Abstract: A system for determining the location of a mobile receiver unit includes static transmitter units, each including a respective clock which it uses to transmit a positioning signal according to a respective transmission schedule. The mobile receiver unit receives a positioning signal from any of the static transmitter units. A first processing means uses information relating to the received positioning signal to determine the location of the mobile receiver unit. A second processing means uses information relating to a respective drift and/or offset of each of the clocks of the static transmitter units to generate transmission schedules for the static transmitter units. Each transmission schedule instructs a respective static transmitter unit to transmit a positioning signal at one or more scheduled times according to the clock of the static transmitter unit.