Abstract: This paper describes an improved Doppler centroid estimation scheme, which determines the pulse repetition frequency (PRF) fractional part of the Doppler centroid and resolves the problem of Doppler ambiguity. Experiments show that the scheme could estimate the Doppler centroid accurately and rapidly for synthetic aperture radar (SAR) under the high velocity diving model. The scheme involves two estimation methods. First, the attitude and velocity data, which is provided by inertial navigation system (INS), are used to resolve the ambiguity. Second, the raw data is used to evaluate the PRF fractional part accurately. Combined together with the above measures, the Doppler centroid estimation for diving model can be achieved quickly with sufficient accuracy.