Satrec Initiative

Daejeon, South Korea

Satrec Initiative

Daejeon, South Korea
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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.

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.

Lee B.-S.,Electronics and Telecommunications Research Institute | Hwang Y.,Electronics and Telecommunications Research Institute | Kim H.-Y.,Yonsei University | Park S.,Satrec Initiative
Advances in Space Research | Year: 2011

The first Korean multi-mission geostationary satellite, Communication, Ocean, and Meteorological Satellite (COMS) will be launched in 2010. The missions of this satellite will be Ka-band communications, ocean color monitoring, and meteorological imaging. The satellite was designed with only one solar array on the south panel. This novel configuration will keep imaging instruments on the north side from heating up. Asymmetry of the spacecraft configuration requires twice-a-day thruster-based Wheel Off-Loading (WOL) operations to keep the satellite attitude for imaging and communication. Thruster firings during the WOL operations cause the satellite orbit to change two times a day. Weekly East-West Station-Keeping (EWSK) and North-South Station-Keeping (NSSK) maneuver operations are planned for the COMS satellite in order to maintain the satellite in ±0.05° box at 128.2°E longitude. The EWSK maneuver is planned to be performed two days after the NSSK maneuver to correct small side effects in East-West direction due to plume impingement against the solar array during NSSK maneuver. Normally thruster firings during the WOL operation affect the satellite orbit only in the North-South direction, but there are also small perturbations in the East-West direction. In this paper, EWSK maneuver strategy for the COMS satellite is presented. This strategy takes twice-a-day thruster-based WOL operations and plume impingement effect of the NSSK maneuver into account. And an iterative process to calculate the velocity changes and maneuver time for the EWSK maneuver is applied. One year of the EWSK and NSSK maneuver simulations are performed with twice-a-day WOL to verify the proposed strategy. The proposed strategy showed that the mean longitude of the satellite is controlled at 128.2° ± 0.007° which is three times tighter than that of non-iterative process. Required Delta velocity (ΔV) for EWSK maneuver is also slightly smaller for iterative process. Tightly controlled mean longitude guarantees a better quality of satellite communications and Earth imaging when the spacecraft attitude is well maintained by on-board control. © 2010 COSPAR. Published by Elsevier Ltd. All rights reserved.

Lee S.,Satrec Initiative | Shin D.,Satrec Initiative
Remote Sensing | Year: 2015

Despite the efforts for precise alignment of imaging sensors and attitude sensors before launch, the accuracy of pre-launch alignment is limited. The misalignment between attitude frame and camera frame is especially important as it is related to the localization error of the spacecraft, which is one of the essential factors of satellite image quality. In this paper, a framework for camera misalignment estimation is presented with its application to a high-resolution earth-observation satellite-Deimos-2. The framework intends to provide a solution for estimation and correction of the camera misalignment of a spacecraft, covering image acquisition planning to mathematical solution of camera misalignment. Considerations for effective image acquisition planning to obtain reliable results are discussed, followed by a detailed description on a practical method for extracting many GCPs automatically using reference ortho-photos. Patterns of localization errors that commonly occur due to the camera misalignment are also investigated. A mathematical model for camera misalignment estimation is described comprehensively. The results of simulation experiments showing the validity and accuracy of the misalignment estimation model are provided. The proposed framework was applied to Deimos-2. The real-world data and results from Deimos-2 are presented. © 2015 by the authors.

Kim H.,Satrec Initiative | Kim M.-G.,Satrec Initiative
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences - ISPRS Archives | Year: 2012

Multiple CCDs are used in space-borne camera system to have multiple bands and/or to have wider swath even in one band. Due to design constraints, the multiple CCDs may be placed at different position on the effective focal plane. In such case, each band or each CCD in a band has different look angle, and hence suffers so-called "parallax effect" when registration between images from different CCD is required. Since the parallax effect is a function of height of the target when the baseline is fixed, the displacement of each target for the registration differs from target to target depending on the height of each target in the images. Hence, the registration between images from different CCDs cannot be achieved using simple affine transform, and rather requires higher order matching methods, such as local warping. In this paper, we suggest the band registration method which includes the compensation of terrain relief by sensor model with DEM data. Since the parallax effect is compensated in this approach, the simpler matching strategy can be adopted for the robustness of algorithm. In the proposed approach, the slave image is resampled to virtual CCD geometry, which has same geometry with master image. In order to realize such approach, accurate camera model for each band and DEM data were used. The difference of proposed approach with conventional ortho-rectification is that the geometry of master image is kept. The experiment results demonstrated that proposed method can effectively correct the displacement caused by parallax effect.

