A differential positioning system (10) includes components of a satellite-based or land-based positioning system (12) and components of a mobile communications network (14). The differential positioning system (10) provides accurate and immediate position information to a mobile unit (17). A transmitter site (40) of a mobile communications network (14) is associated with a reference positioning receiver (38). The reference positioning receiver (38) generates correction data for transmission to the mobile unit (17). The mobile unit (17) includes a mobile communications device (42) for receiving the correction data generated by the reference positioning receiver (38) and a mobile positioning receiver (24) for generating a position fix. The mobile unit (17) refines the position fix generated by the mobile positioning receiver (24) using correction data received by the mobile communications device (42).