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Squint Terrain Observation by Progressive Scans (TOPS) SAR imaging mode is confronted with three issues: azimuth spectrum aliasing, severe range-azimuth coupling and azimuth time output aliasing. To solve these issues, a subaperture imaging algorithm is proposed based on the SPECtral ANalysis (SPECAN) technique. Firstly, the subaperture data is expanded in the azimuth time domain and the two-dimensional spectrum free from aliasing can be obtained. Then the modified Range Migration Algorithm (RMA) is used to complete Range Cell Migration Correction (RCMC) and range compression. After that, the full-aperture azimuth spectrum can be derived by subaperture recombination. Finally, the signal is focused in the Doppler domain by the SPECAN technique. Simulation results demonstrate the effectiveness of the proposed method.