Han J.,Satrec Initiative | Jeon Y.,Satrec Initiative
14th International Conference on Space Operations, 2016 | Year: 2016

At the present day various satellites are observing the Earth by taking images. There are many agencies to provide the image product of each satellite to the customer. If the customer requires the image product, the customer should write the order form of a satellite and send it to the agency. After number of days the customer will take the image product from the agency. In this case the customer should write several order forms to get the image product of different satellites for the same area. The reason is there is no standard for the order form and the image product. ESA proposed the Copernicus Service to solve these kinds of problems. The Copernicus Service has been operating by ESA until now. The Copernicus Service has the standard for the order form and the image product. So, the customer can get the image product of various satellites more easily. Copernicus Contribution Mission (CCM) is an entity of the Copernicus Service. The CCM is a provider of an image product. CCM should satisfy Top Level ICD and Operational scenarios of the Copernicus Service in order to provide an image product. Top Level ICD and Operational scenarios are composed of several sub documents. In this paper, we will describe the development procedure and result of CDWI (Copernicus Data Warehouse Interface) S/W to be a CCM which provides the image products of KOMPSAT-2, 3 and 5 satellites. We expect the analysis steps, which are mentioned in the development procedure, can be the technical assistance for the countries or the companies which want to be a CCM. © 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.

Lee T.,Satrec Initiative | Kwak S.,Satrec Initiative
14th International Conference on Space Operations, 2016 | Year: 2016

The conceptual software design described in this paper establishes a quality control framework for geostationary earth observing satellite’s image production system which requires high availability and reliability. Its main considerations are (1) dynamic quality modification, (2) large quality data manipulation including multiple dimensional analysis and (3) quality simulation. For resolving these consideration, we suggest applying following technologies to the design of quality control system: (a) event driven architecture based on functional language, (b) distributed and parallel data processing framework, (c) simulation platform using container or virtualization. © 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.

Park G.,Satrec Initiative | Shin D.,Satrec Initiative
14th International Conference on Space Operations, 2016 | Year: 2016

This paper introduces an at-sensor radiance estimation model which is quite practical for satellite imaging planning. In order to optimize the imaging planning parameters which determine the dynamic range of images responding to expected ground reflectance (e.g. TDI level, video signal processor gain, CCD read-out line rate of a space-borne electro-optical sensor), we need to predict the energy which arrives at the satellite. The proposed at-sensor radiance estimation model calculates the amount of the spectral radiance for a given ground reflectance, solar zenith angle, satellite viewing angle, and atmospheric conditions. Among the full six energy sources defined in general VIS/NIR radiative transfer modeling (i.e. ground-emitted radiance, thermal path radiance, solar scattered path radiance, directly reflected radiance, thermal diffuse radiance, and solar scattered diffuse radiance), only two dominant energy sources (i.e. solar scattered path radiance and directly reflected radiance) are applied for calculating the at-sensor spectral radiance in the proposed model. In addition, the proposed model derives the spectral radiance by applying actual solar and viewing zenith angles to zero-angle (i.e. normal and nadir) atmospheric spectral transmission database, which can be obtained from any full atmospheric modeling tools. The proposed at-sensor spectral radiance estimation model showed less than 10% difference from MODTRAN-5 in most practical imaging conditions except some extreme cases such as very large solar zenith angles or satellite viewing zenith angles. Considering the uncertainty of atmospheric parameters and ground reflectance as well as the resolutions of the image dynamic range control parameters, the application of the proposed model to satellite imaging planning is regarded as an efficient solution for reducing the complexity of the imaging planning software in an operational ground system. The proposed model was integrated in the imaging planning software in DubaiSat-2 ground system, so that the operational usefulness of the proposed model has been validated successfully. © 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.

Park J.S.,Satrec Initiative
14th International Conference on Space Operations, 2016 | Year: 2016

In this paper, a mission planning logic to solve the problems in urgent condition is introduced. There are some constraints considered for the urgent condition. First, there is only one ground station. Next, the urgent period is defined as the period from the end time of first communication (contact) between satellite and a ground station after the current time to end time of second contact. Last, the new mission should be planned before first contact. Since the telecommand for new mission has to be uploaded during first contact. The mission planning logic before uploading the scenario and the post-processing after uploading the scenario are described in the paper. The urgent condition is to plan a new mission that conflicts with already uploaded mission in urgent period. The conflict between the missions means that the operation time of mission doesnt overlap each other. At this time, the uploaded planning has to be canceled to register the new mission planning. Therefore, mission planning system (MPS) assumes that it is normal to cancel the uploaded planning when the new mission planning is generated. Also, MPS saves the information of canceled missions since it can be restored as the failure of cancellation command. And then, ground station system requests telecommands to cancel and upload the mission planning to the satellite. The post-processing algorithm of the scenario uploading, which depends on the result of the scenario uploading, is designed to handle the anomaly condition as the failure of the scenario uploading partly or all. © 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.

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.

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