An invention is provided for monitoring an item, such as a container. The invention includes a computer in communication with a receiver that receives signals from a remote source. A transmitter in communication with the computer receives values from the computer based on the signals received from the remote source. These values are then transmitted to a security device associated with the item, which includes an authentication table comprising a plurality of initial values. The security device then records values transmitted from the transmitter. The values can be jitter values, with each jitter value being a difference in arrival times of at least two timing signals from the same remote source.