• Journal of Astronomy and Space Sciences
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• 제36권4호
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• pp.293-306
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• 2019

In this study, the observational arc-length effect on orbit determination (OD) for the Korea Pathfinder Lunar Orbiter (KPLO) in the Earth-Moon Transfer phase was investigated. For the OD, we employed a sequential estimation using the extended Kalman filter and a fixed-point smoother. The mission periods, comprised between the perigee maneuvers (PM) and the lunar orbit insertion (LOI) maneuver in a 3.5 phasing loop of the KPLO, was the primary target. The total period was divided into three phases: launch-PM1, PM1-PM3, and PM3-LOI. The Doppler and range data obtained from three tracking stations [included in the deep space network (DSN) and Korea Deep Space Antenna (KDSA)] were utilized for the OD. Six arc-length cases (24 hrs, 48 hrs, 60 hrs, 3 days, 4 days, and 5 days) were considered for the arc-length effect investigation. In order to evaluate the OD accuracy, we analyzed the position uncertainties, the precision of orbit overlaps, and the position differences between true and estimated trajectories. The maximum performance of 3-day OD approach was observed in the case of stable flight dynamics operations and robust navigation capability. This study provides a guideline for the flight dynamics operations of the KPLO in the trans-lunar phase.

• 천문학회보
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• 제40권2호
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• pp.40.3-41
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• 2015

We performed simultaneous observations of H2O and SiO masers toward VX Sgr using the Korean VLBI Network (KVN) and Source Frequency Phase Referencing (SFPR) method. The observations were carried out at 5 epochs from 2014 February to 2015 June. The relative locations of the SiO with respect to the H2O maser emission were determined at two epochs by SFPR for the first time. The H2O masers show well developed asymmetric outflow features which are spread up to ~300 mas in diameter. On the other hand, the SiO masers show a ring-like structure close to the central star with ~ 30 mas diameter. The SFPR observational results at two epochs (${\varphi}=0.83$ and 0.99) provide similar relative locations of H2O and SiO maser features. These superposed maps of H2O and SiO masers lead us to investigate the development of outflow motions from relatively spherical SiO maser regions close to central star to aspherical H2O maser regions according to optical phase of stellar pulsation together with the prediction of the position of central star.

• 천문학회보
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• 제38권2호
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• pp.70.2-70.2
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• 2013

We present collections of optical photometry for a pre-main sequence star HBC 722. It showed large amplitude optical outburst (${\Delta}V=4.7$ mag) in 2010 and classified as a FU Orionis type object. We have been observing HBC 722 from 2011 April to 2013 May, using Camera for QUasars in EArly uNiverse (CQUEAN) attached to the 2.1 m Otto Struve telescope at the McDonald Observatory. Time-series monitoring data (minute-scale interval) were obtained in SDSS r, i and z bands to see short-scale behaviors as well as trace the long-term brightness changes after the eruption in 2010. Interestingly, it started to brighten from 2011 early summer and became brighter than the first outburst peak in our 2013 May observation. We expect that the recovering phase would result from re-increase of disk accretion rate, might attribute to distinctive short-scale color features. In this presentation, we report long- and short-timescale optical behaviors of HBC 722 in the post-outburst phase.

• Journal of Astronomy and Space Sciences
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• 제33권1호
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• pp.45-54
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• 2016

This study developed an approach for improving Carrier-phase Differential Global Positioning System (CDGPS) based realtime satellite relative navigation by applying laser baseline measurement data. The robustness against the space operational environment was considered, and a Synthetic Wavelength Interferometer (SWI) algorithm based on a femtosecond laser measurement model was developed. The phase differences between two laser wavelengths were combined to measure precise distance. Generated laser data were used to improve estimation accuracy for the float ambiguity of CDGPS data. Relative navigation simulations in real-time were performed using the extended Kalman filter algorithm. The GPS and laser-combined relative navigation accuracy was compared with GPS-only relative navigation solutions to determine the impact of laser data on relative navigation. In numerical simulations, the success rate of integer ambiguity resolution increased when laser data was added to GPS data. The relative navigational errors also improved five-fold and two-fold, relative to the GPS-only error, for 250 m and 5 km initial relative distances, respectively. The methodology developed in this study is suitable for application to future satellite formation-flying missions.

