In this study, we proposed an empirical algorithm for significant wave height\n(SWH) retrieval from TerraSAR-X/TanDEM (TS-X/TD-X) X-band synthetic aperture radar (SAR)\nco-polarization (vertical-vertical (VV) and horizontal-horizontal (HH)) images. As the existing\nempirical algorithm at X-band, i.e., XWAVE, is applied for wave retrieval from HH-polarization\nTS-X/TD-X image, polarization ratio (PR) has to be used for inverting wind speed, which is treated as\nan input in XWAVE.Wind speed encounters saturation in tropical cyclone. In our work, wind speed is\nreplaced by normalized radar cross section (NRCS) to avoiding using SAR-derived wind speed, which\ndoes not work in high winds, and the empirical algorithm can be conveniently implemented without\nconverting NRCS in HH-polarization to NRCS in VV-polarization by using X-band PR. A total of\n120 TS-X/TD-X images, 60 in VV-polarization and 60 in HH-polarization, with homogenous wave\npatterns, and the coincide significant wave height data from European Centre for Medium-Range\nWeather Forecasts (ECMWF) reanalysis field at a 0.125ââ??¦ grid were collected as a dataset for tuning\nthe algorithm. The range of SWH is from 0 to 7 m. We then applied the algorithm to 24 VV and 21\nHH additional SAR images to extract SWH at locations of 30 National Oceanic and Atmospheric\nAdministration (NOAA) National Data Buoy Center (NDBC) buoys. It is found that the algorithm\nperforms well with a SWH stander deviation (STD) of about 0.5 m for both VV and HH polarization\nTS-X/TD-X images. For large wave validation (SWH 6ââ?¬â??7 m), we applied the empirical algorithm\nto a tropical cyclone Sandy TD-X image acquired in 2012, and obtained good result with a SWH\nSTD of 0.3 m. We concluded that the proposed empirical algorithm works for wave retrieval from\nTS-X/TD-X image in co-polarization without external sea surface wind information.
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