• Bulletin of the Korean Space Science Society
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• 2005.04a
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• pp.133-137
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• 2005

This paper describes the design and performance verification of a pitch momentum bias control system being built by students at the Space System Research Laboratory(SSRL). HAUSAT-2 ADCS(Attitude Determination and Control of Subsystem) op-elation mode is divided into two parts, initial mode and on-orbit mode. This paper describes design of the HAUSAT-2 performance of attitude control results using pitch momentum bias control method in initial mode and on-orbit mode and momentum dumping method.

• Journal of Astronomy and Space Sciences
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• v.15 no.1
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• pp.183-196
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• 1998

The Synthetic Aperture Radar(SAR) image and the Digital Elevation Model(DEM) of an target area are put into use to generate three dimensional image map. An method of image map generation is explained. The orbit and attitude determination of satellite makes it possible to model signal acquisition configuration precisely, which is a key to mapping image coordinates to geographic coordinates of concerned area. An application is made to RADARSAT in the purpose of testing its validity. To determine the orbit, zero Doppler range is used. And to determine the attitude, Doppler centroid frequency, which is the frequency observed when target is put in the center of antenna's view, is used. Conventional geocoding has been performed on the basis of direct method(mapping image coordinates to geographic coordinates), but in this reserch the inverse method(mapping from geographic coordinates to image coordinates) is taken. This paper shows that precise signal acquisition modeling based on the orbit and attitude determination of satellite as a platform leads to a satellite-centered accurate geocoding process. It also shows how to model relative motion between space-borne radar and target. And the relative motion is described in ECIC(earth-centered-initial coordinates) using Doppler equation and signal acquisition geometry.

• Aerospace Engineering and Technology
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• v.13 no.2
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• pp.7-17
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• 2014

This technical paper deals with development of on-board orbit generation algorithm for GEO Satellite. This paper presents the research analysis results performed in order to improve the accuracy of the existing algorithm used for generating real-time orbit information for GEO satellite. The error impact on orbit accuracy due to the orbit error sources were analyzed with the algorithm suggested by this research. As a result of the analyses, it is found that the initial orbit should be determined with an accuracy of less than 50 m and the reference position angle error for the ground station and the satellite should be maintained within ${\pm}0.0025deg$ in order to meet the orbit accuracy specification. The development of on-board flight software based on the new algorithm was accomplished and the performance verification is ongoing by using a software based performance verification tool.

• Journal of Astronomy and Space Sciences
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• v.38 no.1
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• pp.65-82
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• 2021

For the vast majority of geostationary satellites currently in orbit, station keeping activities including orbit determination and maneuver planning and execution are ground-directed and dependent on the availability of ground-based satellite control personnel and facilities. However, a requirement linked to satellite autonomy and survivability in cases of interrupted ground support is often one of the stipulated provisions on the satellite platform design. It is especially important for a geostationary military-purposed satellite to remain within its designated orbital window, in order to provide reliable uninterrupted telecommunications services, in the absence of ground-based resources due to warfare or other disasters. In this paper we investigate factors affecting the robustness of a geostationary satellite's orbit in terms of the maximum duration the satellite's station keeping window can be maintained without ground intervention. By comparing simulations of orbit evolution, given different initial conditions and operations strategies, a variation of parameters study has been performed and we have analyzed which factors the duration is most sensitive to. This also provides valuable insights into which factors may be worth controlling by a military or civilian geostationary satellite operator. Our simulations show that the most beneficial factor for maximizing the time a satellite will remain in the station keeping window is the operational practice of pre-emptively loading East-West station keeping maneuvers for automatic execution on board the satellite should ground control capability be lost. The second most beneficial factor is using short station keeping maneuver cycle durations.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.42 no.3
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• pp.270-278
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• 2014

This paper describes an improvement of space objects orbit prediction. To screen possible collisions between operational satellites and space objects, the TLE (Two-Line Element) was used as pseudo-measurement and than the orbit determination and orbit prediction were performed through the flight dynamics system. For determining the orbits, the state vectors were assumed by a series of TLEs within a certain period. The propagation error was analyzed according to the fitting period and a number of pseudo-observations. In order to find out the improvement of orbit prediction with the proposed method, KOMPSAT-2, 3 having the precise orbit in the meter-level range were first applied. Then the result applied to space objects under the same conditions was analyzed. As a result of the RMS error comparison with the orbit prediction of space object, the precision of orbit prediction was improved by approximately 90% for seven days prediction. The improved orbit prediction of space objects can be utilized in the daily analysis for initial screening of the close space objects at high risk.

