Patent Description:
An offender may be fitted with an electronic tag in order to enforce conditions prescribed to the offender as a punishment for an offence, as a condition of their release on bail or as a condition of their early release from prison. Examples of conditions that can be enforced by an electronic tag are curfews (which prevent the offender leaving their home at certain times, especially at night) and exclusion zones (which prevent the offender from visiting certain places, such as a victim's home). Electronic tags can also be used to monitor the location of vulnerable patients, such as the elderly or people with mental health disorders.

Electronic tags typically send and receive radiofrequency communications with other devices. For example, the electronic tag may determine its location by exchanging radiofrequency signals between the electronic tag and a fixed monitoring unit, or by receiving GPS signals from GPS satellites. Also, the electronic tag may send radiofrequency communications, for example, to raise an alarm when a prescribed condition is breached.

Sometimes, the person wearing the electronic tag would like to disguise their location, for example, to allow them to break their curfew or visit a location in one of their exclusion zones. Removing the electronic tag is usually not an option as electronic tags tend to be securely attached to the person and have tamper detection, making it impossible to remove the electronic tag without detection. So, the person wearing the tag may instead try to prevent the electronic tag from making and receiving radiofrequency communications in an attempt to disguise their real location or in an attempt to prevent the electronic tag from reporting or raising an alarm.

One way to attempt this is to deliberately block the radiofrequency communications, for example, by wrapping a sheet of metal foil around the electronic tag. <CIT> attempts to determine if the radiofrequency communications are being deliberately blocked by measuring reflections caused by the metal foil. However, this technique cannot distinguish metal foil from other objects surrounding the electronic tag, including the body of the person wearing the electronic tag. This tends to lead to false positives which need to be investigated, wasting resources.

It would, therefore, be desirable to find a way to more accurately identify deliberate blocking caused by metal foil being wrapped around an electronic tag.

<CIT> and <CIT> both disclose tracking devices that are able to determine if radio-frequency communications to or from the tracking device are being deliberately blocked by an object, such as metal foil, placed in proximity to the device.

According to a first aspect of the invention, there is provided an electronic tag configured to be secured to a body of a person. The electronic tag comprises an antenna configured to transmit or receive an electromagnetic signal and an inductive proximity sensor. The inductive proximity sensor is configured to determine a proximity of a metallic object to the electronic tag, based on electromagnetic coupling between the inductive proximity sensor and the metallic object. The inductive proximity sensor is also configured to determine, based on the proximity being below a proximity threshold, that a metallic object is attenuating the electromagnetic signal transmitted or received by the antenna.

By using an inductive proximity sensor, it is possible to distinguish metallic obstructions (such as metal foil wrapped around the electronic tag) which are likely to represent deliberate attempts to block the electromagnetic signal transmitted or received by the antenna, from everyday non-metallic objects in the vicinity of the electronic tag (such as the body of the person wearing the electronic tag). The inductive proximity sensor includes an inductor (such as a coil) configured to generate a first alternating magnetic field which has no effect on a non-metallic object but which is coupled into nearby metallic objects (such as metal foil wrapped around the electronic tag) inducing eddy currents. These eddy currents generate a second alternating magnetic field in the metallic object which is coupled back into the inductor, leading to a measurable change in the inductance of the inductor which can be related to the proximity of the metallic object. Since the inductive proximity sensor can more accurately identify deliberate blocking (such as that caused by metal foil wrapped around the electronic tag), the inductive proximity sensor is much less prone to false positives.

It may be determined that a metallic object is attenuating the electromagnetic signal transmitted or received by the antenna in response to both the proximity being below the proximity threshold and based on a signal strength of the electromagnetic signal. For example, the signal strength may indicate that the electronic tag is unable to receive an electromagnetic signal. Alternatively, the signal strength may have reduced while the proximity has been below the proximity threshold and optionally the signal strength may vary with the value of the proximity.

By using both the proximity being below the proximity threshold and the signal strength of the electronic signal, the electronic tag may more reliably distinguish metallic objects that happen to be in the vicinity of the person (such as furniture, radiators, etc) from deliberate attempts to block transmission or reception of electromagnetic signals caused by, for example, wrapping metal foil around the electronic tag. This is because metallic objects in the vicinity of the person may be detected as having a proximity below the proximity threshold but will tend not to block or significantly attenuate the reception of electromagnetic signals. In contrast, deliberate attempts to block transmission or reception of electromagnetic signals, for example, by wrapping metal foil around the electronic tag, will cause detection of a proximity below the proximity threshold and block (or at least attenuate) the reception of electromagnetic signals. The electronic tag being unable to receive an electromagnetic signal may mean that the antenna received an electromagnetic signal (such as a location signal from a GPS satellite) below a detection threshold or noise floor.

