An improved technique to estimate the Doppler Centroid (DC) for Spaceborne ScanSAR is introduced. The technique estimates unwrapped baseband measures of DC in the subswaths, and then uses PRF (pulse repetition frequency) diversity between different subswaths to solve Doppler Ambiguity by searching proper ambiguity numbers according to the Minimum Mean Square Error (MMSE) rule. This estimator exploits the full datasets but not only the overlap area data, without knowledge about probability distributions of the signals or noises. We assume that changes of azimuth antenna pointing between subswaths are small enough to guarantee that DC changes much tinier than PRF diversity. This algorithm gives efficient and accurate DC estimation, and is easy to implement.