Patent ID: 7629919

Claim:
A method of determining a distance between a first device and a second device, comprising, at the first device, transmitting a signal comprising simultaneous first and second components, wherein the first component comprises a repeated first code and the second component comprises a repeated second code and the first and second codes are of unequal duration, wherein the first component is frequency or phase modulated onto a carrier forming a direct sequence spread spectrum (DSSS) signal and the second component is amplitude modulated and occupies nulls in the DSSS signal, and at the second device: receiving the signal; detecting the first and second codes; determining from the detected first and second codes respective first and second indications of the distance; comparing the first and second indications of the distance; generating a third indication of the distance in response to the first and second indications of the distance being equal within a predetermined tolerance, and initiating an alarm if the third indication of the distance is above a predetermined threshold value.