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Stepped frequency chirps can be used to generate high resolution range profiles (HRRP) by using spectrum synthesis. There are three distributed modes of stepped frequency SAR (SF-SAR). However, the presence of channel phase errors may degrade the performance of HRRP synthesis. This letter presents a unified error estimation method to address this problem. First, to obtain a focused sub-band image, a range phase adjustment by contrast enhancement (RPACE) algorithm is proposed to estimate inner-channel high-order phase errors. Second, a side-lobe balanced model (SLBM) is established to estimate constant phase error from the relationship between the balanced side-lobe and constant phase; the constant phase error can be directly obtained in an efficient manner. Experimental analysis using real data demonstrates the effectiveness of the proposed method.