• International Journal of Aeronautical and Space Sciences
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• v.6 no.1
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• pp.71-76
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• 2005

The KOMPSAT-1 satellite, launched into a circular sun synchronous orbit on Dec. 21, 1999, entered its$6^{th}$year of successful operation this year. The purposes of the mission are to collect earth images (6.6 m resolution), multi-spectral images of the ocean, and to collect information on the particle environment of the low earth orbit. For normal operation, KOMPSAT-1 orbits are determined using GPS navigation solutions. However, at the start of the life of KOMPSAT-1, the 11-year solar activity cycle was at a maximum. Solar flux was maintained at this level until 2002, and thereafter reduced to a moderate level by 2004. Thus, the OD (Orbit Determination) accuracy has varied according to the solar activity. This paper presents the degree to which the OD accuracy could be degraded during a high solar activity period compared with that of a (relatively) low solar activity period. We investigated the effect of the use of solve-for parameters such as a drag coefficient ($C_D$), solar radiation coefficient ($C_R$), and the general accelerations ($G_A$) on OD accuracy with solar activity. For the evaluation of orbit determination accuracy, orbit overlap comparison is used since no independent orbits of comparable accuracy are available for comparison. The effect of the use of a box-wing model instead of a constant cross-sectional area is also investigated.

• Journal of the Korean Society for information Management
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• v.11 no.1
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• pp.17-29
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• 1994

Ths study was prformed to measure the indexmg coverage of the periol~cal literature on a sub'||'&'||' area inKorean language policy. A bibliography of 675 citations dated 1977-1990 was obtained through Xugohaknonpmongnokjip'.Coverage was determined by checlung the citations against 2 indexmg services, Ch-dg~idm"and Xugogungrnunhaknonmunmongrok". For the period 1977-1990, T h o n g g g a n h a n g m s consistentlyprovided coverage of about 63%. For the pencd 1986-1990, Xugogungmunhaknonmunmongrok" providedcoverage of about 33% and the combined coverage of the two services was abut 78%. The extent of overlap anduniqueness of both indexmg services are measured. The analysis also includes the subst of the indexed art~cles andthe type of perid'||'&'||' Omitted.rid& Omitted.

• International Journal of Automotive Technology
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• v.7 no.7
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• pp.763-768
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• 2006

Emission reduction in the cold start period of SI engines is crucial to meet stringent emission regulations such as SULEV Emissoin reduction is the starting point of the study in the which the variable valve timing (VVT) technology may be one promising method to minimize cold start emissions while maintaining engine performance. This is because it is possible to change valve overlap and residual gas fraction during cold start and idle operations. Our previous study showed that spark timing is another important factor for reducing cold-start emissions since it affects warm-up time of close-coupled catalysts (CCC) by changing exhaust gas temperature. However, even though these factors may be favorable for reduction of emissions, they may deteriorate combustion stability in these operating conditions. This means that the two variables should be optimized for best exhaust emissions and engine stability. This study investigated the effects of valve and spark timings in idle performance such as combustion stability and exhaust emissions. Experiments showed that valve timings significantly affected engine stability and exhaust emissions, especially CO and $NO_x$, due to change in residual gas fraction within the combustion chamber. Spark timing also affects HC emissions and exhaust gas temperature. Yet it has no significant effects on combustion stability. A control strategy of proper valve timing and spark timing is suggested in order to achieve a reduction in exhaust emissions and a stable operation of the engine in a cold start and idle operation.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.27 no.7
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• pp.1-7
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• 2013

LED lamps are appropriate to the light source for use in broadcasting lighting. When the lighting frequency of LED lamp is different from the shutter frequency of camera, however, the flicker appears on the moving image which is taken by the camera under the dimmed light of LED lamp with conventional dimming methods. In this study, a new dimming method for the LED lamps used as the broadcasting lighting is proposed to minimize the flicker. A lighting period in the proposed dimming method is divided into the several sub-periods which have weight of the bits. Also, the proposed dimming method has characteristics that are optimizing the turn-on timings of two LED lamps to reduce the turn-off periods of the lamps and not to overlap the turn-on periods of the lamps. In the experiments, the incidence of the flicker is taken by the camera under several lighting conditions of the conventional dimming methods and the proposed dimming method. In addition, the brightness values of the frames are obtained in the moving image, and incidence of the flicker is evaluated and compared with conventional methods. From the results, the incidence of the flicker in the proposed method is more improved than the conventional methods.

