• 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.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2011.11a
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• pp.588-593
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• 2011

In this study, the performance analysis method for ramjet engine system with atmospheric air disturbance was proposed. Flight path was determined to satisfy dynamic pressure constant at each flight altitude. The atmospheric air disturbance incoming into a engine intake was simulated by the model Tank proposed. The performance parameters was investigated at each flight condition with air disturbance. Engine operation stability was evaluated as analysis of the normal shock position.

• Journal of the Korea institute for structural maintenance and inspection
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• v.28 no.3
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• pp.1-9
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• 2024

This study proposes a multi-scale template matching technique with image pyramids (TMI) to measure structural dynamic displacement using a vision sensor under atmospheric turbulence conditions and evaluates its displacement measurement performance. To evaluate displacement measurement performance according to distance, the three-story shear structure was designed, and an FHD camera was prepared to measure structural response. The initial measurement distance was set at 10m, and increased with an increment of 10m up to 40m. The atmospheric disturbance was generated using a heating plate under indoor illuminance condition, and the image was distorted by the optical turbulence. Through preliminary experiments, the feasibility of displacement measurement of the feature point-based displacement measurement method and the proposed method during atmospheric disturbances were compared and verified, and the verification results showed a low measurement error rate of the proposed method. As a result of evaluating displacement measurement performance in an atmospheric disturbance environment, there was no significant difference in displacement measurement performance for TMI using an artificial target depending on the presence or absence of atmospheric disturbance. However, when natural targets were used, RMSE increased significantly at shooting distances of 20 m or more, showing the operating limitations of the proposed technique. This indicates that the resolution of the natural target decreases as the shooting distance increases, and image distortion due to atmospheric disturbance causes errors in template image estimation, resulting in a high displacement measurement error.

• Optical Science and Technology
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• v.16 no.4
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• pp.27-35
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• 2012

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2009.11a
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• pp.546-549
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• 2009

Dynamic model for supersonic engine is proposed to design control law. The model structure is constructed to capture the local characteristics of supersonic and subsonic flow by using conservation equations. To evaluate the stability of control law under the disturbances, the air turbulence model is incorporated with the engine model. The combined model shows analogous results compared to performance analysis model which is good coincidence with CFD results and disturbance effects.

• The Journal of Aerospace Industry
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• s.65
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• pp.89-106
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• 2003

항공기의 자동 착륙 알고리즘을 위한 고 정밀 유도방식에 IBLS(Integrity Beacons Landing System)나 MLS(Microwave Landing System)와 같은 유도 제어 방식을 사용하여 유인 항공기나 무인 항공기의 유도 착륙에 사용하고 있다. 당 연구에서는 무인항공기의 자동 착륙을 위한 실제적인 요구사항들이 분석되었고, 자동 착륙 유도장치로 IBLS와 MLS가 선택되어 각각의 기능과 특성들이 수학적으로 모델링 되었다. 또한 고전제어와 최적제어의 2가지 방식으로 무인항공기의 자동 착륙을 통제하기 위한 autopilot이 설계되어 그 유효성과 특징들이 분석되었다. IBLS, MLS, autopilot, 그리고 이러한 자동 착륙 유도제어 시스템이 적용되는 대상체인 무인항공기와 대기환경 및 외란에 대한 수학적 모델들은 Simulink와 ANSI C를 사용하여 단위 S/W 모듈들로 작성되었고, 여기에 GUI모듈이 추가되어 하나의 통합 시뮬레이션 S/W가 완성되었다. 모의시험평가는 총 2단계로 구성되었는데, 대기 외란이 주어졌을 때 IBLS와 MLS의 유효성을 1차적으로 검증하였고, 2단계 모의수치실험에서는 MLS 유도센서 방식에 따른 고전제어기 및 최적제어기의 항공기 종 방향 운동에 대한 강인성 비교를 시도하였다.

• Korean Journal of Optics and Photonics
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• v.31 no.6
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• pp.274-280
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• 2020

In this paper, the effect of atmospheric turbulence is numerically modeled and analyzed via a phase-screen model, in regard to long-range optical energy transfer using coherent beam combination. The coherent-beam-combination system consists of three channel beams pointing at a target at a distance of 1-2 km. The phase and propagation direction of each channel beam are assumed to be corrected in an appropriate manner, and the atmospheric turbulence that occurs while the beam propagates through free space is quantified with a phase-screen model. The phase screen is statistically generated and constructed within the range of fluctuations of the structure constant Cn2 from 10-15 to 10-13 [m-2/3]. Particularly, in this discussion the shape, distortion, and combining efficiency of the 3-channel combined beam are calculated at the target plane by varying the structure constant used in the phase-screen model, and the effect of atmospheric turbulence on beam-combination efficiency is analyzed. Analysis with this numerical model verifies that when coherent beam combination is used for long-range optical energy transfer, the received power at the target can be at least three times the power obtainable by incoherent beam combination, even for maximal atmospheric fluctuation within the given range. This numerical model is expected to be effective for analyzing the effects of various types of atmospheric-turbulence conditions and beam-combination methods when simulating long-range optical energy transfer.

• Korean Journal of Optics and Photonics
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• v.35 no.5
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• pp.199-209
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• 2024

Predicting the performance of adaptive optics systems is a crucial step in their design and analysis. First-order prediction methods, based primarily on several assumptions and scaling laws, are commonly used. These methods must account for various parameters and error sources, such as the intensity and profile of atmospheric turbulence, fitting errors based on the resolution of the wavefront sensor and deformable mirror, wavefront-sensor noise propagated through the wavefront-reconstruction algorithm, servo lag due to the finite bandwidth of the control loop, and anisoplanatism caused by the arrangement of natural and laser guide stars. However, since first-order performance-prediction methods based on certain assumptions can sometimes yield results that deviate from real-world performance, evaluation through computational simulations and closed-loop tests on a testbed is necessary. Additionally, an atmospheric simulator is required for closed-loop testing, which must adequately simulate the spatial and temporal characteristics of atmospheric disturbances. This paper aims to present an overview of the theory of atmospheric disturbance simulators, as well as their implementation in computational simulation and hardware.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2000.06a
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• pp.1210-1215
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• 2000

Investigation is performed on the stability of general form of dynamic system under colored noise random disturbance whose damping and stiffness are varying in irregular manner along time, which is a preliminary result in the course of research on the characteristic and the control of the stochastic system. Adopted physical model is airfoil under random atmospheric disturbance, which becomes a "time-varying system" whose the governing equation is derived via F-P-K approach in stochastic sense. Control performance and effect of 'Heo-stochastic controller for colored noise' is studied. Also stochastic feature of flutter boundary is discussed as well.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.1
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• pp.75-81
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• 2002

In the re-entry comtrol system, errors apt to induce because the time deriviative of drag acceleration is analytically estimated. Still more, the difficulty of estimation of th exact drag coefficient in hypersonic velocity and the non-reality of the scale height cause a steady-state drag errer. In the Space-Shuttle, a steady-state drag error is reduced by the addition of the integral term of drag acceleation error into the control system. This method, however, induces a difficulties in respect to the modern controller composition due to the multi-poles in a closed-loop system. Thus, this paper proposes the additional method of the disturbance observer. This reduces the steady-state drag error according to the following by the analytic calculation, and then creates the new drag acceleration time derivative using the estimated error. The performance of the re-entry control system is verified about 32 refernce trajectories.