upward velocities around the magnetic equator during evening hours have been used to understand the possibility of impact of irregularities generated over the magnetic equator at latitudes north of 30°N. Post-midnight L-band scintillations at latitudes greater than 30°N without corresponding pre-midnight scintillations present interesting scientific scenario and gives rise to suggestions of 1) any coupling mechanism between the equatorial and mid-latitudes through which irregularities seeded in the mid-latitudes may affect transionospheric satellite links at low latitudes or 2) irregularity generation at mid-latitudes not connected with equatorial instabilities. Long-term analysis of S4 at L-band measured by COSMIC over the Indian longitudes during March 2007–2014 exhibit a well-defined longitude swath around 75°-83°E of reduced (0.2 〈 S4 〈 0.4) or no-scintillations which may be attributed to the longitudinal variability of scintillation occurrence following the global four-cell pattern of ionospheric activity.
    Print ISSN: 0048-6604
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    Topics: Geosciences , Physics
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  • 9
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    Floquet Modes based Asymptotic Analysis of Scattering from FSS-type Reflectarray/Transmitarray for Near-Zone Focused Radiations (2015)
    Wiley-Blackwell
    Details
    Publication Date: 2015-11-15
    Description: This paper presents the asymptotic formulation of ray fields in the decomposition of electromagnetic (EM) scattering mechanisms from a one-dimensional, semi-infinite and periodic array when it is illuminated by a line source. This technique can be applied to analyze the passive FSS (frequency selective surface) type periodic structures with identical elements, or the reflectarray and transmitarray type antennas that are phased to radiate EM fields focused in the near zone of the array aperture. The solutions are built up based on the Floquet mode expansion of the scattering fields, and are obtained by asymptotically evaluating the resulted integrals to express the fields in terms of reflected/transmitted and edge diffracted fields as previously addressed in the framework of uniform geometrical theory of diffraction (UTD). The theoretical investigations over the scattering mechanisms and propagation phenomena are performed. Numerical examinations are presented to demonstrate the utilization of these solutions.
    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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  • 10
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    Channel Characterization in High-Speed Railway Station Environments at 1.89 GHz (2015)
    Wiley-Blackwell
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    Publication Date: 2015-10-31
    Description: Channel characterization is the prerequisite condition for the research and development of the next-generation high-speed railway (HSR) communication system. Train station is one of typical HSR scenarios, where channel characteristics have not yet been investigated sufficiently. In this paper, wideband multi-antenna channel measurements are performed at 1.89 GHz in an open-type train station environment based on long-term evolution (LTE) networks along the Beijing to Tianjin HSR in China. Large-scale characteristics of the HSR station channel, focusing on path loss (PL), shadow fading (SF) and the autocorrelation property of SF, are studied. Moreover, small-scale characteristics, such as Ricean K-factor, delay spread (DS) and spatial correlation (SC), are analyzed and modeled. In addition, the stationary region is characterized using the RUN test method. The obtained results provide useful information for deployment and assessment of the future HSR communication system in the HSR station scenario.
    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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