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In this letter, a new improved step transform (IST) algorithm is developed for highly squint stripmap synthetic aperture radar (SAR) imaging. Different from the previous step transform algorithms, such as the time-varying step transform algorithm, the new algorithm compensates the cubic phase term and the phase term caused by the varying chirp rate using estimation of the target position focused in the coarse-resolution fast Fourier transform, and these undesirable phase terms can be compensated more appropriately. The algorithm can process SAR imaging under a squint angle of up to 75° in X-band. Simulation results show that the IST algorithm is effective.