• Journal of Communications and Networks
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• v.7 no.1
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• pp.76-86
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• 2005

Today's wireless access networks consist of several tiers that overlap each other. Provisioning of real time undisrupted communication to mobile users, anywhere and anytime through these heterogeneous overlay networks, is a challenging task. We extend the end-to-end approach for the handoff management in hybrid wireless data network by designing a fully mobile-controlled handoff for mobile devices equipped with dual mode interfaces. By handoff, we mean switching the communication between interfaces connected to different subnets. This mobile-controlled handoff scheme reduces the service disruption time during both horizontal and vertical handoffs and does not require any modification in the access networks. We exploit the IP diversity created by the dual interfaces in the overlapping area by simultaneously connecting to different subnets and networks. Power saving is achieved by activating both interfaces only during the handoff period. The performance evaluation of the handoff is carried out by a simple mathematical analysis. The analysis shows that with proper network engineering, exploiting the speed of mobile node and overlapping area between subnets can reduce service disruption and power consumption during handoff significantly. We believe that with more powerful network interfaces our proposal of dual interfaces can be realized.

• Journal of Communications and Networks
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• v.17 no.2
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• pp.100-109
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• 2015

Mobile phone carriers face challenges from three synergistic dimensions: Wireless, social, and mobile. Despite significant advances that have been made about social networks and human mobility, respectively, our knowledge about the interplay between two layers remains largely limited, partly due to the difficulty in obtaining large-scale datasets that could offer at the same time social and mobile information across a substantial population over an extended period of time. In this paper, we take advantage of a massive, longitudinal mobile phone dataset that consists of human mobility and social network information simultaneously, allowing us to explore the impact of human mobility patterns on the underlying social network. We find that human mobility plays an important role in shaping both local and global structural properties of social network. In contrast to the lack of scale in social networks and human movements, we discovered a characteristic distance in physical space between 10 and 20 km that impacts both local clustering and modular structure in social network. We also find a surprising distinction in trajectory overlap that segments social ties into two categories. Our results are of fundamental relevance to quantitative studies of human behavior, and could serve as the basis of anchoring potential theoretical models of human behavior and building and developing new applications using social and mobile technologies.

• Korean Journal of Construction Engineering and Management
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• v.8 no.6
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• pp.197-206
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• 2007

The age of synthetic correspondence against "development period shortening, high quality" is about to come. While the product is becoming complexation gradually and also diversification, life cycle of product gets shorted. Under the infinity competition age, the certain answer of "the survival of the fittest" is that the enterprise itself must be able to shorten development and delivery term, reduce product development and life cycle cost, and satisfy customer in performance and value. From this point of view, concurrent engineering's theory, which develops the process of product in the manner of repetition, parallel, and synchronousness, is realized as a important strategy instrument in construction. This study investigates and analyzes several examples in order to apply concurrent engineering successfully, and presents the decision-making model through the simulation about the factor affecting in each process.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.279.2-279.2
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• 2016

InGaN/GaN multiple quantum wells (MQWs) have been attracted much attention as light-emitting diodes (LEDs) in the visible and UV regions. Particularly, quantum efficiency of green LEDs is decreased dramatically as approaching to the green wavelength (~500 nm). This low efficiency has been explained by quantum confined Stark effect (QCSE) induced by piezoelectric field caused from a large lattice mismatch between InGaN and GaN. To improve the quantum efficiency of green LED, several ways including epitaxial lateral overgrowth that reduces differences of lattice constant between GaN and sapphire substrates, and non-polar method that uses non- or semi-polar substrates to reduce QCSE were proposed. In this study, graded short-period InGaN/GaN superlattice (GSL) was grown below the 5-period InGaN/GaN MQWs. InGaN/GaN MQWs were grown on the patterned sapphire substrates by vertical-metal-organic chemical-vapor deposition system. Five-period InGaN/GaN MQWs without GSL structure (C-LED) were also grown to compare with an InGaN/GaN GSL sample. The luminescence properties of green InGaN/GaN LEDs have been investigated by using photoluminescence (PL) and time-resolved PL (TRPL) measurements. The PL intensities of the GSL sample measured at 10 and 300 K increase about 1.2 and 2 times, respectively, compared to those of the C-LED sample. Furthermore, the PL decay of the GSL sample measured at 10 and 300 K becomes faster and slower than that of the C-LED sample, respectively. By inserting the GSL structures, the difference of lattice constant between GaN and sapphire substrates is reduced, resulting that the overlap between electron and hole wave functions is increased due to the reduced piezoelectric field and the reduction in dislocation density. As a results, the GSL sample exhibits the increased PL intensity and faster PL decay compared with those for the C-LED sample. These PL and TRPL results indicate that the green emission of InGaN/GaN LEDs can be improved by inserting the GSL structures.

