🏛️國立陽明交通大學機構典藏

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Zonal and meridional ocean currents at TOPEX/Poseidon and JASON-1 crossovers around Taiwan: Error analysis and limitation
(2008-04-01)
Hwang, Cheinway
;
Shih, Hsuan-Chang
;
Guo, Jinyun
;
Hsiao, Yu-Shen
;
土木工程學系
A crossover method for determining zonal and meridional ocean current components is examined using data at three crossovers of TOPEX/Poseidon and JASON- I ground tracks over 2002 - 2006. To implement this method, a geoid model around Taiwan is constructed using surface and airborne gravity data. The modeled and observed geoidal heights at coastal benchmarks are consistent to 5 cm RMS with the means removed. The error and limitation of this method are discussed, concluding that, in order to obtain current velocities at a 10 cm s(-1) accuracy and a 6-km resolution, the dynamic ocean topography (DOT) at a mm-level accuracy is needed, which is not possible to achieve today. By filtering DOT to a spatial scale of 100 km or coarser, a 10 cm s(-1) accuracy of velocity may be obtained. One crossover (A) is situated south of Taiwan and near the Kuroshio, the second (B) is at the axis of the Kuroshio and the third is located in the northern Taiwan Strait. These three crossovers feature different ocean current patterns. At a spatial scale of 120 km, the agreement among the altimeter, the Princeton Ocean Model (POM), and the drifter-derived velocities is the best at B, followed by that at A, and then C. In fact, at C the altimeter-derived velocities contradict the POM-derived values, and the tide model error is to be blamed. Further improvement on geoid modeling is suggested.
New free-air and Bouguer gravity fields of Taiwan from multiple platforms and sensors
(2014-01-25)
Hwang, Cheinway
;
Hsu, Hung-Jui
;
Chang, Emmy T. Y.
;
Featherstone, W. E.
;
Tenzer, Robert
We construct 1' x 1' grids of free-air and Bouguer gravity anomalies around Taiwan with well-defined error estimates for quality assessment. The grids are compiled from land, airborne and shipbome gravity measurements, augmented with altimeter gravity at sea. Three sets of relative land gravity measurements are network-adjusted and outlier-edited, yielding accuracies of 0.03-0.09 mGal. Three airborne gravity sets are collected at altitudes 5156 and 1620 m with accuracies of 2.57-2.79 mGal. Seven offshore shipborne gravity campaigns around Taiwan and its offshore islands yield shallow-water gravity values with 0.88-2.35 mGal accuracies. All data points are registered with GPS-derived geodetic coordinates at cm-dm accuracies, allowing for precise gravity reductions and computing gravity disturbances. The various datasets are combined by the band-limited least-squares collocation in a one-step procedure. In the eastern mountainous (or offshore) region, Bouguer anomalies and density contrasts without considering the oceanic (or land) topographic contribution are underestimated. The new grids show unprecedented tectonic features that can revise earlier results, and can be used in a broad range of applications. (C) 2013 Elsevier B.V. All rights reserved.
Airborne Gravity Surveys Over Taiwan Island and Strait, Kuroshio Current and South China Sea: Comparison of GPS and Gravity Accuracies at Different Flight Altitudes
(2012)
Hwang, Cheinway
;
Shih, Hsuan-Chang
;
Hsiao, Yu-Shen
;
Huang, Chi-Hsun
;
土木工程學系
We compare the accuracies of GPS and gravity observations obtained from three airborne gravity surveys over Taiwan Island at altitude 5000 m and over Kuroshio Current, Taiwan Strait, and Dongsha Atoll at altitude 1500 m. A kinematic network adjustment was used to determine the positions of the aircrafts. GPS positioning errors are at the decimeter-level, which are not entirely propagated to velocity and acceleration errors due to cancellation of long wavelength errors. Outliers are downweighted in the Gaussian filtering to improve the gravity accuracy, especially at altitude 1500 m. Compared with the upward-continued gravity, the gravity anomalies from the 1500-m surveys show a consistent accuracy of about 3 mgal; the accuracy from the 5000-m survey is degraded, especially over high mountains. The RMS crossover differences at 1500 m and 5000 m are all below 3 mgal, suggesting flight altitudes do not affect the crossover difference. Coherence analysis suggests that the spatial resolvable wavelengths of the airborne gravity range from 4 km (altitude 1500 m) to 6 km (altitude 5000 m).
Data reduction in scalar airborne gravimetry: Theory, software and case study in Taiwan
(2006-12-01)
Hwang, Cheinway
;
Hsiao, Yu-Shen
;
Shih, Hsuan-Chang
;
土木工程學系
;
Department of Civil Engineering
The objective of this paper is to present theories of scalar airborne gravimetry and to publish the related software to interested readers for free access. Numerical procedures and techniques are developed to compute velocities and accelerations from GPS-determined positions. A method based on the correlation analysis of raw gravity reading and vertical acceleration of aircraft is used to correct for gravimeter times. A method and a computer program for crossover adjustment of gravity values along survey lines are developed. This method allows flexible selection of a fixed survey line to overcome the rank defect problem. A computer program is developed to compute gravity anomalies while applying corrections of gravimeter position and filtering. Upward and downward continuations of gravity anomalies are performed using Fast Fourier Transform. Using observed airborne gravity data along a survey line and simulated data in Taiwan, all the programs have been validated and produced reliable results. (c) 2006 Elsevier Ltd. All rights reserved.

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🏛️國立陽明交通大學機構典藏

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