In this letter, we propose a method to improve antenna model accuracy for active phased array SAR antennas. The antenna model is mathematical modeling to represent real radiated patterns of the SAR antenna. Unidentified systematic error contributions degrade the accuracy of the antenna model. Unlike previous approaches that only perform on-ground validation of antenna model accuracy or in-orbit extraction of the beam pattern, the proposed method can be utilized to improve the accuracy using optimization of the error matrix based on a convex algorithm. The proposed method is validated by measured results of a manufactured antenna that is composed of 32 modules and is also analyzed in terms of SAR performance. According to numerical comparisons to known optimal solutions, the results show the proposed method provides high accuracy and less computation time.