Patent ID: 8195422

Claim:
A method of deriving a measure of the distance between first and second devices having respective clocks which may have different clock rates, the method comprising the act of: transmitting a first signal from the first device to the second device, the first signal having a frequency derived from the clock of the first device; in response to receiving the first signal at the second device, transmitting a second signal from the second device to the first device, the second signal having a frequency derived from the clock of the second device; at the second device, calculating a measure of a turnaround time at the second device relative to the clock of the second device, the turnaround time being a time between the first signal being received and the second signal being transmitted; calculating a frequency measure of the frequency of the at the first and second devices relative to each other, wherein the frequency measure comprises a measure of a ratio of the clock frequencies; using the frequency measure to re-calculate the turnaround time relative to the clock of the first device; and at the first device, using the re-calculated turnaround time to derive a measure of the distance between the first and second devices.