Browsing by Author "Hwang Cheinway"

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Oceanic surface geostrophic velocities determined with satellite altimetric crossover method
(2010-11-01)
Guo Jin-Yun
;
Chang Xiao-Tao
;
Hwang Cheinway
;
Sun Jia-Long
;
Han Yan-Ben
If the Coriolis force is balanced by the pressure gradient in the hydrostatic equilibrium. the ocean current is referred to as the geostrophic current Most ocean currents worldwide are approximately geostrophic currents The surface geostrophic velocity is calculated with the satellite altimetric crossover method in this paper The uncertainties of geostrophic velocity determined from satellite altimetric crossover data are analyzed If the accuracy of the sea surface slope along the ascending or descending track of altimetric satellite is up to the 10(-7) level with 50 km of resolution, the geostrophic velocity will have an accuracy of better than 10 cm/s In the areas of low latitude or near the altimetric satellite orbital inclination, the uncertainty of geostrophic velocity determined with the crossover method will be much bigger than that in the low-/mid-latitude areas The Kuroshio area east of Taiwan, China is selected as a testing region A highly accurate and up-to-date geoidal model which is determined using the combined data from the ground-based gravimetry, ship-borne gravimetry, air-borne gravimetry and satellite altimetry, is selected as the reference geoid Sea surface heights from the geophysical data records of TOPEX/Poseidon and Jason-1 from 2002 to 2005 are used to compute highly accurate dynamic heights at three crossover points Geostrophic velocities are then derived at these crossover points and are basically identical to the current velocities provided by the National Center for Ocean Research, Taiwan, China
Oceanic surface geostrophic velocities determined with satellite altimetric crossover method
(2010-11-01)
Guo Jin-Yun
;
Chang Xiao-Tao
;
Hwang Cheinway
;
Sun Jia-Long
;
Han Yan-Ben
If the Coriolis force is balanced by the pressure gradient in the hydrostatic equilibrium. the ocean current is referred to as the geostrophic current Most ocean currents worldwide are approximately geostrophic currents The surface geostrophic velocity is calculated with the satellite altimetric crossover method in this paper The uncertainties of geostrophic velocity determined from satellite altimetric crossover data are analyzed If the accuracy of the sea surface slope along the ascending or descending track of altimetric satellite is up to the 10(-7) level with 50 km of resolution, the geostrophic velocity will have an accuracy of better than 10 cm/s In the areas of low latitude or near the altimetric satellite orbital inclination, the uncertainty of geostrophic velocity determined with the crossover method will be much bigger than that in the low-/mid-latitude areas The Kuroshio area east of Taiwan, China is selected as a testing region A highly accurate and up-to-date geoidal model which is determined using the combined data from the ground-based gravimetry, ship-borne gravimetry, air-borne gravimetry and satellite altimetry, is selected as the reference geoid Sea surface heights from the geophysical data records of TOPEX/Poseidon and Jason-1 from 2002 to 2005 are used to compute highly accurate dynamic heights at three crossover points Geostrophic velocities are then derived at these crossover points and are basically identical to the current velocities provided by the National Center for Ocean Research, Taiwan, China
Optimal threshold algorithm of EnviSat waveform retracking over coastal sea
(2010-04-01)
Guo Jin-Yun
;
Gao Yong-Gang
;
Chang Xiao-Tao
;
Hwang Cheinway
;
土木工程學系
Waveform retracking is an effective method to improve the accuracy of sea surface height measurement with the satellite altimetric technique. The improved threshold retracking algorithm is optimized in the paper. Several waveform retracking algorithms are analyzed. Then a waveform retracking system is programmed. Several retracking methods are used to retrack the EnviSat waveform over the Mediterranean Sea. The improved threshold retracking algorithm is the most robust retracking method and more suitable for the EnviSat waveform retracking over the coastal Mediterranean.
The Shipborne Kinematic Positioning Accuracy of Using GPS Precise Point Positioning (PPP) Technique: A Case Study in Shipborne Gravity Measurement around Taiwan
(2009-01-01)
Chen Kwohwa
;
Chen Yiju
;
Huang Chihsun
;
Hwang Cheinway
;
土木工程學系
From 2004 to present, the GPS phase observation data have been collected in the shipborne gravity measurement around Taiwan, which are used to determine sea surface heights (SSHs) that are equivalent to satellite altimeter-derived SSHs. The ship-based SSHs are corrected for ocean tide and body tide effects, but no tropospheric and ionospheric corrections are needed because they are already applied in GPS positioning. In coastal areas, satellite altimetry is often affected by bad radar waveforms, but ship-based altimetry is not, which makes ship-base altimetry can obtain more accurate SSH than satellite altimetry. In this paper, precise point positioning (PPP) technique is used to determine the ellipsoid heights of ship, and the positioning accuracies are assessed by comparing with DGPS and analyzing the crossover difference of SSHs. This paper will also present the limitation and the prospect of ship-based altimetry.