Hong-Sic Yun

Precision geoid determination by spherical FFT in and around the Korean peninsula

This paper deals with the precision geoid determination by a gravimetric solution in and around the Korean peninsula. A number of data files were compiled for this work, containing now more than 69,900 point gravity data on land and ocean areas. The EGM96 global geopotential model to degree 360 was used in order to determine the long wavelength effect of the geoid surface. By applying the remove-restore technique the geoid undulations were determined by combining a geopotential model, mean free-air gravity anomalies and height in a Digital Elevation Model (DEM). Computation involves a spherical approximation to conduct the Stokes’ integration by a two dimensional spherical Fast Fourier Transform (FFT) with 100% zero-padding. A terrain correction was also computed by FFT with a spherical approximation of the Residual Terrain Model (RTM) terrain correction integration. Accuracy estimates are given for absolute geoid undulations using 78 GPS/Leveling stations. The comparative evaluation gives the bias of 0.187 meters and standard deviation of 0.28 meters, respectively. The relative accuracy achieved was of the order of 3.1 ppm for baselines between 10 and 350 kilometers.

GPS measurements of horizontal deformation across the Lai Chau—Dien Bien (Dien Bien Phu) fault, in Northwest of Vietnam, 2002-2004

Global Positioning System (GPS) measurements from Feb. 2002 through Mar. 2004 were used to estimate the recent crustal movement along the Lai Chau—Dien Bien (Dien Bien Phu) fault (LC-DBF) system in the Northwest of Vietnam. Four GPS campaign data were processed to estimate ITRF2000 and local horizontal velocities, as well as extensive and compressive strain rates across the LC-DBF. ITRF2000 velocities are consistent with east-southeastward movement of Sundaland i.e. Indochina. Local velocities did not reveal much left-lateral strike-slip of the fault system and the derived strain rates were insignificantly different from zero at 95% confidence.

HORIZONTAL DATUM TRANSFORMATION BY DISTORTION MODELLING IN KOREA

Abstract This paper describes the development of a transformation grid for transforming horizontal spatial data from the local geodetic datum to a geocentric datum in Korea. We used least squares collocation as the method for modelling the distortion that remains after the application of a standard seven-parameter similarity transformation. In the experimental tests, the RMSEs in latitude and longitude coordinates are ±0.016m and ±0.007m, respectively, after the distortion modelling, whereas it was indicated that transformation accuracy is improved about 98% by distortion correction. This is a reasonable result, which should give users confidence in transforming data from the local geodetic datum to the geocentric and vice versa for medium scale maps.

Verification of accuracy of precipitable water vapour from GPS during typhoon Rusa

Abstract This paper presents GPS meteorology using GPS observations obtained from the Korean permanent GPS network during a severe weather condition. Our first experimental studies in Korea were based on data collected from seventeen GPS stations and four radiosonde stations during the period of Typhoon Rusa from 31 August to 1 September 2002. The root mean square difference (RMSD) between the GPS Precipitable Water Vapour (PWV) and the radiosonde-PWV was at the 7.2 mm level. In this new study, a least square analysis was used to reduce the large RMSD and without bias, the RMSD was reduced to 5.7 mm. The mean of the RMSD without consideration of Typhoon Rusa effect was 6.9 mm, whereas the RMSD of GPS-PWV with consideration of the Rusa effect was worse with 9.4 mm. The large RMSD during Typhoon Rusa was due to the change of the observation direction of the balloon based radiosonde by high wind speeds.

Optimal GPS positioning solutions interval for a crustal deformation estimation

GPS is an essential tool in positioning which requires high precision such as velocity field estimation of tectonic plates. However it takes long time for this kind GPS solutions because of defective automatic GPS data processing system. Therefore, it is importa nt that the determination of GPS data processing interval to obtain requisite accuracy in a study of crustal deformation parameter estimation. So, the Gipsy-Oasis II software was used to estimate daily deformation of GPS stations with orbital and atmospheric parameters and least square linear fitting technique was used to estimate the velocity of the GPS stations. In this relative solution of daily positioning, the standard deviation which is scale of precision turned out better than 4mm. In this study, to determinet he optimal time-interval for a crustal deformation estimation, three kinds of time-interval was adopted, which is 2-day, 5-day and 10-day time-interval and were compared to daily solutions by the standard deviation of velocity from the calculation results. Researchresults shows that the Std of the horizontal crustal motion parameter was 1.44 times (Std=±0.66mm/yr), 2.24 times (Std=±1.03mm/yr) and 3.20 times (Std=±1.47mm/yr) worse than the Std used to obtain of the daily positioning result (Std=±0.46mm/yr). The crustal motion magnitude varied irregularly for the 2-day interval. So, we made conclusion that the 2-day interval can be applied to process GPS data for rapid velocity field estimation.