Methods and Systems for Estimating Distance of a Radio Frequency Identification Tag

A method for determining a distance between a radio frequency identification (RFID) tag and a reader, comprising:

• determining a first distance estimate of the distance based upon a signal strength measured from a first response signal of a plurality of response signals received by the reader from the RFID tag in response to a plurality of interrogation signals transmitted by the reader;
• based upon the first distance estimate, generating a function of phase data points with respect to a range of frequencies;
• determining a first phase and a second phase associated with the first response signal and a second response signal of the plurality of response signals, respectively;
• correlating the first and second phases with a first phase data point and a second phase data point of the function, respectively;
• and
• determining a second distance estimate of the distance based upon the correlating.

A non-transitory computer readable storage medium having stored thereon instructions that when executed by a machine result in the following operations:

• determining at least two phases associated with at least two response signals transmitted by a radio frequency identification (RFID) tag at different frequencies;
• determining 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 at least two response signals;
• based upon the first distance estimate, generating a set of phase data points that follows a substantially periodic function over a range of frequencies;
• and
• determining a second distance estimate of the distance based upon the at least two phases and the periodic function.