Authentication techniques in a monitoring system

Abstract

An Electronic House Arrest Monitoring (EHAM) system of the present invention includes a transmitter attached to a monitored offender and a receiver positioned in the vicinity of the desired monitoring location. The transmitter and the receiver each have a pattern stored or generated therein for determining the interval of time between signal transmissions and receptions. The pattern is pseudo-random to offer improved protection against imposter transmitters, in other words the time intervals are selected to be unequal and randomly varying but the pattern is repeated or cycled to allow continuing signal transmission by the transmitter. The transmitter transmits signals at varying time intervals according to this pattern, and the receiver authenticates signals it receives as coming from the monitored transmitter based on the expected time intervals from the pattern. The pattern may include any number of time intervals and, in one embodiment, includes more than one subpattern to extend the length of the repeat cycle to increase the difficulty of defeating the monitoring system. As further protection against imposter devices, the receiver may verify the signals based on information unique to the transmitter included in each signal. The receiver may further use the received signals and unique time interval patterns to determine a number of operating states, including New Transmitter ID Received, Transmitter In Range and Locked to Transmitter, Transmitter In Range and Not Locked, Transmitter Out of Range, and Imposter Likely. The monitoring system may also include a remote host computer linked to the receiver or receivers to enable these operating states to be monitored remotely by enforcement personnel.