Fast Factorised Backprojection(FFBP) is a Synthetic Aperture Radar (SAR) focusing technique which uses Filtered Backprojection(FBP) to produce high quality images in the spatial domain at speeds which rival spectral domain SAR focusing methods. Synthetic Aperture Radar (SAR) focusing is fundamentally a matched filtering process which can be performed both in the spatial or spectral domain. Spatial domain SAR focusing techniques like filtered backprojection can, in theory, completely recover the scene from data with an infinite bandwidth acquired using an isotropic antenna along an arbitrary curved flight path of infinite length. This means that the focusing of a image using filtered backprojection is only limited by a finite bandwidth, non isotropic beam width and a finite flight path length. However filtered backprojection with an operation count which is proportional to 0 (n3 ) is computationally inefficient in speed when compared to the frequency domain SAR image reconstruction algorithms with an operational count which is proportional to 0 (n2 10g (n)).