• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.36 no.12
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• pp.1216-1221
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• 2008

A low earth orbit satellite with a fixed solar array always has a sun-pointing attitude during daylight, and changes into a nadir-pointing attitude for a imaging mission. Since external heating sources to the satellite panels are Earth　irradiation and Albedo during most of daylight in a sun-pointing attitude, the thermal environment condition is relatively stable. However, direct sunlight which is the greatest environmental heating has an affect on the satellite panels during a mission period (10% of one orbit) in a nadir-pointing attitude. In satellite thermal design, thermal effects of a nadir-pointing mission attitude due to this thermal environment change need to be evaluated although the duration of a nadir-pointing attitude is short. Therefore, a nadir-pointing attitude during a mission is incorporated into thermal model and by the thermal analysis result, thermal effects on the satellite are investigated.

• International Journal of Aeronautical and Space Sciences
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• v.8 no.1
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• pp.129-139
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• 2007

In this paper, attitude control laws for simultaneous pointing of multiple spacecrafts are considered under a formation flying scenario. The basic approach lies in adaptive feedback gains using relative attitude information or maneuver time approximation for coordinated attitude control. Each control law is targeted to balancing mean motion or to correcting system response to the slowest satellite. The control gain adaptation is constructed by two approaches. The first one is using variable damping gain to manipulate speed of a fast system response, and the second one uses alternate natural frequency of the system under control. The validity and stability of the proposed approaches are examined analytically and tested through numerical simulations.

• International Journal of Aeronautical and Space Sciences
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• v.4 no.1
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• pp.75-87
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• 2003

The primary objective of this study is to demonstrate ground-based experiment for the attitude control of spacecraft. A two-axis rotational simulator with a flexible ann is constructed with on-off air thrusters as actuators. The simulator is also equipped with payload pointing capability by simultaneous thruster and DC servo motor actuation. The azimuth angle is controlled by on-off thruster command while the payload elevation angle is controlled by a servo-motor. A thruster modulation technique PWM(Pulse Width Modulation) employing a time-optimal switching function plus integral error control is proposed. An optical camera is used for the purpose of pointing as well as on-board rate sensor calibration. Attitude control performance based upon the new closed-loop control law is demonstrated by ground experiment. The modified switching function turns out to be effective with improved pointing performance under external disturbance. The rate sensor calibration technique by Kalman Filter algorithm led to reduction of attitude error caused by the bias in the rate sensor output.

• International Journal of Aeronautical and Space Sciences
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• v.9 no.1
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• pp.147-161
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• 2008

In this paper, fine attitude estimation using low-cost sensors for attitude pointing missions of spacecraft is addressed. Attitude kinematics and gyro models including bias models are in general utilized to estimate spacecraft attitude and angular rate. However, a linearized model and a transition matrix are derived in this paper from nonlinear spacecraft dynamics with external disturbances. A Kalman filtering technique is applied and offers relatively high estimation accuracy under dynamic uncertainties. The proposed approach is demonstrated using numerical simulations.

• Aerospace Engineering and Technology
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• v.1 no.1
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• pp.65-71
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• 2002

KOMPSAT-1 AOCS mode divided into three major mode like Sun, Maneuver, Science Mode. The Maneuver mode consist of attitude hold and Δ-V Burn submode. This paper focus on the analysis of AOCS Maneuver Mode characteristics based on on-orbit playback data. The nadir pointing performance of attitude hold submode and the process for Δ-V Burn firing with plus/ minus 90 degree pitch/ roll maneuvering was verified. And also verified that the on-orbit performance meets the AOCS subsystem specification as designed.

• Journal of Astronomy and Space Sciences
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• v.25 no.4
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• pp.445-458
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• 2008

Image chain analysis of synthetic aperture radar (SAR) satellite is one of the primary activities for satellite design because SAR image quality depends on spacecraft bus performance as well as SAR payload. Especially, satellite pointing and stability error make worst effect on the original SAR image quality which is implemented by SAR payload design. In this research, Image chain analysis S/W was developed in order to analyze the SAR image quality degradation due to satellite pointing and stability error. This S/W consists of orbit model, attitude control model, SAR payload model, clutter model, and SAR processor. SAR raw data, which includes total 25 point targets in the scene of $5km{\times}5km$ swath width, was generated and then processed for analysis. High resolution mode (spotlight), of which resolution is 1m, was applied. The results of image chain analysis show that radiometric accuracy is the most degraded due to the pointing error. Therefore, the successful design of attitude control subsystem in spacecraft bus for enhancing the pointing accuracy is most important for image quality.

• Journal of Aerospace System Engineering
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• v.16 no.5
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• pp.49-56
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• 2022

S-STEP is a small SAR satellite mission that monitors time-limited emergency targets and military anomalies in areas of interest, achieving the average revisit in less than 30 minutes by deploying a constellation of 32 satellites in low orbit at an altitude of 510 km. The mission operation mode of S-STEP is divided into normal mode, observation mode, communication mode, and orbit maintenance mode. Further,, the attitude control mode is subdivides into initial detumbling, sun pointing, target pointing, ground station pointing, and thrust direction maintenance. Based on the preliminary mission operational scenario and the satellite's characteristics, this study analyzed the attitude control performance during initial detumbling and observation modes. It verifies that each mode's attitude control accuracy requirements within the time allotted by the scenario of the S-STEP achieved.

• Journal of the Korea Institute of Military Science and Technology
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• v.18 no.5
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• pp.498-509
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• 2015

The existing mobile antenna networks in the military use have been operated by the manual pointing between two antennas. The work presented here describes the study of ATPC(Automatic Tracking and Pointing Control) system between facing antennas and the related tracking and pointing performances. This system is able to automatically track the maximum RSSI(Received Signal Strength Indication) value from the source's RF(Radio Frequency) signal and then control for maintaining the LOS(Line of Sight) between two antennas. The system has three major units; the driving unit consisting of motors, harmonic drives and encoders, the sensor unit with a GPS(Global Positioning System) and AHRS(Attitude and Heading Reference System) and the control unit regulating all the tracking and pointing events. By using PI(Proportional and Integral) controller, this system is able to properly track and point the other antenna under the external disturbance like the wind load. Both the simulation and the experimental works have been successively carried out to prove the performances of the system.

• 제어로봇시스템학회:학술대회논문집
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• 1997.10a
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• pp.692-695
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• 1997

Nonlinear robust attitude controller for 3-axis stabilized spacecraft is designed. Robust stability analysis for nonlinear spacecraft system with disturbance is conducted. External disturbances and parametric uncertainties decrease Spacecraft's attitude pointing accuracy. Sliding Mode Control(SMC) provides stability of system in the face of these disturbances and uncertainties. The concept of quadratic boundedness and quadratic stability are applied to the robust analysis for the nonlinear spacecraft system subject to bounded disturbance torques. Numerical simulation is conducted to compare the analysis result and actual nonlinear simulation. The simulation show that analysis result is valid.

• 제어로봇시스템학회:학술대회논문집
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• 1996.10b
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• pp.1103-1106
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• 1996

Disturbance Accommodating Attitude control logic for 3-axis stabilized spacecraft is designed and compared with PIID control logic. PID controller provide the zero steady error for constant disturbances. PIID controller detect and cancel disturbances upto the ramp input. PID control logic is designed as the main control logic. We designed the disturbance observer to detect the effect of disturbance using the sinusoidal function. The detected disturbance are compensated by the additional control logic. The comparison simulation is conducted between PIID and PIID with DAC. The simulation results show that PIID with DAC shows the better attitude pointing accuracy.