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A position processor for determining the relative position between a first member and a plurality of second members, the first and second members being movable relative to each other and the position of each of the second members relative to the other second members being known and the first and second members being arranged so that upon the transmission of a signal comprising first and second different frequency components between the first member and the plurality of second members, there is generated a plurality of received signals each associated with a respective transmission path between a respective second member and the first member, the position processor comprising:

• a first phase differencing calculator operable to calculate, for each received signal, a phase difference between a phase measurement of the first and second frequency components of the received signal, to generate a first set of phase difference measurements;
• a first position determiner operable to determine a first position measurement of the relative position of the first member and the plurality of second members from the first set of phase difference measurements and the known relative positions of the second members;
• a second phase differencing calculator operable to calculate phase difference measurements between phase measurements of frequency components of different received signals, to generate a second set of phase difference measurements;
• a second position determiner operable to determine a second position measurement of the relative position between the first member and the plurality of second members from the second set of phase difference measurements and the known relative positions of the second members;
• and
• a third position determiner operable to determine the relative position between the first member and the plurality of second members using the first and second position measurements.

A receiver for receiving a plurality of signals transmitted from a plurality of transmitters located at different locations, each transmitter being operable to transmit a signal having first and second different frequency components, the receiver comprising:

• a signal receiver operable to receive the signal transmitted by each transmitter;
• a signal processor operable to process each received signal to determine a corresponding phase measurement for the first frequency component thereof and a corresponding phase measurement for the second frequency component thereof;
• a first phase differencing calculator operable to calculate, for each received signal, a phase difference between the phase measurements of the first and second frequency components of the received signal, to generate a first set of phase difference measurements;
• a second phase differencing calculator operable to calculate phase difference measurements between phase measurements of frequency components of different received signals, to generate a second set of phase difference measurements;
• and
• an output operable to output said first and second set of phase difference measurements for transmission to a central position processor.

A position determining method for determining the relative position between a first member and a plurality of second members, with the first and second members being movable relative to each other and with the position of each of the second members relative to other second members being known;

• the method comprising:
• transmitting a signal comprising first and second different frequency components between the first member and the plurality of second members;
• receiving a plurality of signals, each associated with a respective transmission path between a respective second member and the first member;
• processing each received signal to determine a corresponding phase measurement for the first frequency component thereof and a corresponding phase measurement for the second frequency component thereof;
• calculating, for each received signal, a phase difference between the phase measurements of the first and second frequency components of the received signal, to generate a first set of phase difference measurements;
• determining a first position measurement of the relative position of the first member and the plurality of second members from the first set of phase difference measurements and the known relative positions of the second members;
• calculating phase difference measurements between phase measurements of frequency components of different received signals, to generate a second set of phase difference measurements;
• determining a second position measurement of the relative position between the first member and the plurality of second members from the second set of phase difference measurements and the known relative positions of the second members;
• and
• determining the relative position between the first member and the plurality of second members using the first and second position measurements.

A computer readable medium storing computer executable instructions for causing a programmable computer device to determine the relative position between a first member and a plurality of second members, the first and second members being movable relative to each other and the position of each of the second members relative to the other second members being known and the first and second members being arranged so that upon the transmission of a signal comprising first and second different frequency components between the first member and the plurality of second members, there is generated a plurality of received signals each associated with a respective transmission path between a respective second member an the first member, the computer executable instructions comprising:

• instructions for calculating, for each received signal, a phase difference between a phase measurement of the first and second frequency components of the received signal, to generate a first set of phase difference measurements;
• instructions for determining a first position measurement of the relative position of the first member and the plurality of second members from the first set of phase difference measurements and the known relative positions of the second member;
• instructions for calculating phase difference measurements between phase measurements of frequency components of different received signals, to generate a second set of phase difference measurements;
• instructions for determining a second position measurement of the relative position between the first member and the plurality of second members from the second set of phase difference measurements and the known relative positions of the second member;
• and
• instructions for determining the relative position between the first member and the plurality of second members using the first and second position measurements.