Method and apparatus for determining the position of a mobile communication device using low accuracy clocks

A mobile communication device capable of determining range to a reference communication device by exchanging ranging signals with the reference communication device, comprising:

• means for transmitting to the reference communication device a sequence of outbound ranging signals at different carrier frequencies;
• means for receiving from the reference communication device a sequence of reply ranging signals at the different carrier frequencies in response to the outbound ranging signals;
• means for selecting from among the reply ranging signals a reply ranging signal at a carrier frequency providing a highest signal timing accuracy;
• and
• means for determining a time of arrival of the selected reply ranging signal and the range to the reference communication device from a round-trip signal propagation time of the selected reply ranging signal and a corresponding outbound ranging signal.

A method of determining the range between a mobile communication device and a reference communication device, comprising the steps of:

• (a) transmitting a sequence of outbound ranging signals at different carrier frequencies from the mobile communication device to the reference communication device;
• (b) transmitting a sequence of reply ranging signals at the different carrier frequencies from the reference communication device to the mobile communication device in response to the outbound ranging signals;
• and
• (c) determining the range between the mobile communication device and the reference communication device from a round-trip signal propagation time of a selected outbound ranging signal and a corresponding reply ranging signal transmitted at a carrier frequency providing a highest signal timing accuracy.

A position location system for determining the position of a mobile communication device, comprising:

• a plurality of reference communication devices having known positions, each configured to transmit and receive ranging signals;
• and
• a mobile communication device configured to exchange ranging signals with said reference communication devices, said mobile communication device transmitting to each reference communication device a sequence of outbound ranging signals at different carrier frequencies, each of said reference communication devices transmitting a sequence of reply ranging signals at the different carrier frequencies in response to the outbound ranging signals, wherein;
• said mobile communication device determines the range to each reference communication device from a round-trip signal propagation time of a selected outbound ranging signal and a corresponding reply ranging signal transmitted at a carrier frequency providing a highest signal timing accuracy, and determines the position of said mobile communication device from the known positions of said reference communication devices and the range to each reference communication device.