• Geomechanics and Engineering
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• v.11 no.5
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• pp.713-723
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• 2016

Shear wave velocities of three selected sandy soils subject to drained triaxial compression test were continuously measured using the bender elements. The shear wave velocity during isotropic compression, as widely recognized, increased as confining pressure increased and they were correlated well. However, during drained shearing, the mean effective stress could no further provide a suitable correlation. The shear wave velocity during this stage was almost constant with respect to the mean effective stress. The vertical stress was found to be more favorable at this stage (since confining stress was kept constant). When sample was attained its peak stress, the shear wave velocity reduced and deviated from the previously existed trend line. This was probably caused by the non-uniformity induced by the formation of shear band. Subsequently, void ratios computed based on external measurements could not provide reasonable fitting to the initial stage of post-peak shear wave velocity. At very large strain levels after shear band formation, the digital images revealed that sample may internally re-arrange itself to be in a more uniform loose stage. This final stage void ratio estimated based on the proposed correlation derived during pre-peak state was close to the value of the maximum void ratio.

• Journal of the Society of Naval Architects of Korea
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• v.53 no.6
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• pp.482-493
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• 2016

The identification of modal damping of a segmented hull model with torsional response is difficult task due to the coupling of modal response. This is because the 1st and 2nd torsional vibration modes are closely spaced in frequency domain leading to the situation that the modal decomposition is difficult to achieve by simple band-pass filter. Present study applied several different modal decomposition methods to derive the damping ratio of different modes. The modal decomposition methods considered in this study are simple band-pass filter, Hilbert vibration decomposition, Wavelet transform and proper orthogonal decomposition. Coupled free decay signal obtained from the torsional hammering test on a segmented hull model was processed with four different methods and the derived damping ratios were compared with each other. Discussions also have been made on the pros and cons of the different methodologies.

• Journal of the Korean earth science society
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• v.40 no.4
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• pp.315-328
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• 2019

Since the Coastal Zone Color Scanner (CZCS)/Nimbus-7 was launched in 1978, a variety of studies have been conducted to retrieve ocean color variables from multi-satellites. Several algorithms and formulations have been suggested for estimating ocean color variables based on multi band data at different wavelengths. Chlorophyll-a (chl-a) concentration is one of the most important variables to understand low-level ecosystem in the ocean. To retrieve chl-a concentrations from the satellite observations, an appropriate algorithm depending on water properties is required for each satellite sensor. Most operational empirical algorithms in the global ocean have been developed based on the band-ratio approach, which has the disadvantage of being more adapted to the open ocean than to coastal areas. Alternative algorithms, including the semi-analytical approach, may complement the limits of band-ratio algorithms. As more sensors are planned by various space agencies to monitor the ocean surface, it is expected that continuous monitoring of oceanic ecosystems and environments should be conducted to contribute to the understanding of the oceanic biosphere and the impact of climate change. This study presents an overview of the past and present algorithms for the estimation of chl-a concentration based on multi-satellite data and also presents the prospects for ongoing and upcoming ocean color satellites.

• Current Optics and Photonics
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• v.6 no.6
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• pp.608-618
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• 2022

By analyzing the signal-to-noise ratio (SNR) theory of the conventional true color night vision system, we found that the output image SNR is limited by the wavelength range of the system response λ₁ and λ₂. Therefore, we built a double-channel four-band true color night vision system to expand the system response to improve the output image SNR. In the meantime, we proposed an image fusion method based on principal component analysis (PCA) and nonsubsampled shearlet transform (NSST) to obtain the true color night vision images. Through experiments, a method based on edge extraction of the targets and spatial dimension decorrelation was proposed to calculate the SNR of the obtained images and we calculated the correlation coefficient (CC) between the edge graphs of obtained and reference images. The results showed that the SNR of the images of four scenes obtained by our system were 125.0%, 145.8%, 86.0% and 51.8% higher, respectively, than that of the conventional tri-band system and CC was also higher, which demonstrated that our system can get true color images with better quality.

• Korean Journal of Remote Sensing
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• v.39 no.6_1
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• pp.1299-1308
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• 2023

Forest fires are becoming more frequent and larger around the world due to climate change. Remote sensing such as satellite images can be used as an alternative or assistance data because it reduces various difficulties of field survey. Forest burn severity (differenced normalized burn ratio, dNBR) is calculated through the difference in normalized burn ratio (NBR) before and after a forest fire. The images used in the NBR formula are based on Landsat's near-infrared (NIR) and short-wavelength infrared (SWIR) bands. South Korea's satellite images don't have a SWIR band. So domestic studies related to forest burn severity calculated dNBR using overseas images or indirectly using the normalized difference vegetation index (NDVI) using South Korea's satellite images. Therefore, in this study, dNBR was calculated by substituting the mid-wavelength infrared (MWIR) band of Kompsat-3A (K3A) instead of the SWIR band in the NBR formula. The results were compared with the dNBR results obtained through Landsat which is the standard for dNBR formula. As a result, it was shown that dNBR using K3A's MWIR band has a wider range of values and can be expressed in more detail than dNBR using Landsat's SWIR band. Therefore, it is considered that K3A images will be highly useful in surveying burn areas and severity affected by forest fires. In addition, this study used the K3A's MWIR band images degraded to 30 m. It is considered that much better results will be obtained if a higher-resolution MWIR band is used.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.28 no.3
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• pp.135-139
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• 2018

