Removal of Ambiguous Wind Directions for a Ku-Band Wind Scatterometer Using Three Different Azimuth Angles

The Seasat-A satellite scatterometer (SASS) sensor demonstrated very successfully that Ku-band scatterometers can make accurate synoptic measurements of surface wind speed over the ocean. Because SASS provided normalized radar cross section (NRCS) measurements from only two azimuths, however, the harmonic relationship of NRCS with azimuth results in up to four ambiguous wind directions. The primary improvement to be incorporated in a next-generation scatterometer design such as Navy Remote Ocean Sensing System (NROSS) is the addition of a third azimuth look at each sampled cell. With this and other instrument improvements, preliminary studies indicate that wind-direction ambiguities (aliases) could successfully be removed in at least 80 percent of the cases. Furthermore, these studies show that in over 90 percent of the wind solutions, the two most probable solutions correctly identify the wind streamlines. Methods were studied which could examine typical streamline patterns derived from scatterometers using continuity or pattern-recognition techniques to determine which of the possible two wind directions was correct. In addition, unambiguous solutions were sought for cases where streamlines were not correctly defined. This paper describes several approaches for such alias-removal algorithms. These algorithms were developed with the aid of simulated three-beam scatterometer ambiguous wind-solution data (based on NOSS conditions) over a known windfield. The resulting algorithms were evaluated using a different set of simulated orbital data, but withholding the true winds.