Algorithms widely used for Doppler parameter estimations in SAR processing, show some limitations when platform parameters are time-varying. In this paper an alternative technique, based on the Wigner distribution, will be proposed, to reconstruct the full Doppler history of an emerging point target and then to estimate the Doppler rate in short time intervals (sub-apertures). Moreover, the Doppler centroid will be estimated by using instantaneous frequency corresponding to antenna boresight. Methods for the sub-aperture focalization of emerging target point are also presented. Results obtained with satellite and airborne SAR signals are reported.