Pil-Ho Park

Seasonal variability of GPS-derived zenith tropospheric delay (1994-2006) and climate implications

[1] The total zenith tropospheric delay (ZTD) is an important parameter of the atmosphere and directly or indirectly reflects the weather and climate processes and variations. In this paper the ZTD time series with a 2-hour resolution are derived from globally distributed 150 International GPS Service (IGS) stations ( 1994 - 2006), which are used to investigate the secular trend and seasonal variation of ZTD as well as its implications in climate. The mean secular ZTD variation trend is about 1.5 +/- 0.001 mm/yr at all IGS stations. The secular variations are systematically increasing in most parts of the Northern Hemisphere and decreasing in most parts of the Southern Hemisphere. Furthermore, the ZTD trends are almost symmetrically decreasing with increasing altitude, while the summation of upward and downward trends at globally distributed GPS sites is almost zero, possibly reflecting that the secular ZTD variation is in balance at a global scale. Significant annual variations of ZTD are found over all GPS stations with the amplitude from 25 to 75 mm. The annual variation amplitudes of ZTD near oceanic coasts are generally larger than in the continental inland. Larger amplitudes of annual ZTD variation are mostly found at middle latitudes (near 20 degrees S and 40 degrees N) and smaller amplitudes of annual ZTD variation are located at higher latitudes ( e. g., Antarctic) and the equator areas. The phase of annual ZTD variation is about 60 degrees in the Southern Hemisphere ( about February, summer) and about 240 degrees in the Northern Hemisphere ( about August, summer). The mean amplitude of semiannual ZTD variations is about 10 mm, much smaller than annual variations. The semiannual amplitudes are larger in the Northern Hemisphere than in the Southern Hemisphere, indicating that the semiannual variation amplitudes of ZTD in the Southern Hemisphere are not significant. In addition, the higher-frequency variability (RMS of ZTD residuals) ranges from 15 to 65 mm of delay, depending on altitude of the station. Inland stations tend to have lower variability and sites at ocean and coasts have higher variability. These seasonal ZTD cycles are due mainly to the wet component variations (ZWD).

An improvement of GPS height estimations: stochastic modeling

The results of GPS positioning depend on both functional and stochastic models. In most of the current GPS processing programs, however, the stochastic model that describes the statistical properties of GPS observations is usually assumed that all GPS measurements have the same accuracy and are statistically independent. Such assumptions are unrealistic. Although there were only a few studies modeling the effects on the GPS relative positioning, they are restricted to short baselines and short session lengths. In this paper, the stochastic modeling for IGS long-baseline positioning (with 24-hour session) is analyzed in the GAMIT software by modified stochastic models. Results show that any mis-specifications of stochastic model result in unreliable GPS baseline results, and the deviation of baseline estimations reaches as much as 2 cm in the height component. Using the stochastic model of satellite elevation angle-based cosine function, the precision of GPS baseline estimations can be improved, and the GPS baseline component is closest to the reference values, especially GPS height.

Strain accumulation in South Korea inferred from GPS measurements

Monitoring the variation of the crustal strain is a key issue to understand the physical process of crustal tectonic activities. In this paper, GPS data for the period from March 2000 to February 2004 were analyzed to quantitatively investigate the plate deformation patterns and distributions in the South Korean peninsula. The results show two anomalous rates of strain accumulation in South Korea, a W-E compression accumulation of crustal strain in the East and West parts, and a N-S extension strain accumulation in the middle part along the longitude of about 127.5°E. In addition, the GPS-derived seismic moment accumulation rate is significant and consistent with recent historic earthquakes and fault zones in South Korea. The most anomalous seismic moment rates are in the middle part (about 127.3°E, 35.5°N), North edge (about 128.0°E, 38.0°N) and Northeast part (about 128.5°E, 37.3°N) of South Korea, indicating a high earthquake risk.

Electron Density Profiles Derived From Ground-Based GPS Observations

Nowadays GPS is widely used to monitor the ionosphere. However, the current results from ground-based GPS observations only provide some information on the horizontal structure of the ionosphere, and are extremely restricted in mapping its vertical structure. In this paper, tomography reconstruction technique was used to image 3D ionospheric structure with ground-based GPS. The first result of the 3D images of the ionospheric electron density distribution in South Korea has been generated from the permanent Korean GPS Network (KGN) data. Compared with the profiles obtained by independent ionosondes at or near the GPS receiver stations, the electron density profiles obtained by the GPS tomographic construction method are in better agreement, showing the validity of the GPS ionospheric tomographic reconstruction. It has also indicated that GPS-based 3D ionospheric mapping has the potential to complement other expensive observing techniques in ionospheric mapping, such as ionosondes and radar.

