Methods and systems for determining vehicle position in an automatic vehicle identification system

Methods of estimating vehicle location in a roadway (12) using an automatic vehicle identification system are described. The methods involve receiving a set of response signals from a vehicle-mounted transponder (20, 206) at points in time (t1-t5) and determining a range rate of the transponder (20, 206) relative to the antenna (18, 104, 106, 204, 302, 402a-402e, 602, 604, 606) at each point in time (t1-t5); identifying a minima in the magnitude of the range rate; estimating a first position (pos0) of the transponder (20, 206) at a first time (t0) corresponding to the occurrence of the minima; estimating a velocity (v) of the vehicle (22, 202) based upon one or more of the determined range rates; and estimating a second position (pos1, pos2, pos3) of the transponder (20, 206) based upon the first position (pos0) and the velocity (v).