GB2477917A - Proximity sensor for detecting an attempt to disable a monitoring tag or portable GPS tracking device - Google Patents

Proximity sensor for detecting an attempt to disable a monitoring tag or portable GPS tracking device Download PDF

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Publication number
GB2477917A
GB2477917A GB1002476A GB201002476A GB2477917A GB 2477917 A GB2477917 A GB 2477917A GB 1002476 A GB1002476 A GB 1002476A GB 201002476 A GB201002476 A GB 201002476A GB 2477917 A GB2477917 A GB 2477917A
Authority
GB
United Kingdom
Prior art keywords
antenna
proximity sensor
port
oscillator
signal
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
GB1002476A
Other versions
GB2477917B (en
GB201002476D0 (en
Inventor
John Anderson
Aled Hughes
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Guidance IP Ltd
Original Assignee
Guidance IP Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Guidance IP Ltd filed Critical Guidance IP Ltd
Priority to GB1002476.8A priority Critical patent/GB2477917B/en
Publication of GB201002476D0 publication Critical patent/GB201002476D0/en
Priority to PT100087477T priority patent/PT2290391T/en
Priority to EP10008747.7A priority patent/EP2290391B1/en
Priority to ES10008747T priority patent/ES2859780T3/en
Priority to NZ587538A priority patent/NZ587538A/en
Priority to US12/862,761 priority patent/US8629771B2/en
Priority to IL207866A priority patent/IL207866A0/en
Priority to AU2010214710A priority patent/AU2010214710B2/en
Publication of GB2477917A publication Critical patent/GB2477917A/en
Application granted granted Critical
Publication of GB2477917B publication Critical patent/GB2477917B/en
Expired - Fee Related legal-status Critical Current
Anticipated expiration legal-status Critical

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/02—Systems using reflection of radio waves, e.g. primary radar systems; Analogous systems
    • G01S13/04—Systems determining presence of a target
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/20—Integrity monitoring, fault detection or fault isolation of space segment
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/21—Interference related issues ; Issues related to cross-correlation, spoofing or other methods of denial of service
    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B13/00—Burglar, theft or intruder alarms
    • G08B13/22—Electrical actuation
    • G08B13/24—Electrical actuation by interference with electromagnetic field distribution
    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B21/00—Alarms responsive to a single specified undesired or abnormal condition and not otherwise provided for
    • G08B21/02—Alarms for ensuring the safety of persons
    • G08B21/0202—Child monitoring systems using a transmitter-receiver system carried by the parent and the child
    • G08B21/0275—Electronic Article Surveillance [EAS] tag technology used for parent or child unit, e.g. same transmission technology, magnetic tag, RF tag, RFID
    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B21/00—Alarms responsive to a single specified undesired or abnormal condition and not otherwise provided for
    • G08B21/02—Alarms for ensuring the safety of persons
    • G08B21/0202—Child monitoring systems using a transmitter-receiver system carried by the parent and the child
    • G08B21/0286—Tampering or removal detection of the child unit from child or article

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  • Engineering & Computer Science (AREA)
  • Remote Sensing (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Health & Medical Sciences (AREA)
  • Child & Adolescent Psychology (AREA)
  • General Health & Medical Sciences (AREA)
  • Electromagnetism (AREA)
  • Computer Security & Cryptography (AREA)
  • Business, Economics & Management (AREA)
  • Emergency Management (AREA)
  • Burglar Alarm Systems (AREA)

Abstract

A proximity sensor is provided for use with a monitoring tag or portable tracking device, the tag or device utilising GPS or RF signals for location detection. The proximity sensor determines if signals used by the tag or device are being deliberately blocked by an object, such as metal foil 12 placed in proximity to the tag or device. The proximity sensor includes an oscillator 2, a directional coupler 4, and a stub antenna 8. When GPS or RF signals are lost, the proximity sensor is turned on, and the oscillator 2 provides a transmit signal to the stub antenna 8 via ports 6a and 6b of the directional coupler 4. A third port 6c of the directional coupler 4 acts as a coupled port when RF power that has been transmitted by the antenna 8 is reflected back to the antenna by a blocking object. A detector provides an alert indicating the presence of a blocking object 12 if the magnitude of the transmit signal that is reflected back to the antenna 8 in the form of a receive signal exceeds a set threshold.

