Sub-aperture Doppler rate estimation and SAR data focalization

Synthetic aperture radar (SAR) is an imaging sensor today widely used in remote sensing applications, in order to obtain high resolution ground images. The focusing quality of the SAR image highly depends on the update rate and accuracy of Doppler rate estimations obtained from sensor's raw-data directly. The autofocus algorithm normally used for this purpose works under appropriate conditions only. So, it requires a strictly linear Doppler frequency modulation and that the whole Doppler history may be taken into account. In this paper an alternative technique is proposed, based on the Wigner distribution analysis of SAR data. The new technique allows the azimuth integration time to be divided into some sub-apertures and then follows with good accuracy the platform attitude variations. Results of its application to actual SAR data are reported.