Patent ID: 7228228

Claim:
A position determining system comprising: a tag which is operable to transmit a signal comprising first and second frequency components having a frequency spacing therebetween; a plurality of receivers having known relative positions, each being operable to receive the signal transmitted from the tag; a processor operable to process the signal received at each receiver to determine, for each received signal, a phase measurement for the first frequency component and a phase measurement for the second frequency component; a calculator operable to calculate a phase difference measurement for each received signal by subtracting the phase measurement for the first frequency component from the phase measurement of the second frequency component; and a position determiner operable to determine the relative position between the tag and the receivers on the basis of the calculated phase difference measurements for the received signals and the known relative positions of the receivers; wherein said tag is a transmit only tag that operates asynchronously with respect to said receivers.