A system for estimating distance from a receiver to a communications device, the system comprising, a communications device (1510, 2080, 2090, 2120, 2260, 2330) arranged to periodically transmit a radio frequency signal (1530, 2060, 2070, 2160, 2370, 2480), wherein the periodically transmitted signal includes data; a receiver (1540, 2050, 2170, 2220, 2320) arranged to detect the periodically transmitted signal, to measure the strength of the periodically transmitted signal, and to use the signal strength to provide a first estimate of the receiver's distance from the communications device characterised in that the system further comprises: the communications device including at least one distance sensor (1520, 2030, 2040, 2130, 2270, 2360) for detecting distance information for use in confirming the first estimate, wherein the data includes the distance information, wherein the distance information relates to distance between the communications device and the receiver; the receiver arranged to use the distance information to confirm the first estimate, the confirming the first estimate comprising the distance information indicating a first state; the receiver arranged to perform a function when the first estimate meets at least one criterion and when the distance information indicates the first state. a communications device (1510, 2080, 2090, 2120, 2260, 2330) arranged to periodically transmit a radio frequency signal (1530, 2060, 2070, 2160, 2370, 2480), wherein the periodically transmitted signal includes data; a receiver (1540, 2050, 2170, 2220, 2320) arranged to detect the periodically transmitted signal, to measure the strength of the periodically transmitted signal, and to use the signal strength to provide a first estimate of the receiver's distance from the communications device characterised in that the system further comprises: the communications device including at least one distance sensor (1520, 2030, 2040, 2130, 2270, 2360) for detecting distance information for use in confirming the first estimate, wherein the data includes the distance information, wherein the distance information relates to distance between the communications device and the receiver; the receiver arranged to use the distance information to confirm the first estimate, the confirming the first estimate comprising the distance information indicating a first state; the receiver arranged to perform a function when the first estimate meets at least one criterion and when the distance information indicates the first state. the communications device including at least one distance sensor (1520, 2030, 2040, 2130, 2270, 2360) for detecting distance information for use in confirming the first estimate, wherein the data includes the distance information, wherein the distance information relates to distance between the communications device and the receiver; the receiver arranged to use the distance information to confirm the first estimate, the confirming the first estimate comprising the distance information indicating a first state; the receiver arranged to perform a function when the first estimate meets at least one criterion and when the distance information indicates the first state.