Abstract: This paper concerns the time and frequency domain analysis of the along-track motion error in synthetic aperture radar (SAR). The signal model with motion error is established. The effects on the range migration and azimuth modulation are obtained. Based on the series reversion laws, the two dimensional point target spectrum is derived. Then, the error features are formulated. Analysis and simulation results show that the echo characteristics in frequency domain are more sensitive to the motion error. Above-mentioned study provides a theoretical basis for system analysis and imaging algorithm design of SAR.