• Journal of Astronomy and Space Sciences
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• v.26 no.1
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• pp.89-98
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• 2009

In this study, the YLPODS (Yonsei Laser-ranging Precision Orbit Determination System) is developed for POD using SLR (Satellite Laser Ranging) NP (Normal Point) observations. The performance of YLPODS is tested using SLR NP observations of TOPEX/POSEIDON and CHAMP satellite. JPL's POE (Precision Orbit Ephemeris) is assumed to be true orbit, the measurement residual RMS (Root Mean Square) and the orbit accuracy (radial, along-track, cross-track) are investigated. The validation of POD using GPS (Global Positioning System) raw data is achieved by YLPODS performance and highly accurate SLR NP observations. YGPODS (Yonsei GPS-based Precision Orbit Determination System) is used for generating GPS based precise orbits for TOPEX/POSEIDON. The initial orbit for YLPODS is derived from the YGPODS results. To validate the YGPODS results the range residual of the first adjustment of YLPODS is investigated. The YLPODS results using SLR NP observations of TOPEX/POSEIDON and CHAMP satellite show that the range residual is less than 10 cm and the orbit accuracy is about 1 m level. The validation results of the YGPODS orbits using SLR NP observations of the TOPEX/POSEIDON satellite show that the range residual is less than 10 cm. This result predicts that the accuracy of this GPS based orbits is about 1m level and it is compared with JPL's POE. Thus this result presents that the YLPODS can be used for POD validation using SLR NP observations such as STSAT-2 and KOMPSAT-5.

• Proceedings of the Korean Society of Surveying, Geodesy, Photogrammetry, and Cartography Conference
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• 2007.04a
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• pp.149-152
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• 2007

The horizontal geo-location accuracy of KOMPSAT-2, without GCPs (Ground Control Points) is 80 meters CE90 for monoscopic image of up to 26 degrees off-nadir angle, after processing including POD (Precise Orbit Determination), PAD(Precise Attitude Determination) and AOCS (Attitude and Orbit Control Subsystem) sensor calibration. In case of multiple stereo images, without GCPs, the vertical geometric accuracy is less than 22.4 meters LE 90 and the horizontal geometric accuracy is less than 25.4 meters. There are two types of sensor model for KOMPSAT-2, direct sensor model and Rational Function Model (RFM). In general, a sensor model relates object coordinates to image coordinates The major objective of this investigation is to check and verify the geometrical performance when initial KOMPSAT-2 images are employed and briefly introduce the sensor model of KOMPSAT-2.

• Journal of Astronomy and Space Sciences
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• v.22 no.1
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• pp.21-34
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• 2005

The initial conditions that generate bounded motion in eccentric reference orbit are determined for satellite formation flying. Because Hill's equations cannot describe the relative motion between two satellites in eccentric orbit, a new relative dynamics utilizing the nonlinearity and eccentricity correction for Hill's initial conditions is implemented. The constraint that matches angular rates of chief and deputy satellites is used to obtain the bounded motion between them. The constraint can be applied to satellite formation motions in eccentric orbit, since it implicates J2 perturbation due to the central body's aspherical gravitational forces. The periodic bounded motions are analyzed for the orbit with the eccentricity of less than 0.05 and about 0.5 km relative distance between chief and deputy satellites. It is mainly illustrated that the satellite formations in small eccentric orbits can have hounded motions; consequently, the formation can be kept by matching angular rates of the satellites. These results demonstrate an useful method that reduces the cost for operating satellites by providing effective initial conditions for satellite formation flying in eccentric orbit.

• Journal of Astronomy and Space Sciences
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• v.32 no.4
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• pp.379-386
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• 2015

This work presents fuel-optimal altitude maintenance of Low-Earth-Orbit (LEO) spacecrafts experiencing non-negligible air drag and J2 perturbation. A pseudospectral (direct) method is first applied to roughly estimate an optimal fuel consumption strategy, which is employed as an initial guess to precisely determine itself. Based on the physical specifications of KOrea Multi-Purpose SATellite-2 (KOMPSAT-2), a Korean artificial satellite, numerical simulations show that a satellite ascends with full thrust at the early stage of the maneuver period and then descends with null thrust. While the thrust profile is presumably bang-off, it is difficult to precisely determine the switching time by using a pseudospectral method only. This is expected, since the optimal switching epoch does not coincide with one of the collocation points prescribed by the pseudospectral method, in general. As an attempt to precisely determine the switching time and the associated optimal thrust history, a shooting (indirect) method is then employed with the initial guess being obtained through the pseudospectral method. This hybrid process allows the determination of the optimal fuel consumption for LEO spacecrafts and their thrust profiles efficiently and precisely.

• Journal of Astronomy and Space Sciences
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• v.14 no.2
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• pp.375-380
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• 1997

The accuracy of user position by GPS is heavily dependent upon the accuracy of satellite position which is usually transmitted to GPS users in radio signals. The real-time satellite position information directly obtained from broadcast ephimerides has the accuracy of 3~10 meters which is very unsatisfactory to measure 100km baseline to the accuracy of less than a few mili-meters. There are globally at present seven orbit analysis centers capable of generating precise GPS ephimerides and their orbit quality is of the order of about 10cm. Therefore, precise orbit model and phase processing technique were reviewed and consequently precise GPS ephimerides were produced after processing the phase observables of 28 global GPS stations for 1 day. Initial 6 orbit parameters and 2 solar radiation coefficients were estimated using batch least square algorithm and the final results were compared with the orbit of IGS, the International GPS Service for Goedynamics.