Browsing by Author "Hsu, Hsin-Ying"

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Coastal gravity anomalies from retracked Geosat/GM altimetry: Improvement, limitation and the role of airborne gravity data
(2006-07-01)
Hwang, Cheinway
;
Guo, Jinyun
;
Deng, Xiaoli
;
Hsu, Hsin-Ying
;
Liu, Yuting
We process geophysical and waveform data records of the Geosat/GM (geodetic mission) satellite altimeter mission for waveform retracking and applications. An improved threshold retracker is developed. The performances of the Beta-5, threshold and improved threshold retrackers are assessed over waters around Taiwan. The improved threshold retracker outperforms the other two. The improvement in the accuracy of sea surface height (SSH) is investigated according to marine zone and the distance of waters to the shore. The improvement rate increases closer to the land, with the largest improvement rate of about 20% in waters within 10 km of the shore. Over waters around islands and coasts, there are still retracked SSHs with large errors. Least-squares collocation is used to compute gravity anomalies from the Geosat/GM altimeter data. Use of retracked SSHs improves the accuracy of gravity anomalies by about 11%. Adding airborne gravity data further improves the accuracy, especially in the immediate vicinity of the coasts. Tide model errors over coastal waters remain a problem in altimetry applications, even if the waveforms are properly retracked.
Shallow-water gravity anomalies from satellite altimetry: Case studies in the East China Sea and Taiwan Strait
(2008-07-01)
Hwang, Cheinway
;
Hsu, Hsin-Ying
;
土木工程學系
;
Department of Civil Engineering
The differences between two global satellite altimeter-derived gravity anomaly grids over the East China Sea and the Taiwan Strait are investigated and the causes of the differences are discussed. Difference of gravity anomaly is correlated with tide model error, standard deviation of sea surface heights (SSHs) and ocean depth. We present two new SSH-derived observations of altimetry (differenced height and height slope) for gravity derivation. An iterative method for removing altimeter data outliers is used, and differenced height is found to be most sensitive to this method. For altimeter-gravity conversion, we used three altimeter-derived observations, i.e., deflection of the vertical, differenced height and height slope, in two conversion methods, i.e., the least-squares collocation (LSC) and the inverse Vening Meinesz method. Use of differenced heights in LSC produces the best result when comparing altimeter-derived and shipborne gravity anomalies. Use of land gravity data in the vicinity of coasts enhances the accuracy of altimeter-derived gravity anomalies.