The range Doppler algorithm (RDA) is widely used for synthetic aperture radar (SAR) imaging. Correct estimation for Doppler centroid is essential for range cell migration correction, azimuth compression, secondary range compression in RDA. This paper presents two methods to assess and improve the Doppler parameter estimation results. The symmetry of the Doppler spectrum is examined to evaluate the quality of coarse estimation of baseband Doppler centroid and the weighted least squares algorithm is used to derive the refined results. Selective window and weighted combining based on the quality index of the average power are adopted for detecting Doppler ambiguity. From the simulation results, we show that the proposed techniques with the appropriate quality index can improve the performance of estimation compared to the conventional schemes for real-time SAR signal processing.