• 보존과학연구
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• s.7
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• pp.11-23
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• 1986

Korea has a long history from several thousand years ago. Specially stonecultural properties were used for the long period in Korea. Ancient tombs of prehistoric age, Sukgul-am (stone cave), Chumsung-dae (astronomical observatory)and stone statue of Buddha in the period of three dynasties were made of stone. There after the foundation stone, mortarstone, millstone, and other stone facilities at home have been also used for a long time. For the conservation of the cultural properties, the ecological distribution of lichens was investigated with the stone buildings around tombs and multistory stonepagodas.1. Yoo-nung (The tomb of Emperor Sungjong) : The stone buildings of granitearound tomb about 80 years old are clean and solid in quality, but they were covered in places with three kinds of lichens as the circle forms of pale-green and dark-gray colored thallus in crustose type. They were examined ecological distribution, identification and measurement of lobe size of lichens.2. Yoong-nung (The tomb of son of Emperor Yung-jong) : Granite buildings around tomb about 200 years old are also clean, moreover, they were also covered with the same kinds of crustose lichens in overlap.3. Yung-nung (The tomb of Emperor Sejong) : More than seven species of crustose, foliose and one species of fruticose lichens were growing on thegranite stone buildings built in about 500 years ago.4. Shilluk-sa multistory brick pagoda : It was built at the Koryo dynasty more than about 700 years. More than 5 species of crustose folios and two species of fruticose lichens were growing well on the granite stone at shade area. It was assumed that foliose lichens as dominant species with the respect of ecological aspect.5. Shilluk-sa multistory marble pagoda : This marble pagoda was built at the middle period of the Koryo dynasty as the same age of multistory brick pagoda. It was covered with black colored thallus such as crustose lichens. It is seemed that only crustose lichens on the stone were due to the drymarble stone. From the results described above, distribution of lichens showed that only crustose lichens on the stone from the short history (about 100-200 years) were grown with small size of circle form, stone buildings over 500 years were covered with crustose, folios and fruticose lichens, and the lichens distribution on the stonemore than 500 years appeared more foliose and fruticose than crustose type. Therefore, it is suggested that the succession of lichens on the stone buildings iscrustose$\rightarrow$foliose$\rightarrow$fruticose.Parmelia mexicana, Cladonia floerkeana, Ramalina yasudae identified are described in this paper and other unidentified species of saxicolous lichens will bedescribed later.

• Journal of Conservation Science
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• v.29 no.3
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• pp.223-229
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• 2013

It is not known in details for the A.D. period as the archaeomagnetic dating method to be fully facilitated in Korea but it has prepared for the revised shape of standard curve to trace the geomagnetic field variation, and there were cases to increase the survey on relics on the B.C. period to find out for the detailed archaeomagnetic field variation on the Bronze Age to the Early Iron Age. Furthermore, the survey cases on the relics on the Neolithic Age began to emerge a little by little archaeomagnetic field variation of the Neolithic Age through 34 pieces of the archaeomagnetic measurement data as making active advancement around mid-western region. Data is insufficient yet that it is difficult to find out the detailed trend of modification but it is estimated for approximate appearance. The archaeomagnetic field variation of the Neolithic Age made changes without breaking away from the scope of changes in the A.D. period as in the same way with the Bronze Age, and comparing to the variation of archaeomagnetic field for the Bronze Age, the magnetic inclination shifted within the scope of having almost no difference, but the declination is shown to skewed toward the east in its overall appearance. In addition, the comparison was made with the data of the Jomon Age in Japan and the archaeomagnetic measurement data of Korea has a little bit more depth for while the declination is skewed toward the east for 10 degree or more compared to those of Japan. However, in the part where the data is concentrated most intensely, the data for both countries has significant part to overlap to each other that the archaeomagnetic field variation of the Neolithic Age of Korea showed overall similar variation with certain partial changes when compared to those of Japan.