Abstract: A tag transmits signals having at least two different frequency components. The signals are receivable by at least some of a plurality of spaced receivers. Each receiver, in response to receiving a signal, measures respective phases of the first and second frequency components and a frequency offset between the tag and the receiver. The frequency offset is dependent upon a difference in frequencies of respective clocks in the tag and the receiver. A position processor obtains a difference in phases of the first and second frequency components for a signal, a frequency offset between the receiver and tag and determines the position of the tag in dependence upon the difference between respective phases of the first and second frequency components and in dependence upon the frequency offset.