A novel RFID synthetic aperture radar (SAR) based localization method using an antenna trajectory estimated using known reference tags of known location is proposed and demonstrated to find the location of target tags who's location is not known. A robot with an integrated RFID reader and antenna is used to obtain phase measurements of both the reference and target tags in an indoor environment. The trajectory of the moving RFID antenna is estimated using an Inverse SAR (ISAR) sensing algorithm. The trajectory is then combined with the phase measurements of the target tags to compute their location using a conventional SAR algorithm. The performance is compared to target tags located by SAR where the antenna trajectory is measured by LiDAR. Experimental results show similar performance (15cm mean absolute error) using both LiDAR-SAR and ISAR-SAR algorithms. Compared to previous SAR based systems, the ISAR-SAR based RFID system is a cost-effective solution which is more commercially attractive for inventory tracking applications.