The inductive proximity sensor may be activated in response to the electronic tag being unable to receive an electromagnetic signal for a period of time. The inductive proximity sensor may be activated in response to the electronic tag being unable to receive a reply to a transmitted electromagnetic signal within a period of time.

The inductive proximity sensor may check for obstructions only if the electronic tag is unable to communicate with the outside world, that is, if the electronic tag does not receive an electromagnetic signal for a period of time, or does not receive a reply to a transmitted electromagnetic signal within a period of time, as the lack of communications may indicate that an attempt is being made to deliberately block electromagnetic signals to/from the electronic tag. For example, the proximity sensor may be activated if no external location signal has been received for a particular period of time which might indicate that the electronic tag has been wrapped in metal foil. By providing an initial check for whether the electronic tag might be obstructed by an object before activating the inductive proximity sensor to confirm whether that object is metallic, power consumption may be reduced and battery life may be improved.

The electronic tag may be configured to determine the location of the electronic tag based on an external location signal. The inductive proximity sensor may be activated in response to the electronic tag being unable to receive the external location signal for a period of time. The external location signal may be a GPS signal (for example, from a GPS satellite). The electronic tag may determine the location of the electronic tag based on the GPS signal.

The inductive proximity sensor determines periodically the proximity of the metallic object to the electronic tag. It may be desirable to check periodically for metallic objects in proximity of the electronic tag, regardless of whether or not the electronic tag is able to transmit/receive electromagnetic signals (for example, even if the electronic tag is able to receive an external location signal) because an attempt may have been made to block the electromagnetic signals with a metallic object which does not completely block receipt of electromagnetic signals.

The electronic tag further comprises a computer-readable storage medium (such as a memory) configured to store a blocked state. The blocked state indicates that the electronic tag is blocked based on a plurality of proximity determinations being below the proximity threshold. The blocked state indicates that the electronic tag is unblocked based on a plurality of proximity determinations being above the proximity threshold.

The blocked state is based on a value of a counter. The value of the counter is configured to be changed (for example, incremented) towards a blocked threshold in response to a proximity determination of the plurality of proximity determinations being below the proximity threshold. The value of the counter is configured to be changed (for example, decremented) towards an unblocked threshold in response to a proximity determination of the plurality of proximity determinations being above the proximity threshold.

The blocked state is set to blocked in response to the value of the counter reaching the blocked threshold. The blocked state is set to unblocked in response to the value of the counter reaching the unblocked threshold.

By determining the blocked state in this way, using a plurality of proximity determinations recorded, for example, with a counter, the electronic tag is better able to distinguish between deliberate attempts to block the electromagnetic tag from metallic objects which are detected that happen to be in the vicinity of the electronic tag.

The electronic tag may be configured to generate a message indicating a change in the blocked state. The message may be transmitted to a monitoring station in response to the electronic tag being able to transmit and/or receive an electromagnetic signal. It may be determined that the electronic tag is able to transmit and/or receive an electromagnetic signal based on determining that the proximity is above the proximity threshold. While the metallic object, such as the metal foil, is obstructing the antenna, the electronic tag may be prevented from transmitting an alarm signal to the monitoring centre. At these times, a record of the times when the proximity was below the proximity threshold is recorded on the computer-readable storage medium. As soon as the metallic object obstructing the antenna is removed, for example, as soon as the person wearing the electronic tag removes the metal foil they had wrapped around it, the proximity changes to being above the proximity threshold which indicates to the electronic tag to send the record to the monitoring centre for investigation.

It may be determined that a metallic object is attenuating the electromagnetic signal in response to the proximity being below a proximity threshold for one or more of: a period of time; a plurality of proximity measurements; and a plurality of failed attempts to obtain an external location signal.

The metallic object may be metal foil, for example, aluminium foil.

The invention shall now be described, by way of example only, with reference to the accompanying drawings in which:.

