Tag tracking

A position processor for determining the position of a transmit-only tag relative to a plurality of receivers, the receivers having known relative positions and being operable to receive a signal comprising first and second frequency components having a frequency spacing therebetween transmitted from the tag which operates asynchronously with respect to the receivers, the position processor comprising:

• a processor operable to receive a plurality of sets of phase measurements, each set associated with a respective one of the receivers and each set comprising a phase measurement for the first frequency component and a phase measurement for the second frequency component of the signal received at the corresponding receiver;
• a calculator operable to calculate a respective phase difference measurement for each set of phase measurements;
• and
• a position determiner operable to determine the relative position of the tag on the basis of the calculated phase difference measurements and the known relative positions of the receivers.

A position determining method for determining the relative position between a transmit only tag and a plurality of receivers whose relative positions are known, the method comprising the steps of:

• operating the tag asynchronously with respect to the receivers;
• transmitting from the tag a signal comprising first and second frequency components having a frequency spacing therebetween;
• receiving at each receiver the signal transmitted by the transmitter;
• processing 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;
• calculating 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
• determining 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.