• Current Optics and Photonics
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• 제6권5호
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• pp.445-452
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• 2022

We developed an adaptive optics test bench using an optical simulator and two rotating phase plates that mimicked the atmospheric turbulence at Bohyunsan Observatory. The observatory was reported to have a Fried parameter with a mean value of 85 mm and standard deviation of 13 mm, often expressed as 85 ± 13 mm. First, we fabricated several phase plates to generate realistic atmospheric-like turbulence. Then, we selected a pair from among the fabricated phase plates to emulate the atmospheric turbulence at the site. The result was 83 ± 11 mm. To address dynamic behavior, we emulated the atmospheric disturbance produced by a wind flow of 8.3 m/s by controlling the rotational speed of the phase plates. Finally, we investigated how closely the atmospheric disturbance simulation emulated reality with an investigation of the measurements on the optical table. The verification confirmed that the simulator showed a Fried parameter of 87 ± 15 mm as designed, but a little slower wind velocity (7.5 ± 2.5 m/s) than expected. This was because of the nonlinear motion of the phase plates. In conclusion, we successfully mimicked the atmospheric disturbance of Bohyunsan Observatory with an error of less than 10% in terms of Fried parameter and wind velocity.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2011년도 제37회 추계학술대회논문집
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• pp.588-593
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• 2011

대기외란을 적용한 램제트 엔진 시스템의 성능계산 기법을 제안 하였다. 비행 궤도는 동압력이 일정한 궤적을 사용하였다. 고도에 따른 동압력을 설정하고, 이에 따른 비행 조건에서 대기 외란을 계산하였다. 엔진 흡입구로 유입되는 대기 외란을 계산하기 위하여 Tank의 외란 모델을 사용하였으며, 외란을 고려한 비행 조건에 대한 성능 계산을 수행하고 추력과 종말 충격파의 위치를 파악함으로써 설계된 비행 궤도 내에서 램제트 엔진 시스템의 비행 안정성을 확인하였다.

• 한국구조물진단유지관리공학회 논문집
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• 제28권3호
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• pp.1-9
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• 2024

본 연구는 대기외란 조건에서 비전센서를 활용하여 구조물의 동적 변위 측정을 위하여 멀티스케일 템플릿 매칭 기법 (TMI: Template Matching with Image pyramids)을 제안하고 제안기법의 변위 측정 성능을 조사하기 위해 진행되었다. 촬영거리에 따른 변위 측정 성능을 평가하기 위해 3층 전단 구조물을 설계하였으며, FHD(1920×1080)급 카메라를 준비하여 변위 계측에 사용하였다. 최초 촬영거리를 10m로 설정하였고, 10m씩 멀어지면서 최대 40m까지 변위 측정 실험을 진행하였다. 실내 조도 조건(450lux)에서 발열 기구를 활용하여 대기외란을 발생시켰으며, 대기외란으로 이미지를 왜곡시켰다. 사전실험을 통해 대기외란시 특징점 기반 변위 측정 방법과 제안기법의 변위 측정 타당성을 비교 검증하였으며, 검증 결과 제안기법의 낮은 측정 에러율을 나타냈다. 대기외란 환경에서 변위 측정 성능평가 결과, 인공 타겟을 활용한 TMI는 대기외란 유무에 따라 변위 측정 성능에 큰 차이가 없었다. 하지만 자연 타겟을 활용하였을 때, 20m 이상의 촬영거리에서 RMSE가 크게 상승하여 제안기법의 운용 한계를 보여줬다. 이는 촬영거리 증가에 따라 자연 타겟의 해상도가 저하되며, 대기외란으로 인한 이미지 왜곡이 템플릿 이미지 추정에 오류가 발생 되어 변위 측정 오차가 높게 발생하는 경향을 나타냈다.

• Current Optics and Photonics
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• 제8권2호
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• pp.138-150
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• 2024

This study is essential for advancing our knowledge about the interaction between long-range high-power lasers and energetic materials, with a particular emphasis on understanding the response of a 155-mm shell under various surface irradiations, taking into account external factors such as atmospheric disturbances. The analysis addresses known limitations in understanding the use of non-realistic targets and the negligence of ambient conditions. The model employs the three-dimensional level-set method, computer-aided design (CAD)-based target design, and a message-passing interface (MPI) parallelization scheme that enables rapid calculations of the complex chemical reactions of the irradiated high explosives. Important outcomes from interaction modeling include the accurate prediction of the initiation time of ignition, transient pressure, and temperature responses with the location of the initial hot spot within the shell, and the relative magnitude of noise with and without the presence of physical ambient disturbances. The initiation time of combustion was increased by approximately a factor of two with atmospheric disturbance considered, while slower heating of the target resulted in an average temperature rise of approximately 650 K and average pressure increase of approximately 1 GPa compared to the no ambient disturbance condition. The results provide an understanding of the interaction between the high-power laser and energetic target at a long distance in an atmospheric condition.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2000년도 춘계학술대회논문집
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• pp.1210-1215
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• 2000

일반적인 형태의 동적 시스템에 가해지는 외란이 유색잡음이며, 외란의 형태가 시간에 따라 불규칙하게 변할 때와 감쇠계수와 강성계수가 시간에 따라 불규칙하게 변할 때의 그 확률 시스템의 특성과 제어기법에 대하여 연구하였다. 고려된 물리적인 모델은 불규칙한 대기교란을 받는 항공익으로써 이는 "시변계"가 되며 그 지배방정식이 확률론적 관점에서 F-P-K 접근법으로 유도되었으며, "유색잡음용 허-확률제어기"의 효과를 확률 영역 및 시간영역에서 고찰하였다. 또한 확률론적 플러터경계에 대해서도 고찰하였다.