<FIG> illustrates an electronic tag <NUM> with a tether <NUM> for securing the electronic tag <NUM> to an offender <NUM>, typically securing the electronic tag <NUM> around their ankle (as shown in <FIG>). The electronic tag <NUM> monitors the position of the offender <NUM>, so it can be determined whether the offender <NUM> is adhering to one or more prescribed conditions, such as remaining outside an exclusion zone <NUM> around a victim's house <NUM> prescribed as a condition of the early release of offender <NUM> from prison.

<FIG> shows how the electronic tag <NUM> determines the position of the offender <NUM>. The electronic tag <NUM> has a GPS receiver <NUM> which periodically receives GPS signals <NUM> from GPS satellites <NUM>. The GPS receiver <NUM> uses the received GPS signals <NUM> to determine the location of the electronic tag <NUM>, and hence the location of the offender <NUM>. The electronic tag <NUM> sends location information <NUM> indicating the location of the offender <NUM> to a monitoring station <NUM> via antenna <NUM> as a radiofrequency signal, so that the monitoring station <NUM> can monitor where the offender <NUM> is in relation to their exclusion zone <NUM>.

The offender <NUM> may attempt to disguise their location (for example, to allow the offender <NUM> to visit a location inside the exclusion zone <NUM>). As shown in <FIG>, the offender <NUM> may do this by wrapping the electronic tag <NUM> in metal foil <NUM> (such as aluminium foil) to prevent the electronic tag <NUM> from receiving the GPS signals <NUM> and to prevent the electronic tag <NUM> from sending location information <NUM> to the monitoring station <NUM>.

<FIG> illustrates an inductive proximity sensor <NUM> for detecting whether there is metal in proximity to the electronic tag <NUM> which might indicate that metal foil <NUM> has been wrapped around the electronic tag <NUM> in an attempt to block the receipt of GPS signals <NUM> and the transmission of location information <NUM>.

The inductive proximity sensor <NUM> has a coil <NUM> connected to a capacitor <NUM> to form a resonant circuit <NUM>. A resonant drive circuit <NUM> provides an alternating current drive signal to match the frequency of the resonant circuit <NUM> which generates a first alternating magnetic field <NUM> which is coupled into any metallic object (such as metal foil <NUM>) in the vicinity, inducing eddy currents <NUM> in the metallic object. These eddy currents <NUM> generate a second alternating magnetic field <NUM> which is coupled back into the coil <NUM>, which increases the load on the resonant circuit <NUM> influencing the amplitude and/or frequency of the resonant circuit <NUM>. The closer the metal object is, the greater the induced eddy currents <NUM> and the more effect the second magnetic field <NUM> has on the magnitude and frequency of oscillation of the resonant circuit <NUM>. The changes in amplitude and/or frequency of the resonant circuit <NUM> can be determined by detector <NUM> to give an output which is related to the proximity of the metal object.

A proximity threshold can be determined by determining a first output of the proximity sensor <NUM> with metal foil <NUM> wrapped around it and determining a second output without any metal nearby. The proximity threshold is, for example, the average of these first and second outputs. When it is determined that the output of the proximity sensor <NUM> is below the proximity threshold, it can be confirmed that a metallic object is in the vicinity of the electronic tag <NUM>, which could be metal foil <NUM> which the offender has wrapped around electronic tag <NUM> in an attempt to block the electronic tag <NUM> from receiving the GPS signal <NUM> or transmitting the location information <NUM>.

However, it is possible that the metallic object detected by the electronic tag <NUM> as being in the vicinity of the electronic tag <NUM> might not be metal foil <NUM> but could instead be some incidental metallic object that happens to be in the vicinity (such as furniture, a radiator, etc.) that has not been placed there deliberately in an attempt to block the electronic tag <NUM>. To distinguish between deliberate attempts to block the electronic tag <NUM> with metal foil <NUM> and incidental metallic objects in the vicinity of the electronic tag <NUM>, it may only be determined that metal foil <NUM> is in proximity of the electronic tag <NUM> when the output of the proximity sensor <NUM> is below the proximity threshold on a number of occasions when the GPS receiver <NUM> also fails to receive GPS signals <NUM> from GPS satellites <NUM>, or when a reduction in the strength of the GPS signals <NUM> coincides with the output of the proximity sensor <NUM> falling below the proximity threshold.

To prevent false alarms, for example, due to noise causing the output of the proximity sensor <NUM> to fluctuate above/below the proximity threshold or due to transient metallic objects in the vicinity of the electronic tag <NUM>, it may be necessary for the output of the proximity sensor <NUM> to be below the proximity threshold for a minimum number of occasions before it can be established that a deliberate attempt is being made to block the electronic tag <NUM> with metal foil <NUM>.