Description

TITLE
Proximity sensors
DESCRIPTION
Technical Field
The present invention relates to proximity sensors, and in particular to proximity sensors that can be used to detect the close proximity of an object that might be used to block an electromagnetic signal.
The proximity sensor can be designed to be incorporated into a monitoring tag that might be secured around the appendage of an individual, for example. The monitoring tags can include a location detection means for detecting if the individual has moved outside a designated area within which electromagnetic (e.g. radio-frequency or RF) signals can be exchanged between the monitoring tag and a base unit. The monitoring tags can also include a global positioning system (GPS) device that uses electromagnetic signals (e.g. GPS signals) to determine the location of the individual and to track and record their movement. The proximity sensor can also be designed to be incorporated into a portable tracking device that is carried or worn by the individual and which receives RF signals from a separate monitoring tag that is secured to the individual. The portable tracking device will normally include a GPS device that uses GPS signals to determine the location of the individual and to track and record their movement. As such, the tags and/or portable tracking devices can be worn by hospital patients, the elderly, children or offenders and can raise an alarm or notify an authorised person if they move into or outside a designated area.
It is possible to deliberately block the electromagnetic signals by placing an object such as a sheet of metal foil, for example, around the housing of the monitoring tag or the portable GPS tracking device. There is therefore a need for a proximity sensor to determine if the electromagnetic signals are being deliberately blocked in an attempt to incapacitate or disable the monitoring tag or portable GPS tracking device.
Summary of the Invention
The present invention provides a proximity sensor used with a device that relies on the transmission andlor receipt of a main electromagnetic signal that can be blocked by an object being placed in proximity to the device, the proximity sensor comprising: an oscillator; an antenna for transmitting and receiving electromagnetic signals; a directional coupler having a first port and a second port connected together by a main line and a third port and a fourth port connected together by a coupled line, the first port being connected to the oscillator, the second port being connected to the antenna, and the third port being connected to a detector and acting as an isolated port of the directional coupler when a transmit signal is supplied from the oscillator to the antenna along the main line and as a coupled port when a receive signal is supplied from the antenna to the oscillator along the main line; wherein the oscillator is controlled to provide a transmit signal to the antenna along the main line of the directional coupler and wherein the detector provides an alert indicating the presence of a blocking object if the magnitude of the power of the transmit signal that is reflected back to the antenna in the form of a receive signal exceeds a threshold; and wherein the oscillator is controlled to provide a transmit signal on detection that the main electromagnetic signal is not being transmitted andlor received.
When there is no blocking object in the proximity of the device then the transmit signal that is provided from the oscillator to the antenna through the directional coupler will be radiated, approximately omni-directionally, away into free space.
However, if a blocking object such as a sheet of metal foil, for example, is placed in close proximity to the antenna then a significant proportion of the radiated power of the transmit signal will be reflected back to the antenna and onto the third port (coupled port) of the directional coupler. The return loss of the antenna is therefore effectively used as the means for detecting the presence or absence of a blocking object.
In the case where the proximity sensor is intended to be incorporated into a device such as a monitoring tag or a portable tracking device, for example, that might be carried or worn close to the body of an individual then it will be readily appreciated that the body can produce unwanted reflections of the transmit signal back to the antenna even when no blocking object is present. If such reflections are detected then they can lead to false positive alerts. The proximity sensor is therefore preferably designed to provide an alert only if an object that might be used to block the transmission and/or receipt of the main electromagnetic signal is detected to be in the proximity of the device. The positioning of the antenna within the device can be an important factor in minimising reflections from the body. For example, the antenna may be positioned so that it is spaced away from the body when the device is in use.
Spacing the antenna about a quarter wavelength (or a significant part thereof) of the transmit signal away from the body may have the effect of minimising the proportion of the radiated power of the transmit signal that is reflected back to the antenna by the body of the individual. The detector threshold or any other structural or operational feature of the proximity sensor or its associated device can be selected to enable the proximity sensor to distinguish properly between reflections from the body and those from a blocking object.
The magnitude of the reflected power is monitored by the detector that is connected to the third port of the directional coupler. If the magnitude of the reflected power exceeds a set threshold then the detector will provide an alert. The alert can take any suitable form including an audible or visual alert, the setting of an alert flag, or the transmission of an alert signal to a base unit or control centre, for example. The transmission of the alert signal may be delayed until it has been determined that the blocking object has been removed.
The oscillator can be configured to supply a continuous or pulsed transmit signal to the antenna. To minimise temperature drift issues that are often present in conventional RF detectors, and enhance detection sensitivity, then the detector can be configured to receive a pulsed receive signal. The oscillator can be onloff modulated such that the receive signal that is coupled to the detector is a pulsed signal.
The detector can include a detector diode that converts the power of the continuous or pulsed receive signal to a dc or ac voltage. The detector can also include a Schmitt trigger circuit that monitors the voltage provided by the detector diode and provides an alert if the voltage exceeds a set threshold. The threshold may be fixed or variable and can be set with regard to the design of the proximity sensor and its intended operating parameters.
The proximity sensor preferably uses radio-frequency or RF components.
The proximity sensor can be configured such that the oscillator is controlled to provide a continuous or pulsed transmit signal. The proximity sensor is turned on only once a preliminary detection has been made that the main electromagnetic signal is not being properly transmitted or received. That preliminary detection may be carried out by any associated detection circuit. The proximity sensor is therefore used to check if the main electromagnetic signal is not being properly transmitted or received because it is being deliberately blocked due to the proximity of a blocking object such as a sheet of metal foil, for example, or for some other reason.
