An Improved Doppler Centroid Estimator Meeting the Cramér–Rao Bound for Ocean SAR Application

The Doppler information in complement to the intensity of radar backscatter has been verified useful for ocean synthetic aperture radar (SAR) applications. An accurate estimate of the ocean surface motions requires an accurate estimate of the Doppler centroid. When we process the real SAR data, we find that different methods yield different mean values of the estimates. After a careful analysis, we find that it is caused by the underestimation of the centroids for conventional methods. The estimate bias depends on the Doppler centroid itself. To overcome this problem, we carefully study the conventional methods and the Cramér–Rao bound of the estimation. Based on the maximum-likelihood principle, a novel method is proposed. By adaptively adjusting the center and the bandwidth of the weighting function after a coarse estimation of the spectrum, the symmetry of the spectrum is maintained. The estimate bias of conventional methods is eliminated. It is a fully optimal method meeting the Cramér–Rao bound no matter whether the true Doppler centroid is high or low. Both the simulated and the real SAR data validate our novel method. In the simulation, an estimate error of tens of Hz could be avoided when the true Doppler center is hundreds of hertz. In real SAR estimation, an accuracy of 3–6 Hz can be improved. The result of the estimation is consistent with the simulation and theoretical analysis, validating the efficiency of our improved method.