Accuracy of scatterometer-derived winds using the Cramer-Rao bound

A wind scatterometer makes measurements of the normalized radar-backscatter coefficient /spl sigma//spl deg/ of the ocean surface. To retrieve the wind, a geophysical model function (GMF), which relates /spl sigma//spl deg/ to the near-surface wind, is used. The wind vector can be estimated using maximum-likelihood techniques from several /spl sigma//spl deg/ measurements made at different azimuth angles. The probability density of the measured /spl sigma//spl deg/ is assumed to be Gaussian with a variance that depends on the true /spl sigma//spl deg/ and therefore, depends on the wind through the GMF. With this model for wind estimation, the Cramer-Rao (C-R) bound is derived for wind estimation, and its implications for wind retrieval are discussed. As part of this discussion, the role of geophysical modeling error is considered and shown to play a significant role in the performance of near-surface wind estimates. The C-R bound is illustrated using parameters from the ERS AMI, NSCAT, and Sea Winds scatterometers.