Horizontal deformation in South Korea from permanent GPS network data, 2000–2003

Analysis of continuous GPS data 2000–2003 at 50 stations of the Korean GPS Network (KGN) suggests that the southern part of the Korean peninsula is tectonically more stable than other regions in the Eastern Asia. The average velocity was ∼1.5 mm/yr and the average overall strain rate was around −0.3 × 10−9str/yr. The obtained velocity field indicates the presence of anticlockwise rotation of the whole region with respect to Daejeon station (DAEJ), in the central part of South Korea. It also showed that KM-OB (Kyonggi Massif and the Okchon Basin) and YM-TB (Yeongnam Massif and Taebaeksan Basin) have left-lateral shearing movements. Both movements were confirmed by the deformation analysis of the KGN horizontal velocity field using the infinitesimal plate theory. The results show that South Korea moves toward the WNW direction with a velocity of 0.9 mm/yr with a slow anticlockwise rotation. The strain field in South Korea indicates the existence of both compression and tension. The compression and extension axes have WSW-EWE and NNW-SSE directions, respectively.

Preliminary crustal movement study around the Honam shear zone and Okchon Belt (South Korea) using GPS observations

Crustal movement around the Honam shear zone and the Okchon Belt was estimated by using five GPS stations data. The data had been collected from two campaigns; each of them had five days long continuous observation and separated by one-year time span. The average velocity in this area is less than 2.4 mm/year and the depicted strain is less than 0.053 micro-strain/year. The fault line structure together with the principal axis of compression was used to determine the nature of strain and the direction of movements; the results demonstrate the presence of relative movement between the Okchon Belt and its surrounding blocks “Yeongnam Massif and Kyonggi Massif”. In spite of the fact that these results are obtained from one-year time span, they suggest the presence of relative movement between Okchon Belt and its surrounding massifs.

Seismicity and GPS constraints on crustal deformation in the southern part of the Korean Peninsula

Three years of continuous observations at 45 GPS sites in South Korea show horizontal deformation velocities are less than 3 mm/yr with respect to the stable South Korea block. These velocities and the associated horizontal strain rate reveal that South Korea is dominated by both ENE-WSW compression and NNW-SSE extension. Compared to the seismic strain rate in South Korea derived from recently recorded earth-quake data (Mw>4.0, 1936–2004), the principal horizontal axes of both strain rate tensors are nearly consistent, indicating that the seismicity can be used to improve GPS-derived deformation style and orientation. In addition, it also reflects that the occurrence of shallow earthquakes in South Korea is closely related with the horizontal strain.

Present-day relative displacements between the Jeju Island and the Korean peninsula as seen from GPS observations

We use three years data from six permanent GPS stations, in the Korean peninsula and Jeju Island, to examine the relative displacement between Jeju Island and the Korean peninsula and whether the Jeju Island lies on the Amurian Plate or not. Out of these stations, three sites, considered to be on the stable interior of the Amurian Plate (the Korean peninsula), showed relative velocities less than 1.3 mm/yr. On the other hand, the maximum velocity of the Jeju Island stations was 2.1 mm/yr with respect to DAEJ station, and the average relative velocity between the island stations and those in the main land are less than 1.7 mm/yr. By comparing these results with other velocities within the Amurian Plate, we found that the observed velocities are intra-plate. Our results are supported by geological evidences and seismological records, which claimed that the Korean peninsula and the Jeju Island are on the same plate. Hence, the Amurian Plate boundary line is considered to lie somewhere to the south of Jeju Island.

ORBIT DETERMINATION OF GPS AND KOREASAT 2 SATELLITE USING ANGLE-ONLY DATA AND REQUIREMENTS FOR OPTICAL TRACKING SYSTEM

Gauss method for the initial orbit determination was tested using angle-only data obtained by orbit propagation using TLB and SGP4/SDP4 orbit propagation model.. As the analysis of this simulation, a feasible time span between observation time of satellite resulting the minimum error to the true orbit was found. Initial orbit determination is performed using observational data of GPS 26 and Koreasat 2 from 0.6m telescope of KAO(Korea Astronomy Observatory) and precise orbit determination is also performed using simulated data. The result of precise orbit determination shows that the accuracy of resulting orbit is related to the accuracy of the observations and the number of data.

TRACKING CONTROL DESIGN USING SLIDING MODE TECHNIQUES FOR SATELLITE FORMATION FLYING

Satellite formation flying is currently an active area of research in the aerospace engineering. So it has been researched by various authors. In this study, a tracking controller using sliding mode techniques was designed to control a satellite for the satellite formation flying. In general, Hill`s equations are used to describe the relative motion of the follower satellite with respect to the leader satellite. However the modified Hill`s equations considering the perturbation were used for the design of sliding mode controller. The extended Kalman filter was applied to estimate the state vector based on the measurements of relative distance and velocity between two satellites. The simulation results show that the follower satellite tracks the desired trajectory well by thruster operations based on the sliding mode control law.