A method of determining the position of a plurality of a radio transmitter units relative to a master unit is disclosed in which a control signal is provided from the master unit commanding each of the radio transmitter units to transmit a test signal and the remaining units to receive. The arrival times of the test signals are measured at the receiving radio transmitter units, and the position of each radio transmitter unit relative to the master unit is calculated solely on the basis of the measured arrival times, and an approximate initial position for each unit.