Yong-Sik Chun

Geostationary orbit determination for time synchronization using analytical dynamic models

A real time analytical orbit determination method has been developed for precision national time synchronization. The one-way time transfer technique via a geostationary TV satellite standard time and frequency signal (STFS) dissemination system was considered. The differential method was also applied for mitigating errors in geostationary satellite STFS dissemination system. Analytical dynamic orbit determination with extended Kalman filter (EKF) was implemented to improve differential mode STFS (DSTFS) service accuracy by acquiring better accuracy of a geostationary satellite position. The perturbation force models applied for satellite dynamics include the geopotential perturbation up to fifth degree and order harmonics, luni-solar perturbations, and solar radiation pressure. All of the perturbation effects were analyzed by secular, short, and long period variations for equinoctial orbit elements such as semimajor axis, eccentricity vector, inclination vector, and mean right ascension of the geostationary satellite. The reference stations for orbit determination were composed of four calibrated stations. Simulations were performed to evaluate the performance of real time analytical orbit determination in Korea. The simulation results demonstrated that it is possible to determine real time position of geostationary satellite with the accuracy of 300 m rms. This performance implies that the time accuracy is better than 25 ns all over the Korean peninsula. The real time analytical orbit determination method developed in this research can provide a reliable, extremely high accurate time synchronization service through setting up domestic-only benchmarks.

Optical Modeling-Assisted Characterization of Polymer:Fullerene Photodiodes

In this paper, the practical viability of an organic bulk heterojunction (BHJ)based photodiode is studied, including the analysis of dark current density (J_d in A/cm²), external quantum efficiency (EQE in percent), responsivity (R in A/W), noise-equivalent power (in W/Hz^1/2), and specific detectivity (Jones in cmHz^1/2/W). The dark current was minimized down to 90 pA/cm² in P3HT : PC₆₀BM BHJ photodiodes by increasing the thickness, whereas the EQE maintained high values. The measured noise current was 2:20 χ 10^-13, 1:13 χ 10^-13, and 1:94 χ 10^-14 A/Hz^1/2 for 45-, 55-, and 65-nm BHJ photodiodes, respectively. With those values, the calculated detectivity obtained, given light of a 550 nm wavelength, was 2:55 χ 10¹¹, 5:93 χ 10¹¹, and 4:16 χ 10¹² Jones, respectively. The results demonstrated a performance of polymer:fullerence photodiode near equivalent to that of Si-based photodiodes.

RETRIEVAL OF ELECTRON DENSITY PROFILE FOR KOMPSAT-5 GPS RADIO OCCULTATION DATA PROCESSING SYSTEM

The AOPOD (Atmosphere Occultation and Precision Orbit Determination) system, the secondary payload of KOMPSAT (KOrea Multi-Purpose SATellite)-5 scheduled to be launched in 2010, shall provide GPS radio occultation data. In this paper, we simulated the GPS radio occultation characteristic of KOMPSAT-5 and retrieved electron density profiles using KROPS (KASI Radio Occultation Processing Software). The electron density retrieved from CHAMP (CHAllenging Minisatellite Payload) GPS radio occultation data on June 20, 2004 was compared with IRI (International Reference Ionosphere) - 2001, PLP (Planar Langmuir Probe), and ionosonde measurements. When the result was compared with ionosonde measurements, the discrepancies were 5 km on the peak height () and on the electron density of the peak height (). By comparing with the Laugmuir Probe measurements of CHAMP satellite (PLP), both agrees with at the height of 365.6 km.

Analysis of X-Band Link Performance Degradation Caused by Adjacent Satellite

Satellite Operation Division, Korea Aerospace Research Institute, Daejeon 305-333, KoreaAs more satellites are designed to downlink their observed image data through the X-band frequency band, it is inevitable that the occupied bandwidth of a target satellite will overlap with that of other X-band downlink satellites. For sun-syn-chronized low earth orbit satellites, in particular, it can be expected that two or more satellites be placed within the look -ing angle of a ground station antenna at the same time. Due to the overlapping in the frequency band, signals transmitted from the adjacent satellites act as interferers, leading to degraded link performance between target satellite and ground station. In this paper, link analysis was initiated by modeling the radiation pattern of ground station antenna through a validated Jet Propulsion Laboratory peak envelope model. From the relative antenna gain depending on the offset angle from center axis of maximum antenna directivity, the ratio of received interference signal level to the target signal level was calculated. As a result, it was found that the degradation increased when the offset angle was within the first null point of radiation pattern. For a 7.3 m antenna, serious link degradation began at an offset angle of 0.4 degrees. From this analysis, the link performance of the coming satellite passes can be recognized, which is helpful to establish an operat-ing procedure that will prevent the ground station from receiving corrupted image data in the event of a degraded link.

The KOMPSAT-5 Image Reception and Processing Element

KOREAs first synthetic aperture radar (SAR) satellite, KOMPSAT-5, is now ready to launch in the middle of 2011. The KOMPSAT-5 will be delivered to low Earth orbit for all-weather day-night monitoring of the Korean peninsula. The primary mission goal is to provide GOLDEN mission: Geographic information system, Ocean management, Land management, Disaster monitoring, and ENvironment monitoring. In order to fulfill the GOLDEN mission, the KOMPSAT-5 will use COSI (COrea SAR Instrument) payload to provide high-resolution mode SAR images of 1 m resolution, standard mode SAR images of 3 m resolution, and wide-swath mode SAR images of 20 m resolution, all with the viewing condition of a 45-degree incidence angle. The KOMPSAT-5 Ground Segment (KGS) constitutes three central elements—Mission Control Element, Calibration and Verification Element, and Image Reception and Processing Element (IRPE)—for controlling and operating the KOMPSAT-5 satellite; for calibrating its COSI payload; for archiving the SAR data; and for generating and distributing basic products. This paper outlines the KGS layout, describes the IRPE in more detail, and presents the overall operational workflow.

Statistical texture classifcation via histograms of wavelet filtered images

We show how the first order statistic, i.e. the histogram, of the wavelet filtered image is related to the higher order statistic of the original image. We then propose a texture classification method that uses the histogram information of the filtered images as the feature vector. The histogram based features are able to provide richer description of the texture information. Classification experiments show that the proposed method achieves significantly better classification result than the traditional Gabor wavelet filtering method that uses only the mean and standard deviation of the filtered image as feature vectors.

Radio Occultation Mission in Korea Multi-Purpose Satellite KOMPSAT-5

The first occultation mission in the Korean space program, KOMPSAT-5, is scheduled for launch in 2010. KOMPSAT-5 will have a dual frequency GPS receiver to generate precision orbit determination data and occultation data. The KOMPSAT-5 spacecraft and the characteristics of its occultation payload are described in this paper. An occultation processing system developed by KASI is also introduced.

Analysis of SAR Image Quality Degradation due to Pointing and Stability Error of Synthetic Aperture Radar Satellite

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 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.