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An effective method is proposed to remove the flat-Earth phase (i.e., flattening) in airborne single-pass interferometric synthetic aperture radar (InSAR) imaging. Two conventional flattening methods, namely, the orbit-equation algorithm and the fringe-frequency algorithm, are used for comparison. The orbit-equation algorithm is theoretically accurate, but the orbit ephemeris that it requires is maybe inaccurate or even unknown. The fringe-frequency algorithm is effective in spaceborne InSAR flattening, but in airborne cases, a phase trend will remain in the residual interferogram with this method. To overcome these limitations, this letter presents a novel flattening algorithm by combining the two conventional methods for airborne single-pass InSAR flattening. By exploiting a reasonable model of the flat-Earth phase and the spectrum information of the practical interferogram, the proposed algorithm is able to flatten the interferogram accurately without knowledge of the airplane trajectory (except for the airplane height). Finally, some experimental results demonstrate the effectiveness of the proposed flattening algorithm.