In the Internet-of- Things paradigm, the knowledge of item/people position becomes crucial to develop new services and applications. Autonomous vehicles, robots and drones rep-resent key enablers to implement cutting-edge solutions among with localization of items or people. In this paper, we present the adoption of an Unmanned Aerial Vehicle (UAV) equipped with a Radio Frequency Identification (RFID) system to locate passive RFID tags in outdoor scenario. In particular, we apply a synthetic-aperture-radar approach and we discuss how the position uncertainty of the reader-antenna trajectory affects the tag localization uncertainty.