Swell parameters retrieval using ALOS/PALSAR data and its application

We have developed retrieval methods for swell parameters such as wave height, wavelength, and wave propagation direction from images of PALSAR on board ALOS taken over coastal seas around Japan. The retrieval methods for wavelength and wave direction use the PALSAR image spectrum. The SAR image spectrum has an inherent 180° direction ambiguity, which can be solved using the fact that the swells only propagate toward the coast. Using the coupling swell spectra and the PALSAR image spectra, an empirical L-band modulation transfer function was obtained for the wave height retrieval through linear regression analysis. A comparison between SAR-derived and in situ wave parameters showed close agreement. For the wavelength and wave direction, the biases were 10.4 m and 1.3° the root mean-square errors (RMSE) were 18.3 m and 15.5° and the correlation coefficients were 0.93 and 0.94, respectively. These results demonstrate that the L-band SAR imaging mechanism for swells is mostly linear. For the wave height, the bias was 0.08 m, the RMSE was 0.30 m, and the correlation coefficient was 0.80. The coastal water depth was calculated from the retrieved wavelength. The comparison shows that the result is better when the water depth is less than 50 meters.