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Phase adjustment by contrast enhancement (PACE) is an autofocus algorithm that is capable of performance that is unattainable by conventional techniques. It is a nonparametric method that requires no constraints on the type of phase error to be measured. The algorithm does not require special data culling techniques or the presence of isolated scatterers. However, the drawback of PACE algorithm is that the number of estimated variables is very large; it leads to a long computational time. The azimuth sampling frequency is commonly much bigger than the bandwidth of phase error in SAR image so that we can estimate part of the phase error variables and then obtain the whole variables by interpolation; this induces the interpolated phase adjustment by contrast enhancement (IPACE) algorithm. The IPACE algorithm can remarkably reduce the computational time while maintaining the accuracy. This letter has derived the detailed processing of IPACE, and the results of the experiments using real SAR data are presented to show the validity of the proposed algorithm.