Browsing by Author "E, Dongchen"

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A fixed full-matrix method for determining ice sheet height change from satellite altimeter: an ENVISAT case study in East Antarctica with backscatter analysis
(2014-09-01)
Yang, Yuande
;
Hwang, Cheinway
;
E, Dongchen
;
土木工程學系
;
Department of Civil Engineering
A new method, called the fixed full-matrix method (FFM), is used to compute height changes at crossovers of satellite altimeter ground tracks. Using the ENVISAT data in East Antarctica, FFM results in crossovers of altimeter heights that are 1.9 and 79 times more than those from the fixed half method (FHM) and the one-row method (ORM). The mean standard error of height changes is about 14 cm from ORM, which is reduced to 7 cm by FHM and to 3 cm by FFM. Unlike FHM, FFM leads to uniform errors in the first-half and second-half height-change time series. FFM has the advantage in improving the accuracy of the change of height and backscattered power over ORM and FHM. Assisted by the ICESat-derived height changes, we determine the optimal threshold correlation coefficient (TCC) for a best correction for the backscatter effect on ENVISAT height changes. The TCC value of 0.92 yields an optimal result for FFM. With this value, FFM yields ENVISAT-derived height change rates in East Antarctica mostly falling between and 3 cm/year, and matching the ICESat result to 0.94 cm/year. The ENVISAT result will provide a constraint on the current mass balance result along the Chinese expedition route CHINARE.
A subwaveform threshold retracker for ERS-1 altimetry: A case study in the Antarctic Ocean
(2012-04-01)
Yang, Yuande
;
Hwang, Cheinway
;
Hsu, Hung-Jui
;
E, Dongchen
;
Wang, Haihong
Based on a correlation analysis method, a subwaveform threshold retracker is developed and coded in FORTRAN for satellite altimetry to determine the leading edge and retracking gate, and to improve the precision of sea surface heights (SSHs) and gravity anomalies (GAs). Using ERS-1/ERM waveforms, the subwaveform threshold retracker outperforms full-waveform threshold retrackers at the tide gage Port Station. A direct comparison between retracked SSHs and in situ SSHs is made at tide gage Port Station. Here the subwaveform retracking improves SSH precision from 0.241 to 0.193 m, yielding an improvement percentage (IMP) of 20%. Using ERS-1/GM waveforms, the subwaveform threshold retracker outperforms the Beta-5 and full-waveform threshold retrackers over the Bellingshausen and Amundsen Seas (BAS) in the Antarctic Ocean. The standard deviations of raw and retracked SSHs are 0.157 and 0.070 and 1.836 and 0.220 m over the ice-free and ice-covered oceans, corresponding to IMPs of 54.4% and 88%, respectively. Use of retracking improves the precision of GAs by up to 46.6% when comparing altimeter-derived and shipborne GAs. (C) 2011 Elsevier Ltd. All rights reserved.
GPS-derived velocity and strain fields around Dome Argus, Antarctica
(2014-01-01)
Yang, Yuande
;
Sun, Bo
;
Wang, Zemin
;
Ding, Minghu
;
Hwang, Cheinway
Knowledge of the surface topography, velocity field and strain field at an ice-core site is critical to the accurate interpretation of ice-core records. At Dome Argus (Dome A), where a Chinese deep ice-core drilling project is being carried out, we have produced an accurate surface digital elevation model from GPS measurements in January 2013 at 47 sites. We identify two peaks at Dome A, with the northern peak similar to 7 cm higher than the southern peak. Repeat GPS measurements at 12 sites in 2008 and 2013 provide a surface velocity field around the dome. The surface velocity ranges from 3.1 +/- 2.6 to 29.4 +/- 1.2 cm a(-1), with a mean of 11.1 +/- 2.4 cm a(-1). The surface flow directions are near perpendicular to the surface elevation contours. Velocities from GPS are lower than derived from satellite radar interferometry (InSAR). From GPS velocities, the accuracy of velocity from the existing InSAR velocity field is determined, resulting in a standard deviation of 0.570 m a(-1) in speed and 117.5 degrees in direction. This result is consistent with the reported accuracy of InSAR, showing the value of in situ GPS measurements for assessing and correcting remote-sensing results. A surface strain field for the drilling site over Dome A is calculated from 24 strain triangles, showing north-south extension, east-west compression and vertical layer thinning.