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Baseline is the key factor for highly accuracy terrain elevation measurement in InSAR system. Variation of baseline attitude induced by aircraft's unstable motion leads to phase errors varying along the aperture. The variation will generate prodigious deterioration in DEM's precision. Therefore, in order to reduce the phase errors, critical motion compensation has to be implemented. Here, the sub-aperture method is introduced to resolve the above issue. The new method is described with mathematical analysis and modelling. The influence of the new method on the sensitivity of roll estimation is also analysed. The simulation and real testing show a very positive result. The roll errors are eliminated greatly in case the sub-aperture length is optimised. Moreover, the requirements of SNR and azimuth resolution are satisfied.