Precise positioning of RFID tags using a phased array antenna

A means of precisely determining the position of an RFID tag, relative to the tag reader, has been developed and applied to the problem of determining the location of an urban rail train along its guideway. With this scheme the position can, in principle, be determined to centimeter accuracy using a sensing antenna array consisting of two identical elements which measures the abrupt 180 degree phase change that occurs in the differential phase of the sum and difference signals formed by the array as the array passes over a tag. Electromagnetic modelling confirms that a reliable phase detection signal can be derived from an array located under a train with RF tags located on the guideway.