• 제어로봇시스템학회논문지
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• 제11권3호
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• pp.262-269
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• 2005

Automatic landing of UAVs receives increasing interest these days, with increasing number of the developed UAV systems. In this paper, a glidepath tracking algorithm of the subscale UAV was proposed and the performance was analyzed. Flight data analysis shows that the existing autolanding flight control algorithm has a classical type glidepath control. This paper presents an alternative glidepath tracking strategy based on embedded flight control law. The performance of the proposed strategy was investigated through the TDP(Touch Down Point) error analysis with regard to various flight environment: steady headwind, atmospheric disturbance, communication transfer delay. It was verified that the proposed glidepath tracking strategy can be successfully applied to the practical autolanding of UAV systems.

• 제어로봇시스템학회논문지
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• 제13권10호
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• pp.962-969
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• 2007

In this paper, an approach is developed for relative state estimation of satellite formation flying. To estimate relative states of two satellites, the Extended Kalman Filter Algorithm is adopted with the relative distance and speed between two satellites and attitude of satellite for measurements. Numerical simulations are conducted under two circumstances. The first one presents both chief and deputy satellites are orbiting a circular reference orbit around a perfectly spherical Earth model with no disturbing acceleration, in which the elementary relative orbital motion is taken into account. In reality, however, the Earth is not a perfect sphere, but rather an oblate spheroid, and both satellites are under the effect of $J_2$ geopotential disturbance, which causes the relative distance between two satellites to be on the gradual increase. A near-Earth orbit decays as a result of atmospheric drag. In order to remove the modeling error, the second scenario incorporates the effect of the $J_2$ geopotential force, and the atmospheric drag, and the eccentricity in satellite orbit are also considered.

• Current Optics and Photonics
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• 제8권3호
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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.

• Current Optics and Photonics
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• 제1권2호
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• pp.107-112
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• 2017

An adaptive optics system can be simulated or analyzed to predict its closed-loop performance. However, this type of prediction based on various assumptions can occasionally produce outcomes which are far from actual experience. Thus, every adaptive optics system is desired to be tested in a closed loop on an optical test bench before its application to a telescope. In the close-loop test bench, we need an atmospheric simulator that simulates atmospheric disturbances, mostly in phase, in terms of spatial and temporal behavior. We report the development of an atmospheric turbulence simulator consisting of two point sources, a commercially available deformable mirror with a $12{\times}12$ actuator array, and two random phase plates. The simulator generates an atmospherically distorted single or binary star with varying stellar magnitudes and angular separations. We conduct a simulation of a binary star by optically combining two point sources mounted on independent precision stages. The light intensity of each source (an LED with a pin hole) is adjustable to the corresponding stellar magnitude, while its angular separation is precisely adjusted by moving the corresponding stage. First, the atmospheric phase disturbance at a single instance, i.e., a phase screen, is generated via a computer simulation based on the thin-layer Kolmogorov atmospheric model and its temporal evolution is predicted based on the frozen flow hypothesis. The deformable mirror is then continuously best-fitted to the time-sequenced phase screens based on the least square method. Similarly, we also implement another simulation by rotating two random phase plates which were manufactured to have atmospheric-disturbance-like residual aberrations. This later method is limited in its ability to simulate atmospheric disturbances, but it is easy and inexpensive to implement. With these two methods, individually or in unison, we can simulate typical atmospheric disturbances observed at the Bohyun Observatory in South Korea, which corresponds to an area from 7 to 15 cm with regard to the Fried parameter at a telescope pupil plane of 500 nm.

• 한국지구과학회지
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• 제35권5호
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• pp.333-341
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• 2014

A new set of basis functions was constructed using the Rossby-Haurwitz waves, which are the eigenfunctions of nondivergent barotropic vorticity equations on the sphere. The basis functions were designed to be non-separable, that is, not factored into functions of either the longitude or the latitude. Due to this property, the nodal lines of the functions are aligned neither along with the meridian nor the parallel. The basis functions can be categorized into groups of which members have the same degree or the total wavenumber-like index on the sphere. The orthonormality of the basis functions were found to be close to the machine roundoffs, giving the error of $O(10^{-15})$ or $O(10^{-16})$ for double-precision computation (64 bit arithmetic). It was demonstrated through time-stepping procedure that the basis functions were also the eigenfunctions of the non-divergent barotropic vorticity equations. The projection of the basis functions was carried out onto the low-resolution geopotential field of Gaussian bell, and compared with the theory. The same projections were performed for the observed atmospheric-geopotential height field of 500 hPa surface to demonstrate decomposition into the fields that contain disturbance of certain range of horizontal scales. The usefulness of the new basis functions was thus addressed for application to the eigenmode analysis of the atmospheric motions on the global domain.