Effect of Faraday Rotation on L-Band Ocean Normalized Radar Cross Section and Wind Speed Detection

Abstract

Observational evidence that L -band backscatter measurements are distorted by Faraday rotation (FR) is presented using match-ups consisting of the phased-array type L -band synthetic aperture radar-2 (PALSAR-2) data, scatterometer winds, and FR estimated from total electron contents (TEC) and magnetic fields. Investigating these data reveals that the PALSAR-2-derived ocean surface HH normalized radar cross section (NRCS) tends to decrease slightly as the FR angle increases and the HV one tends to increase the most. At an incidence angle of 27.5°, the measured HV NRCS increases by about 15 dB or more, whereas the FR angle increases from 0° to 15°. The variation of the PALSAR-2 observations with FR and its incident angle dependence are roughly explained by a model based on the scattering matrix of linearly polarized backscatter signatures. The error of wind speeds estimated by the L- band HH geophysical model function of ocean surface winds from the match-ups shows a tendency to underestimate wind speeds as FR increases. This is explained by the fact that the HH NRCS decreases as FR increases. Meanwhile, it is suggested that because ocean cross-polarized (HV) data with large co- and cross-polarization ratio has strong sensitivity to FR due to the inclusion of the co-polarization component, correction for the influence of the FR angle is essential for accurate ocean wind estimation.