Method and apparatus for identifying remote sending units in a tyre pressure monitor system of a vehicle using secondary modulation of wheel rotation

A remote tire monitoring system (10) includes tire monitors (12) positioned at respective wheels of a vehicle (V) for transmitting radio signals modulated by tire data and modulation means positioned above near the tire monitors for imposing a secondary modulation on the radio signals. A receiver (14) receives the radio signals and recovers the tire data (26) from the radio signals and associates the tire monitor with the respective wheels of the vehicle using the secondary modulation.