• Journal of the Institute of Convergence Signal Processing
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• v.24 no.1
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• pp.1-14
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• 2023

Underwater images carry information that is useful in the fields of aquaculture, underwater military security, navigation, transportation, and so on. In this research, we transmitted an underwater image through various underwater mediums in the presence of underwater turbulence and beam attenuation effects using a high-speed visible optical carrier signal. The optical beam undergoes scintillation because of the turbulence and attenuation effects; therefore, distorted images were observed at the receiver end. To understand the behavior of the communication media, we obtained the bit error rate (BER) performance of the system with respect to the average signal-to-noise ratio (SNR). Also, the structural similarity index (SSI) and peak SNR (PSNR) metrics of the received image were evaluated. Based on the received images, we employed suitable nonlinear filters to recover the distorted images and enhance them further. The BER, SSI, and PSNR metrics of the specific nonlinear filters were also evaluated and compared with the unfiltered metrics. These metrics were evaluated using the on-off keying and binary phase-shift keying modulation techniques for the 50-m and 100-m links for beam attenuation resulting from pure seawater, clear ocean water, and coastal ocean water mediums.

• Atmosphere
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• v.24 no.2
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• pp.235-243
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• 2014

Sources of aviation turbulence vary through the seasons, especially in the East Asia including Korean peninsula, associated primarily with the changes in the jet/front system and convective activities. For this reason, a seasonal Korean aviation Turbulence Guidance (KTG) system (seasonal-KTG) is developed in the present study by using pilot reports (PIREPs) and analysis data of the operational Unified Model (UM) of the Korea Meteorological Administration (KMA) for two years between June 2011 and May 2013. Twenty best diagnostics of aviation turbulence in each season are selected by the method of probability of detection (POD) using the PIREPs and UM data. After calculating a weighting value of each selected diagnostics using their area under curve (AUC), the 20 best diagnostics are combined with the weighting scores into a single ensemble-averaged index by season. Compared with the current operational-KTG system that is based on the diagnostics applying all seasons, the performances of the seasonal-KTG system are better in all seasons, except in fall.

• Atmosphere
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• v.19 no.3
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• pp.269-287
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• 2009

CAT (clear-air turbulence) forecasting algorithm, the Graphical Turbulence Guidance (GTG) system developed at NCAR (national center for atmospheric research), is evaluated with available observations (e.g., pilot reports; PIREPs) reported in South Korea during the recent 5 years (2003-2008, excluding 2005). The GTG system includes several steps. First, 44 CAT indices are calculated in the domain of the Regional Data Assimilation and Prediction System (RDAPS) analysis data with 30 km horizontal grid spacing provided by KMA (Korean Meteorological Administration). Second, 10 indices that performed ten best forecasting scores are selected. Finally, 10 indices are combined by measuring the score based on the probability of detection, which is calculated using PIREPs exclusively of moderate-or-greater intensity. In order to investigate the best performance of the GTG system in Korea, various statistical examinations and sensitivity tests of the GTG system are performed by yearly and seasonally classified PIREPs. Performances of the GTG system based on yearly distributed PIREPs have annual variations because the compositions of indices are different from each year. Seasonal forecasting is generally better than yearly forecasting, because selected CAT indices in each season represent meteorological condition much more properly than applying the selected CAT indices to all seasons. Wintertime forecasting is the best among the four seasonal forecastings. This is likely due to that the GTG system consists of many CAT indices related to the jet stream, and turbulence associated with the jet stream can be activated mostly in wintertime under strong jet magnitude. On the other hand, summertime forecasting skill is much less than other seasons. Compared with current operational CAT prediction system (KITFA; Korean Integrated Turbulence Forecasting System), overall performance of the GTG system is better when CAT indices are selected seasonally.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.6 no.2
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• pp.159-164
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• 2005

When an optical pulse propagates through the atmospheric channel, the atmospheric turbulence attenuates and spreads this pulse. This attenuation and broadening of pulse are occurred by the fluctuation in the arrival time of pulse at the optical receiver. This pulse broadening induces the intersymbol interference (ISI) between the adjacent pulses. finally, the adjacent pulses are overlapped and the bit rate and the repeaterless transmission length are limited by the ISI. In digital communication system, therefore, the pulse broadening is more important factor than the attenuation. In this paper, thus, we find the ISI in the atmospheric turbulence as the function of the structure constant for the refractive index fluctuation that presents the strength of turbulence using the temporal momentum function and present it by numerical analysis.

• Current Optics and Photonics
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• v.8 no.3
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• pp.259-269
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• 2024

Optical imaging systems that operate through atmospheric pathways often suffer from image degradation, mainly caused by the distortion of light waves due to turbulence in the atmosphere. Adaptive optics technology can be used to correct the image distortion caused by atmospheric disturbances. However, there are challenges in conducting experiments with strong atmospheric conditions. An optical phase plate (OPP) is a device that can simulate real atmospheric conditions in a lab setting. We suggest a novel two-step process to fabricate an OPP capable of simulating the effects of atmospheric turbulence. The proposed fabrication method simplifies the process by eliminating additional activities such as phase-screen design and phase simulation. This enables an efficient and economical fabrication of the OPP. We conducted our analysis using the statistical fluctuations of the refractive index and applied modal expansion using Kolmogorov's theory. The experiment aims to fabricate an OPP with parameters D/r₀ ≈ 30 and r₀ ≈ 5 cm. The objective is defined with the strong atmospheric conditions. Finally, we have fabricated an OPP that satisfied the desired objectives. The OPP closely simulate turbulence to real atmospheric conditions.

