This paper investigates the issue of phase offset calibration in the localization of Radio Frequency Identification (RFID) tags by using Synthetic Aperture Radar (SAR) methods. Although these techniques demonstrate robustness in handling challenges like multipath interference, the phase offset calibration represents an open issue which deserves further investigation. Indeed, it can be affected by antenna radiation pattern and polarization mismatch between reader antenna and tag. In this paper, we propose a phase offset calibration method based on polynomial fitting and we analyse its effectiveness by using an antenna motion system implemented through a Cartesian gantry portal in indoor scenario.