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Keystone formatting [Doerry 2012] for spotlight synthetic aperture radar is an alternative to the popular polar format and reduces the computational burden of resampling the polar data to a rectangular grid before inverse Fourier transformation by a fast Fourier transform—-the polar format algorithm. After a brief review of relevant geometry and signals including linear frequency modulation and stepped sines, it is shown how to convert a polar-format radar into a keystone-format radar. Reconstructions from simulated data in both formats are shown and the differing kinds of image aliasing are examined.