Preliminary Study on Developing an L-band Wind Retrieval Model Function Using Alos/palsar

The relationship between ocean wind vectors and Lband normalized radar cross sections (NRCS) is examined using the Phased-Array L-band Synthetic Aperture Radar (PALSAR). We used PALSAR ScanSAR images with a wide range of incidence angles from 17o to 43o. More than 6,000 match-ups, each consisting of the NRCS, incidence angles, wind speeds and wind directions, were collected. The NRCS exhibits a power-law relationship with respect to the wind speed. The coefficients of the power law can be derived as a function of the incidence angles. Based on this relation, the wind speeds are then inversely estimated from the NRCS and the incidence angles. A comparison with the truth winds reveals -0.68m/s bias and 2.99m/s rms error, demonstrating the feasibility of L-band scatterometry. Collecting more match-ups and further considering wind-direction dependence, which was not included in this study, would lead to the derivation of a robust L-band model function. Recently, SAR-measured NRCS have been directly compared with buoy and scatterometer-estimated winds to establish an L-band model function [4]. However, it does not take account of incidence angle dependency due to constraints on JERS-1 operation. In January 2006, the Japan Aerospace Exploration Agency (JAXA) launched the Advanced Land Observing Satellite (ALOS), which carries the Phased-Array L-band Synthetic Aperture Radar (PALSAR). PALSAR can observe the Earth with various off-nadir angles and has a ScanSAR function that enabled us to investigate incidence angle dependency and to develop an L-band model function. In this study, we investigate the relationship between NRCS from PALSAR and co-located wind data.