In this study, we report the effect of VI/III ratio on ${\alpha}-Ga_2O_3$ epilayer on sapphire substrate by halide vapor phase epitaxy. The surface of ${\alpha}-Ga_2O_3$ epilayer grown with various VI/III ratios was flat and crack-free. To analyze the optical properties of the ${\alpha}-Ga_2O_3$ epilayers, the transmittance and an optical band gap were measured. The optical band gap was shown to be around 5 eV and showed a proportional increase in VI/III ratios. To determine the crystal quality of alpha gallium oxide grown with a ratio of 23, closed to the theoretical optical band gap, the FWHM was measured by HR-XRD. The calculated dislocation density of screw and edge were $1.5{\times}10^7cm^{-2}$ and $5.4{\times}10^9cm^{-2}$, respectively.

• Phonetics and Speech Sciences
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• v.14 no.3
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• pp.87-101
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• 2022

In addition to administering a questionnaire (J-survey), which questions individuals on subjective vocal fatigue, voice samples were collected before and after speech-language pathology sessions from 50 female speech-language pathologists in their 20s and 30s in the Daejeon and Chungnam areas. We identified significant differences in Korean Vocal Fatigue Index scores between the fatigue and non-fatigue groups, with the most prominent differences in sections one and two. Regarding acoustic phonetic characteristics, both groups showed a pattern in which low-frequency band energy was relatively low, and high-frequency band energy was increased after the treatment sessions. This trend was well reflected in the low-to-high ratio of vowels, slope LTAS, energy in the third formant, and energy in the 4,000-8,000 Hz range. A difference between the groups was observed only in the vowel energy of the low-frequency band (0-4,000 Hz) before treatment, with the non-fatigue group having a higher value than the fatigue group. This characteristic could be interpreted as a result of voice abuse and higher muscle tonus caused by long-term voice work. The perturbation parameter and shimmer local was lowered in the non-fatigue group after treatment, and the noise-to-harmonics ratio (NHR) was lowered in both groups following treatment. The decrease in NHR and the fall of shimmer local could be attributed to vocal cord hypertension, but it could be concluded that the effective voice use of speech-language pathologists also contributed to this effect, especially in the non-fatigue group. In the case of the non-fatigue group, the rhamonics-to-noise ratio increased significantly after treatment, indicating that the harmonic structure was more stable after treatment.

• Proceedings of the IEEK Conference
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• 2001.06a
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• pp.233-236
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• 2001

Orthogonal freqency division multiplexing (OFDM) is an attractive technique for achieving high-bit-rate wireless data transmission. However, the potentially large peak-to-average power ratio (PAPR) has limited its application; An OFDM signal with the large PAPR can cause power degradation (In-band distortion) and spectral spreading (Out-of-band distortion) by being clipped passing through a power amplifier. Thus, we propose the combining algorithm of Hadamard transform and phase shift, which is ascribed to the relation between the correlation of the IFFT input sequence function and PAPR. Extensive computer simulations show that the combining algorithm is an effective technique to reduce PAPR.

• Bulletin of the Korean Chemical Society
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• v.19 no.1
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• pp.83-86
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• 1998

The luminescence spectra of Eu(Ⅲ) complexes have been measured as a function of molar metal-to-ligand ratio and pH. The ligands used in this study are tridentate iminodiacetate (IDA) and methyliminodiacetate (MIDA). The 620 nm emission band, attributed to the $^5D_0{\to}^7F_2$ transition, is monitored to investigate the optimum condition for $EuL_3^{3-}$ complex formation in aqueous state. In addition, the sensitivity of the 465 nm absorption band, attributed to the $^7F_0{\to}^5D_2$ transition, to the ligand environment has been also investigated.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.16 no.9 s.100
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• pp.893-902
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• 2005

We propose a novel single-feed triangular patch antenna with bar-type slots for dual-band circular polarization operation. And also propose a H-type slotted triangular patch antenna to enhance the ratio of the resonance frequencies. 3 cases of desist examples with bar-type slots were investigated and also 4 cases of H-slotted antennas were investigated to meet the dual-band requirement for GPS(Global Positioning System) and DMB(Digital Multimedia Broadcasting). One suitable design example was implemented and measured. Simulation results of the design example for GPS and DMB system show that axial ratio and maximum gain are 3.80 dB, 8.85 dBi for low-band at 1.575 GHz and 2.02 dB, 8.60 dBi for high-band at 2.642 GHz, respectively. Measured results of the implemented antenna show that $S_{11}$ is -12.43 dB for low-band at 1.575 GHz and less than -18.75 dB for high-band, respectively.