• Geophysics and Geophysical Exploration
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• v.13 no.4
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• pp.315-322
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• 2010

The phase-screen and the split-step Fourier migrations, which are implemented in both the frequency-wavenumber and frequency-space domains by using one-way scalar wave equation, allow imaging in laterally heterogeneous media with less computing time and efficiency. The generalized-screen migration employs the series expansion of the exponential, unlike the phase-screen and the split-step Fourier migrations which assume the vertical propagation in frequency-wavenumber domain. In addition, since the generalized-screen migration generalizes the series expansion of the vertical slowness, it can utilize higher-order terms of that series expansion. As a result, the generalized-screen migration has higher accuracy in computing the propagation with wide angles than the phase-screen and split-step Fourier migrations for media with large and rapid lateral velocity variations. In this study, we developed a 2D prestack generalized-screen migration module for imaging a complex subsurface efficiently, which includes various dips and large lateral variations. We compared the generalized-screen propagator with the phase-screen propagator for a constant perturbation model and the SEG/EAGE salt dome model. The generalized-screen propagator was more accurate than the phase-screen propagator in computing the propagation with wide angles. Furthermore, the more the higher-order terms were added for the generalized-screen propagator, the more the accuracy was increased. Finally, we compared the results of the generalizedscreen migration with those of the phase-screen migration for a model which included various dips and large lateral velocity variations and the synthetic data of the SEG/EAGE salt dome model. In the generalized-screen migration section, reflectors were positioned more accurately than in the phase-screen migration section.

• The Magazine of the IEIE
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• v.16 no.5
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• pp.38-46
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• 1989

• Information and Communications Magazine
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• v.14 no.8
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• pp.91-104
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• 1997

• Standardization Trend in Telecommunications
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• s.30
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• pp.1-18
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• 1993

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.12 no.6
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• pp.1002-1008
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• 2001

The absorbing structure composed of multi-layered fiber reinforced composite materials was designed and microwave absorbing properties are investigated. On the basis of transmission line theory, the theoretical equations to predict the reflection loss and the appropriate composite material for each functional layer are suggested. The most significant result of this study is the successful design and fabrication of triple-layered composite laminates which has the superior microwave absorbing porperties (more than 10 dB in 4∼12 GHz range), without using the ferrite filler in the impedance transforming layer. In the two-layered composite laminate (absorber/substrate), however, the use of ferrite filler (about 40 wt %) in the absorbing layer is necessary to obtain the certain level of microwave absorbance. By combining the glass-fiber composite with ferrite filler and carbon-fiber composite substrate, the microwave absorbing properties more than 10 dB in 4∼12 GHz frequencies than be obtained.

• The Transactions of the Korea Information Processing Society
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• v.3 no.1
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• pp.144-152
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• 1996

Recently, as the demands for radio spectrum are growing and the number of cell sites is increasing rapidly, the spectrum engineering plays an important role in estimating frequency sharing and reuse. The radio propagation analysis is essential in the basic technology of radio network design such as deciding the service area and selecting the position of the base station. But, domestic propagation environment in which mountainous region is occupying over 70% of our terrain does not allow us to apply foreign studies which are deduced in highly different environments. Therefore, we need to have our propagation analysis system derived from our own terrain condition. In this paper, we propose the propagation prediction model which issuitable toour propagation environment, and also usinghis model, we implement thesimulator based on GIS(Geographic Information System)which can be applied to both spectrum engineering and radio propagation analysis. We showed that this simulator can well be applied to frequency assignment, propagation network design as well as other radio services. Considering the results of our analysis, we could guarantee the standard deviation of error between the measured data and predicted results as 5 to 7 dB.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.34 no.11
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• pp.1015-1025
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• 2010

In this present study, we experimentally investigated the effects of electric fields on the characteristics of flames spreading over electric-wires with AC fields. The dependence of the rate at which a flame spreads over polyethylene-insulated wires on the frequency and amplitude of the applied AC electric field was examined. The spreading of the flame can be categorized into linear spreading and non-linearly accelerated spreading of flame. This categorization is based on the axial distribution of the field strength of the applied electric field. The rate at which the flame spreads is highly dependent on the inclined direction of the wire fire. It could be possible to explain the spreading of the flame on the basis of thermal balance.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.9
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• pp.999-1004
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• 2014

In the present paper, the effects of initial crack location on spatial randomness of fatigue crack growth resistance (FCGR) in friction stir welded (FSWed) AA7075-T651 plates were studied. The objective of this study is to characterize the statistical properties of FCGR for three different types of initial crack location (ICL) specimens. In this work, the FCGR coefficients were treated as a spatial random process. It was found that the FCGR coefficients for all initial crack location specimens closely followed a two parameter Weibull distribution. The shape parameter of the Weibull distribution for BM-ICL specimens showed the largest value of 7.50, and that for the WM-ICL specimens showed the smallest value of 2.61. In addition, the autocorrelation functions for all the ICL specimens followed the exponential function.

• Journal of Advanced Navigation Technology
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• v.21 no.6
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• pp.561-571
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• 2017

Radio navigation systems are already used as important national infrastructures in various fields such as air traffic, land transportation, geodetic survey, broadcasting communication and national defense, and are used in various industries. Since these systems are fusion of various technologies, it takes much time and cost in the development stage. In the early stages of development of the system, developed countries are establishing a national mid-term plan that meets international compatibility and standards. Korea did not develop the system due to the lack of national mid- and long-term plan, and it is not contributing effectively to industries. This paper analyzes the technical principles and technology trend of radio navigation system in the aviation sector to establish mid - and long - term plan. Based on the analyzed technology trends, the future prospect of technological development of the domestic navigation system and the development of related industries will be presented.

• Korean Journal of Optics and Photonics
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• v.15 no.2
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• pp.89-99
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• 2004

At the interface of two materials with frequency-dependent material-parameters of permittivity and permeability, there may exist two kinds of surface polaritons: surface electric-polaritons(SEPs) and surface magnetic-polaritons(SMPs). Possible combinations of the material-parameters to support propagation of the two surface polaritons are suggested at the interface between metals and metamaterials such as a left-handed material. Dispersion relations are also derived in order to characterize frequency dependence of propagation of the SEP and SMP. It is found that only one propagation mode of SEP or SMP is allowed at a given set of four material parameters, and that counter-propagation of the phase and group velocities of the propagation mode can be observed even in the case when there are no double negative(or, negative-index) materials. Physical origin of the counter-propagation of the group velocity is proposed by evaluating the ratio of two electromagnetic-energy densities of a surface polariton propagating along within the two interface media, and it is confirmed by the dispersion relations.