• Journal of IKEEE
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• v.1 no.1 s.1
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• pp.156-163
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• 1997

Since the wireless optical communications system transmits informations through the atmosphere, it is subject to many effects of the constituent materials of atmosphere. The turbulence effect always exists in both clear and cloudy days. It causes a beam wandering, breathing, and scintillation. These disadvantageous phenomena degrade the performance of an optical communications system. In this paper, I designed a refractive index measuring system and subcarrier ASK optical communications system. Through this system I measured refractive index in May and in August. From these measurements, the minimum value of the refractive index in these period was about ${\approx}10^{-15}[m^{-2/3}]$ at night time and the maximum value was about ${\approx}10^{-12}[m^{-2/3}]$ at day time. The refractive index structure parameter. BER(bit error rate), and the burst length were measured simultaneously in these measurements. the theoretically predicted BER and the measured values showed a good agreement.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.39B no.12
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• pp.856-863
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• 2014

In the terrestrial optical wireless communication(OWC), the performance is affected by atmospheric turbulence and particles in the air. The received signal power loss mainly is caused by turbulence and scattering. To minimize the adverse atmospheric effects, the OWC used optical signal modulation, such as OOK, PPM and DPIM. In this paper, the packet error rate(PER) was analyzed above three modulation methods to ground optical link in atmospheric turbulence, scattering and link distance. The OWC system used three wavelengths which are 850nm, 1310nm and 1550nm. I assumed the atmospheric turbulence intensity is weak, so the refractive index is $Cn2{\approx}10-14m-2/3$ and the visibility is 2km. The numerical results shown that the L-DPIM scheme and the wavelength 1550nm are better than other modulation methods and wavelengths.

• Journal of Korean Society of Water and Wastewater
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• v.28 no.2
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• pp.225-233
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• 2014

This paper presents a Computational Fluid Dynamics(CFD) based simulation and experimental tracer test of flow pattern and turbulent energy dissipation inside a serpentine flocculation basin with continuous operation. Research focused on the evaluation of a specific flow pattern on the hydraulic behavior on the flocculation basin. From the results of CFD simulation and actual tracer test, both results were in good accordance with each other. Also, each Morill index were calculated as 1.5 from CFD simulation and 1.7 from actual tracer test, respectively. Especially, turbulence energy was dissipated relatively higher in the vicinity of inlet to the flocculation basin than other region. The differences between the CFD simulation and actual tracer test were 1.4 min in $T_{50}$, and 1.3 min in $T_p$, respectively.

• International Journal of Naval Architecture and Ocean Engineering
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• v.7 no.6
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• pp.1020-1033
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• 2015

In order to design a compact urea selective catalytic reduction system, numerical simulation was conducted by computational fluid dynamics tool. A swirl type static mixer and a mixing chamber were considered as mixing units in the system. It had great influence on flow characteristics and urea decomposition into ammonia. The mixer caused flow recirculation and high level of turbulence intensity, and the chamber increased residence time of urea-water-solution injected. Because of those effects, reaction rates of urea decomposition were enhanced in the region. When those mixing units were combined, it showed the maximum because the recirculation zone was significantly developed. $NH_3$ conversion was maximized in the zone due to widely distributed turbulence intensity and high value of uniformity index. It caused improvement of $NO_x$ reduction efficiency of the system. It was possible to reduce 55% length of the chamber and connecting pipe without decrease of $NO_x$ reduction efficiency.

• Korean Journal of Optics and Photonics
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• v.31 no.6
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• pp.247-258
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• 2020

In this paper, based on a stochastic parallel gradient descent (SPGD) algorithm we study phase control of a coherent-beam-combining system under turbulent atmospheric conditions. Based on the statistical theory of atmospheric turbulence, we carry out the analysis of the phase and wavefront distortion of a laser beam propagating through a turbulent atmospheric medium. We also conduct numerical simulations of a coherent-beam-combining system with 7- and 19-channel laser beams distorted by atmospheric turbulence. Through numerical simulations, we characterize the phase-control characteristics and efficiency of the coherent-beam-combining system under various degrees of atmospheric turbulence. It is verified that the SPGD algorithm is capable of realizing 7-channel coherent beam combining with a beam-combining efficiency of more than 90%, even under the turbulent atmospheric conditions up to cn2 of 10-13 m-2/3. In the case of 19-channel coherent beam combining, it is shown that the same turbulent atmospheric conditions result in a drastic reduction of the beam-combining efficiency down to 60%, due to the elevated impact of the corresponding refractive-index inhomogeneity. In addition, by putting together the number of iterations of the SPGD algorithm required for phase locking under atmospheric turbulence and the time intervals of atmospheric phenomena, which typically are of the order of ㎲, it is estimated that hundreds of MHz to a few GHz of computing bandwidth of SPGD-based phase control may be required for a coherent-beam-combining system to confront such turbulent atmospheric conditions. We expect the results of this paper to be useful for quantitatively analyzing and predicting the effects of atmospheric turbulence on the SPGD-based phase-control performance of a coherent-beam-combining system.