Browsing by Author "Chao, Benjamin Fong"

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Gravity monitoring of Tatun Volcanic Group activities and inference for underground fluid circulations
(2016-12-15)
Mouyen, Maxime
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Chao, Benjamin Fong
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Hwang, Cheinway
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Hsieh, Wen-Chi
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土木工程學系
The Tatun Volcano Group (TVG), located on the northern coast of Taiwan adjacent to the city of Taipei, experiences active hydrothermalism but has no historical record of volcanic eruption. Yet recent studies suggest that TVG is dormant-active rather than extinct. To monitor mass transfers and to gain further understanding of this volcanic area, gravity variations have been recorded continuously since 2012 using a superconducting gravimeter, and once every few months since 2005 using absolute gravimeters. We analyze the continuous gravity time series and propose a model that best explains the gravity variations due to local groundwater redistribution. By correcting these variations, we identify gravity changes as large as 35 Gal that occurred concomitantly to fluid pressure-induced earthquakes and changes in the gas composition at Dayoukeng, one of TVG\'s fumaroles, over 2005-2007. We examine several fluid movements that can match the gravity observations, yet too few additional constraints exist to favor any of them. In particular, no significant ground displacements are observed when these gravity variations occurred. On the other hand, the model of gravity changes due to local groundwater redistribution can be routinely computed and removed from the ongoing time gravity measurements in order to quickly identify any unusual mass transfer occurring beneath TVG. (C) 2016 Elsevier B.V. All rights reserved.
Modeling Orbit Dynamics of FORMOSAT-3/COSMIC Satellites for Recovery of Temporal Gravity Variations
(2008-11-01)
Hwang, Cheinway
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Lin, Ting-Jung
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Tseng, Tzu-Pang
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Chao, Benjamin Fong
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土木工程學系
The precise GPS high-low tracking data from the joint Taiwan-USA mission FORMOSAT-3/COSMIC (COSMIC) can be used for gravity recovery. The current orbital accuracy of COSMIC kinematic orbit is 2 cm and is better than 1 cm for 60-s normal points. We model the perturbing forces acting on the COSMIC spacecraft based on standard models of orbit dynamics. The major tool for the numerical work of force modeling is NASA Goddard's GEODYN II software. Considering that COSMIC spacecrafts are not equipped with accelerometers, the accelerations due to atmospheric drag, solar radiation pressure, and other minor surface forces are modeled by estimating relevant parameters over one orbital period from COSMIC's kinematic and reduced dynamic orbits. We carry out experimental solutions of time-varying geopotential coefficients using one month of COSMIC kinematic orbits (August 2006). With the nongravity origin forces properly modeled by GEODYN II, residual orbital perturbations (difference between kinematic and reference orbits) are assumed to be linear functions of time-varying geopotential coefficients and are used as observations to estimate the latter. Both COSMIC and combined COSMIC and GRACE gravity solutions are computed. The COSMIC solution shows some well-known temporal gravity signatures but contains artifacts. The combined COSMIC and GRACE solution enhances some local temporal gravity signatures in the GRACE solution.
Preface to the Special Issue on Geodynamic and Climate-Change Processes over Tibet, Xinjiang and Siberia
(2011-04-01)
Hwang, Cheinway
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Chao, Benjamin Fong
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Freymueller, Jeffrey T.
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Shen, Wenbin
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Shum, C. K.
Quality assessment of FORMOSAT-3/COSMIC and GRACE GPS observables: analysis of multipath, ionospheric delay and phase residual in orbit determination
(2010-01-01)
Hwang, Cheinway
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Tseng, Tzu-Pang
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Lin, Ting-Jung
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Svehla, Drazen
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Hugentobler, Urs
The precise orbit determination antennas of F3/C and GRACE-A satellites are from the same manufacturer, but are installed in different configurations. The current orbit accuracy of F3/C is 3 cm at arcs with good GPS data, compared to 1 cm of GRACE, which has a larger ratio of usable GPS data. This paper compares the qualities of GPS observables from F3/C and GRACE. Using selected satellites and time spans, the following average values for the satellite F3/C and satellite A of GRACE are obtained: multipath effect on the pseudorange P1, 0.78 and 0.38 m; multipath effect on the pseudorange P2, 1.03 and 0.69 m; occurrence frequency of cycle slip, 1/29 and 1/84; standard error of unit weight, 4 and 1 cm; dynamic-kinematic orbit difference, 10 and 2 cm. For gravity determination using F3/C GPS data, a careful selection of GPS data is critical. With six satellites in orbit, F3/C's large amount of GPS data will make up the deficiency in data quality.
Vertical Displacement due to Ocean Tidal Loading Around Taiwan Based on GPS Observations
(2011-08-01)
Yeh, Ta-Kang
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Hwang, Cheinway
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Huang, Jiu-Fu
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Chao, Benjamin Fong
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Chang, Ming-Han
Ocean tidal loading (OTL) is an important factor in GPS positioning, especially along the vertical direction. OTL influences the precision of GPS positioning and produces height variations of up to 12 cm. In this study, daily UPS data obtained from 27 GPS tracking stations around Taiwan were collected, and four OTL models were applied to relative static GPS positions derived from these data. The UPS data were obtained from 8 - 14 August 2006 (summer) and 1 - 30 January 2007 (winter). The software Bernese 5.0 was utilized for data processing. During the summer, the average GPS error along the vertical axis was 2.72 cm. After correcting the measured heights with the Schwiderski, NAO.99b, CSR4.0 and TPXO.6.2 models, the average errors became 2.30, 2.30, 2.30 and 2.31 cm, respectively. In winter, the average UPS error in the vertical direction was 1.32 cm. After correcting the heights using the above models, the average errors became 0.86, 0.85, 0.89 and 0.84 cm, respectively. The precision of the vertical direction was improved by approximately 15 - 36% on average. In eastern Taiwan, all UPS stations in the vertical direction have amplitudes smaller than 1 cm, because the ocean is more than 2000 m deep and with less shallow and narrow bights and estuaries. On the other hand, the amplitudes of GPS stations in northwestern Taiwan are larger than 1 cm because the ocean is only approximately 200 m deep. The amplitudes in the vertical direction of UPS stations are approximately 1.2 - 2.7 cm on the offshore islands of Taiwan, larger than average in Taiwan. In the future, local corrections from an OTL model are necessary for high accurate GPS positioning.