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Remote Sens. 2017, 9(7), 711; https://doi.org/10.3390/rs9070711

An Empirical Algorithm for Wave Retrieval from Co-Polarization X-Band SAR Imagery

1
Marine Science and Technology College, Zhejiang Ocean University, Zhoushan 316000, China
2
Physical Oceanography Laboratory, Ocean University of China, Qingdao 266100, China
3
Global Science and Technology, National Oceanic and Atmospheric Administration (NOAA)-National Environmental Satellite, Data, and Information Service (NESDIS), College Park, MD 20740, USA
*
Author to whom correspondence should be addressed.
Academic Editors: Xiaofeng Yang, Ferdinando Nunziata and Alexis Mouche
Received: 17 April 2017 / Revised: 28 June 2017 / Accepted: 6 July 2017 / Published: 11 July 2017
(This article belongs to the Special Issue Ocean Remote Sensing with Synthetic Aperture Radar)
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Abstract

In this study, we proposed an empirical algorithm for significant wave height (SWH) retrieval from TerraSAR-X/TanDEM (TS-X/TD-X) X-band synthetic aperture radar (SAR) co-polarization (vertical-vertical (VV) and horizontal-horizontal (HH)) images. As the existing empirical algorithm at X-band, i.e., XWAVE, is applied for wave retrieval from HH-polarization TS-X/TD-X image, polarization ratio (PR) has to be used for inverting wind speed, which is treated as an input in XWAVE. Wind speed encounters saturation in tropical cyclone. In our work, wind speed is replaced by normalized radar cross section (NRCS) to avoiding using SAR-derived wind speed, which does not work in high winds, and the empirical algorithm can be conveniently implemented without converting NRCS in HH-polarization to NRCS in VV-polarization by using X-band PR. A total of 120 TS-X/TD-X images, 60 in VV-polarization and 60 in HH-polarization, with homogenous wave patterns, and the coincide significant wave height data from European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis field at a 0.125° grid were collected as a dataset for tuning the algorithm. The range of SWH is from 0 to 7 m. We then applied the algorithm to 24 VV and 21 HH additional SAR images to extract SWH at locations of 30 National Oceanic and Atmospheric Administration (NOAA) National Data Buoy Center (NDBC) buoys. It is found that the algorithm performs well with a SWH stander deviation (STD) of about 0.5 m for both VV and HH polarization TS-X/TD-X images. For large wave validation (SWH 6–7 m), we applied the empirical algorithm to a tropical cyclone Sandy TD-X image acquired in 2012, and obtained good result with a SWH STD of 0.3 m. We concluded that the proposed empirical algorithm works for wave retrieval from TS-X/TD-X image in co-polarization without external sea surface wind information. View Full-Text
Keywords: SAR; significant wave height; co-polarization; TerraSAR-X/TanDEM-X SAR; significant wave height; co-polarization; TerraSAR-X/TanDEM-X
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Shao, W.; Wang, J.; Li, X.; Sun, J. An Empirical Algorithm for Wave Retrieval from Co-Polarization X-Band SAR Imagery. Remote Sens. 2017, 9, 711.

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