Effect of Faraday Rotation on L-Band NRCS and Wind Speed Detection

Abstract

The relationship between ocean wind vectors and L-band normalized radar cross sections (NRCS) is examined to modify the L-band geophysical model function (GMF) using the Phased-Array L-band Synthetic Aperture Radar 2 (PALSAR-2) and scatterometer wind data. Since the PALSAR-2 has a function of dual polarized ScanSAR, the sensitivity of L-band HV on wind fields is examined. Though the HV signal shows wind speed and incidence angle dependencies, they are too noisy to be used for wind detection. In order to seek the possible cause, the L-band NRCS is compared with Faraday rotation (FR) angle, indicating a clear positive relationship between the HV NRCSs and with the FR angle. The HH/HV ratio from the PALSAR-2 observation is compared with theoretical ones calculated from FR angle. The observation and theory correspond in terms of the tendency that the HH/HV decreases with increasing the FR angle and its sensitivity decreases with incidence angle. On the other hand, the decreasing ratio of the observation exceeds the theoretical one resulting in increasing discrepancy with the FR angle. Since the HH NRCS is not significantly affected by FR, the L-band HH GMF is modified in order to cover the wider incidence angle ranges from 17° to 50°. Based on the derived GMF, wind speeds are estimated and compared with the buoy-measured winds. The comparison shows the −0.75 m/s bias and 2.07 m/s root mean square (rms) error.