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The authors' objective is to develop an efficient algorithm to focus two-dimensional and three-dimensional SAR images under the following extreme conditions: large coherent integration angles and high bandwidth-to-centre-frequency ratios. These conditions constitute a common situation in wideband high-resolution SAR configurations, as in anechoic chambers and in ground-based systems. The algorithm presented is an extension of the well known chirp scaling algorithm (CSA). First, the original 2-D CSA is generalised by extending the formulation from data obtained with a linear aperture to data measured by a planar synthetic aperture. Moreover, a compact generalised formula of the impulse response function in the three-dimensional frequency domain is presented. These extensions enable the development of a 3-D algorithm under the aforementioned conditions. Finally, the algorithm has been implemented and tested with satisfying results.