Underwater topography detection of Taiwan Shoal with SAR images |
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Authors: | Jungang Yang Jie Zhang Junmin Meng |
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Affiliation: | (1) Geospatial and Earth Monitoring Division, Geoscience Australia, 2601 Canberra, ACT, Australia |
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Abstract: | Under suitable conditions of tidal current and wind, underwater topography can be detected by synthetic aperture radar (SAR)
indirectly. Underwater topography SAR imaging includes three physical processes: radar ocean surface backscattering, the modulation
of sea surface short wave spectrum by the variations in sea surface currents, and the modulation of sea surface currents by
the underwater topography. The first process is described usually by Bragg scattering theory because the incident angle of
SAR is always between 20°–70°. The second process is described by the action balance equation. The third process is described
by an ocean hydrodynamic model. Based on the SAR imaging mechanism for underwater topography, an underwater topography SAR
detection model and a simplified method for its calculation are introduced. In the detection model, a two-dimensional hydrodynamic
model — the shallow water model is used to describe the motion of tidal current. Due to the difficulty of determining the
expression of SAR backscattering cross section in which some terms can not be determined, the backscattering cross section
of SAR image used in the underwater topography SAR detection is pro-processed by the simulated SAR image of the coarse-grid
water depth to simplify the calculation. Taiwan Shoal, located at the southwest outlet of Taiwan Strait, is selected as an
evaluation area for this technique due to the occurrence of hundreds of sand waves. The underwater topography of Taiwan Shoal
was detected by two scenes of ERS-2 SAR images which were acquired on 9 January 2000 and 6 June 2004. The detection results
are compared with in situ measured water depths for three profiles. The average absolute and relative errors of the best detection
result are 2.23 m and 7.5 %, respectively. These show that the detection model and the simplified method introduced in the
paper is feasible. |
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