The correct determination of the antenna phase center is a key point when performing UHF-RFID localization through a phase-based method. In this paper, we investigate the effect of a wrong knowledge on the reader antenna phase center in phase-based methods exploiting the relative motion of the reader antenna with respect to the stationary tags through a Synthetic Aperture Radar approach. An experimental analysis is carried out in a real scenario with the reader antenna attached on a robotic-wheeled walker.