IMPROVEMENTS IN OR RELATING TO TIME-OF-FLIGHT RANGING SYSTEMS

摘要

A time-of-flight ranging system, such as a keyless access control system, comprises a first part (10) and a second part (12), which may implemented as a portable device such as a key fob. Both parts have signal transmitting and receiving means (14, 22 and 26, 32) for effecting communication with each other. At least the first part comprises means for determining the distance between the first and second parts based on time-of­flight. In order to save power, when that first and second parts are a relatively great distance apart, the time-of-flight measuring means computes the time based on a relatively coarse algorithm, and when the parts are relatively close, the computation is carried-out using a more precise algorithm. In variants and refinements of the system the clock frequency may be reduced when the first and second parts are a relatively great distance apart and increased when they are closer and the transmitter power may be reduced when the first and second parts are relatively close together and increased when they are a relatively great distance apart. The time-of-flight ranging system may be applied to not only entry security systems but also to tracking systems such as systems for tracking toddlers, personnel and equipment.