Monitoring systems in which a receiver monitors the presence or absence of a transmitter in the vicinity of the receiver are known in the art. Such monitoring systems have been used for monitoring persons or things in various applications, including monitoring criminal justice offenders under house arrest, monitoring locations of trucks or rail cars, monitoring movements of wild animals, and the like.
Electronic house arrest monitoring (EHAM) systems allow for monitoring and some limited control of criminal justice system offenders without the much greater expenses associated with incarceration of such offenders. In a typical EHAM system, criminal justice system offenders are under parole or probation requirements to remain at certain locations during specified times. For example, an offender may be ordered to remain at his/her home from 7 p.m. to 7 a.m. on weekdays and all day on weekends. In other words, the offender in this example could be required to be at home at all times other than between 7 a.m. and 7 p.m. on weekdays, but he/she could be allowed to travel to and from work at his/her place of employment during that period away from home.
A typical EHAM system includes a transmitter (sometimes called a “tag”) attached in some manner to the offender, such as by a bracelet or band semi-permanently affixed to the wrist or ankle of the offender. A receiver, also known as a field monitoring device (FMD), is centrally located in the offender's home and is provided with or connected to some communication system for communicating with a remote computer or host computer located at a central monitoring facility, where the compliance of the offender with the location and travel restrictions imposed on him or her is monitored. The receiver (FMD) is programmed to flag the presence or the absence of the transmitter, thus also the offender, in the vicinity of the receiver (FMD) based on the reception or the absence of a reception by the receiver (FMD) of signals generated by the transmitter. For example, the receiver (FMD) in the offender's home may be connected to a standard telephone line so that the receiver (FMD) can place calls to or receive calls from the host computer at the central monitoring facility. Alternatively, the receiver (FMD) at the offender's home may communicate with the host computer at the central monitoring facility via a cellular telephone link, a dedicated RF link, a cable television (CATV) link, an optical link, or some other communication link. Wireless communications links are especially useful in more sophisticated systems in which the receiver (FMD) may be mobile and tracked by a tracking or locating system, such as a global positioning satellite (GPS) system, a ground-based RF tower locating system, or the like.
Typically, the transmitter on the person or device being monitored is powered by a battery and transmits a RF signal at intermittent time intervals, which RF signal is detectable and read by the receiver (FMD). Such RF signals from the transmitter are typically not transmitted continuously in order to reduce the power consumed by the transmitter and to conserve power available from the battery. While not continuous, the RF signal transmissions must, at a minimum, be often enough to enable reasonable monitoring of the offender's presence or absence from home or from other places where the offender's presence or absence is being monitored. For example, depending to some extent on remoteness of the place monitored and other circumstances, monitoring may not be effective with RF signal transmissions from the transmitter on the offender that occur only once every ten hours, but transmitting RF signals once every ten seconds may be superfluous for monitoring. At the same time, it is desirable for the transmitter battery to have a sufficiently long life to minimize the amount of time that probation or parole officers spend replacing transmitter batteries, and superfluous transmissions would consume power and shorten transmitter battery life.
The receiver (FMD) includes a RF receiver circuit for receiving the relatively low power transmissions from the transmitter on the offender. Simple loop-type antennas are typically used in the transmitters for cost and weight reasons. Since many antennas, including loop-type antennas, have inherent null points at particular angular directions, it is common for each transmitter to include two antennas oriented differently in relation to the transmitter circuitry to make the transmissions more detectable regardless of angular orientation in relation to the receiver (FMD). The transmitter may also alternate sequential transmissions between the two differently-oriented antennas.
It is desirable to design electronic house arrest monitoring (EHAM) systems so that they are not foiled easily by transmissions from impostor transmitters. For example, there are consumer devices available that have the ability to record and then duplicate transmissions from remote controls for other consumer devices, such as infrared control signal transmissions for audio and video equipment, RF transmissions for garage door openers, remote keyless entry system transmissions for vehicles, and the like. Such devices, also known as code grabbers, may be useful as an all-in-one remote control. While most such code grabbers available to consumers work only in the infrared region or for amplitude-modulated (AM) RF signals, it is conceivable that similar code grabber devices could be constructed to record and duplicate frequency-modulated (FM) RF signals, which may be used in EHAM systems. Such a FM code grabber signal duplicating device could conceivably be used by a confined offender, who is wearing a transmitter, to fool an EHAM system into indicating that the transmitter worn by the offender is still in the vicinity of the receiver (FMD) in the offender's home, when the receiver (FMD) is instead actually receiving duplicated signal transmissions from the FM code grabber signal duplicating device positioned near the receiver (FMD).
At least one attempt has been made to authenticate signal transmissions to ensure that such signal transmissions are sent from a particular transmitter and not from some other source, as disclosed in U.S. Pat. No. 5,032,823. That patented system utilizes a single bit in each transmission bit-string or alternates its state (i.e., from zero to one or vice versa) in consecutive transmissions. Additionally, each transmission is sent at fixed time intervals. Unfortunately, such a system is easily fooled by an impostor transmitter that produces a similar bit in each transmission and which toggles such bit as well. In other words, such a signal transmission authentication scheme is too simple.
It is against this background and with the desire to provide better protection against non-authentic transmissions, i.e., to detect possible imposter transmissions, that the present invention has been developed.