A global positioning system -based access control method and apparatus are disclosed for limiting access to a device or secure facility by verifying the location of an authorized user. While an annular magnetic body having a plurality of annular, unmagnetized magnetic encoder tracks integral therewith and juxtaposed relative to each other is rotated, the tracks of the magnetic encoder are individually magnetized by a magnetizing head, made up of a magnetizing yoke and an exciting coil, to thereby provide the magnetic encoder. A locking member secures the plate member to the coupling member.