Time filter

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Daejeon, South Korea

Lee B.-S.,Electronics and Telecommunications Research Institute | Hwang Y.,Electronics and Telecommunications Research Institute | Lee U.-S.,Satrec Initiative | Kim B.-Y.,Korea Aerospace Research Institute
Proceedings of the International Astronautical Congress, IAC | Year: 2013

Conjunction assessment and collision avoidance of the Communications Ocean and Meteorological Satellite (COMS) in Geostationary Earth Orbit (GEO) were investigated. An abandoned inclined geosynchronous orbit satellite was approached to COMS. Backward and forward orbit propagation based on the Conjunction Summary Message (CSM) from Joint Space Operations Center (JSpOC) was carried out. COMS satellite orbit change due to the station-keeping maneuver and momentum dumping operation was considered to generate Owner/Operator ephemeris. Differences of orbital position in radial, along-track, and cross-track were analyzed for conjunction assessment. In this case, regular East-West station-keeping maneuver and North-South station-keeping maneuver act like collision avoidance maneuvers for along-track and cross-track separation. Source

Choi E.-J.,Satrec Initiative | Choi E.-J.,Yonsei University | Yoon J.-C.,Korea Aerospace Research Institute | Lee B.-S.,Electronics and Telecommunications Research Institute | And 2 more authors.
Advances in Space Research | Year: 2010

Spaceborne GPS receivers are used for real-time navigation by most low Earth orbit (LEO) satellites. In general, the position and velocity accuracy of GPS navigation solutions without a dynamic filter are 25 m (1σ) and 0.5 m/s (1σ), respectively. However, GPS navigation solutions, which consist of position, velocity, and GPS receiver clock bias, have many abnormal excursions from the normal error range for space operation. These excursions lessen the accuracy of attitude control and onboard time synchronization. In this research, a new onboard orbit determination algorithm designed with the unscented Kalman filter (UKF) was developed to improve the performance. Because the UKF is able to obtain the posterior mean and covariance accurately by using the second-order Taylor series expansion through the sampled sigma points that are propagated by using the true nonlinear system, its performance can be better than that of the extended Kalman filter (EKF), which uses the linearized state transition matrix to predict the covariance. The dynamic models for orbit propagation applied perturbations due to the 40 × 40 geo-potential, the gravity of the Sun and Moon, solar radiation pressure, and atmospheric drag. The 7(8)th-order Runge-Kutta numerical integration was applied for orbit propagation. Two types of observations, navigation solutions and C/A code pseudorange, can be used at the user's discretion. The performances of the onboard orbit determination were verified using real GPS data of the CHAMP and KOMPSAT-2 satellites. The results of the orbit determination were compared with the precision orbit ephemeris (POE) of the CHAMP and KOMPSAT-2 satellites. The comparison of the orbit determination results using EKF and UKF shows that orbit determination using the UKF yields better results than that using the EKF. In addition, the estimation of the accuracy using the C/A code pseudorange is better than that using the navigation solutions. The absolute position and velocity accuracies of the UKF using GPS C/A code pseudorange were 12.098 m and 0.0159 m/s in the case of the CHAMP satellite, and 8.172 m and 0.0085 m/s in the case of the KOMPSAT-2 satellite. Moreover, the abnormal excursions of navigation solutions can be eliminated. These results verify that onboard orbit determination using GPS C/A code pseudorange, which is based on the UKF can provide more stable and accurate orbit information in the spaceborne GPS receiver. © 2010 Elsevier Ltd. All rights reserved. Source

Lim Y.-M.,Korea Advanced Institute of Science and Technology | Choi Y.-J.,Korea Advanced Institute of Science and Technology | Jo Y.-S.,Korea Advanced Institute of Science and Technology | Lim T.-H.,Korea Advanced Institute of Science and Technology | And 3 more authors.
Journal of the Korean Physical Society | Year: 2013

A prototype hyper-spectral imager in the visible spectral band was developed for the planned Korean lunar missions in the 2020s. The instrument is based on simple refractive optics that adopted a linear variable filter and an interline charge-coupled device. This prototype imager is capable of mapping the lunar surface at wavelengths ranging from 450 to 900 nm with a spectral resolution of ∼8 nm and selectable channels ranging from 5 to 252. The anticipated spatial resolution is 17.2 m from an altitude of 100 km with a swath width of 21 km. © 2013 The Korean Physical Society. Source

Cho D.-H.,KAIST | Chung Y.,Satrec Initiative | Bang H.,KAIST
Acta Astronautica | Year: 2012

A spacecraft for interplanetary mission is usually perturbed by some disturbance sources. The trajectory correction maneuver (TCM) is required to adjust this trajectory error, and the B-plane targeting method is widely used in this field. However, this B-plane targeting method is based on the differential correction algorithm, and a numerical Jacobian matrix is usually used for this algorithm. Therefore, our main goal in this paper is to suggest the improved B-plane targeting method to overcome the disadvantages of the conventional B-plane method which requires a numerical Jacobian matrix for the initial perturbation selection and iterations. For this improvement, an analytical Jacobian matrix is introduced instead of the numerical Jacobian matrix. Then, another B-plane approach that offers an analytical solution is suggested using the target eccentricity instead of the target time of closest approach (TCA). Using a modified Keplers equation, the previous B-plane targeting approach can be replaced with the new method through the analytical solution. Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved. Source

Yoon H.,Satrec Initiative | Yoon H.,Korea Advanced Institute of Science and Technology | Lim Y.,Korea Advanced Institute of Science and Technology | Bang H.,Korea Advanced Institute of Science and Technology
Journal of Spacecraft and Rockets | Year: 2011

A new star-pattern identification algorithm using correlation pattern-matching is proposed in this study. The new approach is based upon maximizing the target cost function, which is formed by the correlation between an original image and a target image. The image is reconstructed from the centroid positions of stars that are modeled as twodimensional Gaussian functions. The correlation function in the form of cross-convolution in the image plane can be expressed by Fourier transform, so itisconstructed analytically using only the centroid positionsofstarsinthe image plane. The proposed algorithm compared with conventional pattern-matching techniques is simpler and more reliable, as verified by simulation study. Source

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