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Improvement of Envisat Altimetric Measurements in Taiwan Coastal Oceans by a Developed Waveform Retracking System
A new waveform retracking system applied on 18-Hz Envisat RA-2 data (cycles 10 to 90) was successfully developed in this study and aimed to improve the accuracy of measuring sea surface heights (SSHs) in coastal oceans around Taiwan. The specialized retracking system comprised the CurveFit retracker of fitting altimetry subwaveforms to the Brown model along with Gaussian peak models and a technique of removing non-ocean waveforms. Therefore, our system can favorably retain ocean-reflected waveforms and effectively reduce the land contamination in reflected waveforms. Results demonstrated that the CurveFit retracker performed best within 0 to 5 km offshore in all the study areas, with improvement percentages (IMPs) reaching a maximum of 92%. Within 5 to 10 km from the coast, the CurveFit retracked SSHs considerably improved with an IMP of up to 93%. Given that most waveforms, particularly of ground tracks from ocean to land, conformed to a typical ocean model in the 5 to 10 km zone, all the used retrackers similarly achieved satisfactory and equivalent improvements. This study also manifested that the accuracy of retracked SSHs largely depends on both land topography and ground tracks approaching or leaving land. The amount and accuracy of valid retracked measurements along ground tracks from land to ocean were remarkably lower than those of ground tracks across the coastline reversely. Moreover, the altimetric waveforms were highly contaminated by coastal land topography when ground tracks approached coastal flat land from the ocean or passed through coastal undulating topography toward the ocean.
Keywords: satellite altimetry, waveform retracking, coastal ocean, land topography, ground track
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