A localization method and system, including determining phases of a plurality of response signals at a plurality of frequencies received from a radio frequency identification (RFID) tag in response to interrogation signals by a reader at the plurality of frequencies; calculating a first distance estimate of a distance between the RFID tag and the reader based on a signal strength measured from a first response signal of the plurality of response signals; based on the phases and the first distance estimate, calculating a phase slope; and determining a second distance estimate of the distance based upon the phase slope.