The inductive proximity sensor <NUM> may be activated periodically (for example, on a regular schedule) to check for metal in proximity of the electronic tag <NUM>. The inductive proximity sensor <NUM> may be activated when the strength of the GPS signals <NUM> is suddenly reduced, or when the GPS receiver <NUM> has failed to receive any GPS signals <NUM> for a selected period of time or on a number of occasions when the GPS receiver <NUM> has polled for GPS signals <NUM>. The inductive proximity sensor <NUM> may be activated when the electronic tag <NUM> has been unable to receive a reply to a transmitted signal (such as a reply from the monitoring station <NUM> to confirm that the monitoring station <NUM> has received location information <NUM>) within a selected period of time or on a number of occasions.

Memory <NUM> (such as flash memory or another computer-readable storage medium) stores a counter which is initially set to zero when the electronic tag <NUM> is manufactured or reset. Each time the inductive proximity sensor <NUM> is activated, the output of the inductive proximity sensor <NUM> is compared with the proximity threshold and it is confirmed whether the GPS receiver <NUM> is able to receive GPS signals <NUM>. If the output from the proximity sensor <NUM> is below the proximity threshold and the GPS receiver <NUM> is unable to receive a GPS signal <NUM>, the electronic tag <NUM> increments the counter. If the output from the proximity sensor <NUM> is above the proximity threshold and the GPS receiver <NUM> is able to receive a GPS signal <NUM>, the electronic tag <NUM> decrements the counter. When the counter reaches an upper (blocked) threshold, this indicates that it is likely that metal foil <NUM> has been detected wrapped around the electronic tag <NUM> and a blocked state stored in memory <NUM> is set to blocked, and a record of the blocking start time is recorded in the memory <NUM>.

The electronic tag <NUM> continues to activate the inductive proximity sensor <NUM> periodically, to check for changes in the blocked state. Again, each time the inductive proximity sensor <NUM> is activated, the output of the inductive proximity sensor <NUM> is compared with the proximity threshold and it is confirmed whether the GPS receiver <NUM> is able to receive GPS signals <NUM>. If the output from the proximity sensor <NUM> is below the proximity threshold and the GPS receiver <NUM> is unable to receive a GPS signal <NUM>, the electronic tag <NUM> increments the counter but caps the counter at the upper threshold. If the output from the proximity sensor <NUM> is above the proximity threshold and the GPS receiver <NUM> is able to receive a GPS signal <NUM>, the electronic tag <NUM> decrements the counter. When the counter reaches a lower (unblocked) threshold, this indicates that it is likely that the metal foil <NUM> has now been removed from the electronic tag <NUM> and the blocked state stored in memory <NUM> is set to unblocked, and a record of the blocking end time is recorded in the memory <NUM>.

In response to the blocked state changing to blocked, the electronic tag <NUM> will generate a message to be sent to the monitoring station <NUM> via antenna <NUM> the next time the blocked state changes to unblocked. The message may include the blocking start time and the blocking end time. The message will alert operators at the monitoring station <NUM> that the offender <NUM> was attempting to block the electronic tag <NUM> using metal foil <NUM> so that they can investigate and, if necessary, reprimand the person for tampering with their electronic tag <NUM>.

Claim 1:
An electronic tag (<NUM>) configured to be secured to a body of a person, the electronic tag (<NUM>) comprising:
an antenna (<NUM>) configured to transmit or receive an electromagnetic signal; and
an inductive proximity sensor (<NUM>) configured to:
determine a proximity of a metallic object to the electronic tag (<NUM>), based on electromagnetic coupling between the inductive proximity sensor (<NUM>) and the metallic object; and
determine, based on the proximity being below a proximity threshold, that a metallic object is attenuating the electromagnetic signal transmitted or received by the antenna (<NUM>); wherein the inductive proximity sensor (<NUM>) periodically determines the proximity of the metallic object to the electronic tag (<NUM>);
characterised by:
a counter configured to have a value that is changed towards a blocked threshold in response to a proximity determination being below the proximity threshold and changed towards an unblocked threshold in response to a proximity determination being above the proximity threshold, and
a computer-readable storage medium configured to store a blocked state, wherein the blocked state is set to blocked in response to the value of the counter reaching the blocked threshold and the blocked state is set to unblocked in response to the value of the counter reaching the unblocked threshold.