• 전력전자학회논문지
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• 제9권4호
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• pp.303-310
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• 2004

본 연구에서는 순시전압 sag 및 고조파 전류 보상을 위한 공간벡터 검출법을 기반으로 한 3상 하이브리드 직렬형 능동전력필터 시스템을 제안하고 있다. 순시전안 sag 및 고조파 전류를 검출하기 위한 공간벡터 검출법은 종전의 이론에 비해 곱셈 연산 과정을 감소할 수 있고 좌표 변환이 필요치 않기 때문에 간략화한 보상 알고리즘 구현이 가능하다. 본 연구의 타당성은 전력전자전용 시뮬레이터 PSIM에 의해 정상상태와 과도상태에서 입증하였다. 그 결과 3상 교류 전원 모두에 순간적인 전압 sag가 발생되거나, 임의의 상에 왜형 및 sag가 있는 경우, 전압 보상 및 고조파 전류 보상이 모두 가능함을 입증하였다.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2003년도 한국우주과학회보 제12권2호
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• pp.48-48
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• 2003

A precision orbit determination (POD) system of low Earth orbiter using the GPS dual frequency measurements has been developed. It is an option of KOMPSAT-2 POD process system. In this research, the orbit determination using the real dual frequency carrier phase measurements of the SAC-C satellite was conducted to verify KOMPSAT-2 POD system reliability. The SAC-C satellite is an international cooperative mission between NASA, the Argentine Commission on Space Activities (CONAE), Centre National d'Etudes Spatiales (CNES or the French Space Agency), Instituto Nacional De Pesquisas Espaciais (Brazilian Space Agency), Danish Space Research Institute, and Agenzia Spaziale Italiana (Italian Space Agency). The SAC-C was launched at November 21, 2000. The altitude of SAC-C is 702 km and it carries a TurboRogue III GPS and four high gain antennas developed by the JPL. The receiver is able to generate the dual frequency code and carrier phase data. Double-differenced carrier phase measurements were formed using 25 IGS stations. The data were sampled at 30 seconds interval. Fully dynamic approach was adopted for POD. The POD results were compared with those of JPL using GOA n software. The comparison verifies that deci-meter level 3D position accuracy of low Earth orbiting satellite could be achieved. The POD system has been developed successfully.

• Journal of Power Electronics
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• 제5권1호
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• pp.29-35
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• 2005

This paper presents a novel prototype of an active auxiliary quasi-resonant snubber(Auxiliary Quasi-Resonant Commutation Block-Link; ARCB)-assisted three phase voltage source soft switching space voltage vector modulated PFC rectifier, which uses Zero Voltage Soft Switching (ZVS) commutation. The operating principles of this digitally-controlled three phase soft switching PWM-PFC rectifier system with an instantaneous power feedback scheme are illustrated and its steady-state performance is evaluated using computer-aided simulation analysis.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1996년도 하계학술대회 논문집 A
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• pp.333-335
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• 1996

A method for detecting the three-phase currents of a voltage-fed pulsewidth modulated(PWM) inverter is proposed by utilizing only one current sensor placed on the dc-link. The proposed space vector PWM technique is two phase modulated PWM, this enables to detect the phase currents from only one DC link current sensor. The proposed method is simple, reduces the cost, and provides the small detection errors.

• Journal of Astronomy and Space Sciences
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• 제37권3호
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• pp.187-197
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• 2020

In order to avoid the high cost and high risk of demonstration mission of rendezvous-docking technology, missions using nanosatellites have recently been increasing. However, there are few successful mission cases due to many limitations of nanosatellites like small size, power limitation, and limited performances of sensor, thruster, and controller. To improve the probability of rendezvous-docking mission success using nanosatellite, a rendezvous-docking phase analysis tool for nanosatellites is developed. The tool serves to analyze the relative position and attitude control of the chaser satellite at the docking phase. In this tool, the Model Predictive Controller (MPC) is implemented as a controller, and Extended Kalman Filter (EKF) is adopted as a filter for noise filtering. To verify the performance and effectiveness of the developed tool for nanosatellites, simulation study was conducted. Consequently, we confirmed that this tool can be used for the analysis of relative position and attitude control for nanosatellites in the rendezvous-docking phase.

• 한국세라믹학회지
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• 제33권3호
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• pp.362-370
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• 1996

Kalsilite (KAlSiO4) system undergoes a displacive phase transition from hexagonal phase with p63 space group to the phase with P63mc at 886$^{\circ}C$. The flux composition having kalsilite :K2O:B2O3=1:2:2 has enabled the growth of hexagonal kalsilite with the size of 0.5~1 mm at a slow cooling rate (0.3$^{\circ}C$/hr). On decreasing the cooling rate the size has increased and pyramidal (1011) faces are newly developed with the shape of (0001) and (1010) faces. Upon stirring (1011) faces are degraded. The space group of O1 and O2 are P21221 and C2221 respectively. Their orthorombic modification O1 and O2 are synthesized at relatively low and high temperature respectively.