Once the proximity sensor has been turned on then it may continue to operate and provide a continuous or pulsed transmit signal continually or at timed intervals until the proximity sensor detects that the blocking object is no longer present (e.g. until the magnitude of the reflected power at the third port of the directional coupler falls below a set threshold). The proximity sensor can also be turned off or placed in a standby mode after a predetermined amount of time has lapsed or if the associated detection circuit detects that the main electromagnetic signal is being properly transmitted or received again, for example.
The device that is associated with the proximity sensor preferably includes a primary antenna that is used to transmit and/or receive the main electromagnetic signal. The primary antenna can be used as the antenna of the proximity sensor. For example, if the oscillator is only controlled to provide a transmit signal on detection that the main electromagnetic signal is not being transmitted and/or received (e.g. the proximity sensor is turned on once a preliminary detection has been made by a detection circuit) then the oscillator can supply the transmit signal to the primary antenna of the device.
The proximity sensor can therefore use the primary antenna of the device to determine if the loss of the main electromagnetic signal is the result of it being blocked by a blocking object by detecting the proportion of radiated power of the transmit signal that is reflected back to the primary antenna. If the oscillator is controlled to provide a transmit signal either continually or at timed intervals without any preliminary detection that the main electromagnetic signal is not bring transmitted andlor received then the proximity sensor will normally require its own dedicated antenna to avoid interfering with the main electromagnetic signal.
The proximity sensor is particularly suitable to be incorporated into a monitoring tag or a portable tracking device that comprises a primary antenna for transmitting andlor receiving a main electromagnetic signal (e.g. a GPS signal). In this case the antenna of the proximity sensor is preferably located substantially adjacent to the primary antenna of the monitoring tag or portable tracking device. The alert provided by the proximity sensor may be provided to the control unit of the monitoring tag or portable tracking device and used for any suitable purpose. For example, the detector may cause an alert flag to be set within the control unit of the monitoring tag or portable tracking device.
However, it will be readily appreciated that the proximity sensor may be used with, or incorporated into, any suitable device that relies on the transmission andlor receipt of a main electromagnetic signal. Other suitable devices might, for example, include alarm systems and tracker systems, such as car alarms that use GPS or GSM signals to track the location of vehicles, mobile telephones and other portable electronic devices.
Drawings Figure 1 is a schematic diagram showing a foil sensor according to the present invention; and Figure 2 is a schematic diagram showing a detector.
A foil sensor according to the present invention will be described with reference to Figures 1 and 2. To aid integration, the foil sensor is preferably small, low cost and uses surface mount technology. A preferred operating frequency for the foil sensor is 2.45 GHz because this is a popular ISM band and suitable RF components are readily available from commercial suppliers.
The foil sensor includes a low power RF oscillator 2. An RF directional coupler 4 has a first port 6a and a second port 6b. A third port 6c operates either as an isolated port or a coupled port depending on the direction of RF power flow through the directional coupler 4. This will be explained in more detail below.
The first port 6a is connected to the oscillator 2 by a 50 transmission line.
The second port 6b is connected to a stub antenna 8 such as a printed quarter wave stub antenna.
The foil sensor is incorporated into a monitoring tag (e.g. an offender tag) or a portable tracking device that includes a GPS antenna (not shown) for receiving the GPS signals that allow the monitoring tag or portable tracking device to track its location. The stub antenna 8 is positioned adjacent the GPS antenna within the housing 10 of the monitoring tag or portable tracking device. In some arrangements the stub antenna 8 can be omitted completely and the second port 6b is connected to the UPS antenna instead. The foil sensor will normally be turned off or in a standby mode (e.g. a low quiescent current mode) to minimise battery load. When the GPS signals are lost then the foil sensor is turned on to determine if the GPS signals are being deliberately blocked. However, the foil sensor can also be configured to operate continually or at time intervals.
To minimise battery loading the power chosen for the oscillator 2 is around I mW.
When the foil sensor has been turned on, the oscillator 2 provides a transmit signal to the stub antenna 8 and this passes along a main line of the directional coupler 4 which connects the first port 6a to the second port 6b. The transmit signal can be continuous or pulsed. At the operating frequency of the foil sensor the match of the stub antenna 8 is typically better than -15 dB so very little RF power is reflected back to the directional coupler 4.
If the GPS signals are unexpectedly lost then there is a possibility that a sheet of metal foil 12 (e.g. aluminium foil) has been wrapped around the housing 10 of the monitoring tag or portable tracking device. To test for this the foil sensor is turned on by the control circuit of the monitoring tag or portable tracking device and a transmit signal is supplied to the stub antenna 8 from the oscillator 2. RF power is radiated by the stub antenna 8. If metal foil is in close proximity to the stub antenna 8 then it will cause an RF short circuit at the stub antenna. In practice, the presence of the metal foil changes the return loss for the stub antenna 8 from better than -15 dB to worse than -3 dB. The short circuit reflects RF power back to the stub antenna 8 in the form of a receive signal and along the main line of the directional coupler 4. It will be readily appreciated that the receive signal is effectively that part of the transmit signal which has been reflected back to the stub antenna 8 by the metal foil 12.
A proportion of the reflected RF power that passes along the main line from the second port 6b to the first port 6a is coupled to the third port 6c which is operating as a coupled port. A fourth port (not shown) in this case operates as an isolated port and typically has an isolation of better than -30dB and is terminated in a 5011 load. (It will be appreciated that when the transmit signal passes along the main line of the directional coupler 4 from the oscillator 2 to the stub antenna 8 then the third port 6c operates as an isolated port.) The third port 6c is connected to an RF detector diode 14 that is shown in Figure 2.
The detector diode 14 converts the coupled RF power at the third port 6c to a dc or ac voltage that is proportional to the coupled RF power. This voltage is monitored by a Schmitt trigger circuit 16 the output of which changes state to provide an alert in the form of a foil detection flag if the voltage exceeds a set threshold.

Claims (12)

  1. CLAIMS1. A proximity sensor used with a device that relies on the transmission and/or receipt of a main electromagnetic signal that can be blocked by an object being placed in proximity to the device, the proximity sensor comprising: an oscillator; an antenna for transmitting and receiving electromagnetic signals; a directional coupler having a first port and a second port connected together by a main line and a third port and a fourth port connected together by a coupled line, the first port being connected to the oscillator, the second port being connected to the antenna, and the third port being connected to a detector and acting as an isolated port of the directional coupler when a transmit signal is supplied from the oscillator to the antenna along the main line and as a coupled port when a receive signal is supplied from the antenna to the oscillator along the main line; wherein the oscillator is controlled to provide a transmit signal to the antenna along the main line of the directional coupler and wherein the detector provides an alert indicating the presence of a blocking object if the magnitude of the power of the transmit signal that is reflected back to the antenna in the form of a receive signal exceeds a threshold; and wherein the oscillator is controlled to provide a transmit signal on detection that the main electromagnetic signal is not being transmitted and/or received.
  2. 2. A proximity sensor according to claim 1, wherein the detector includes a detector diode that converts the power of the receive signal to a dc or ac voltage.
  3. 3. A proximity sensor according to claim 2, wherein the detector includes a Schmitt trigger circuit that monitors the voltage provided by the detector diode and provides an alert if the voltage exceeds a set threshold.
  4. 4. A proximity sensor according to any preceding claim, wherein the transmit signal is a continuous transmit signal.
  5. 5. A proximity sensor according to any of claims 1 to 3, wherein the transmit signal is a pulsed transmit signal.
  6. 6. A proximity sensor according to any preceding claim, wherein the antenna is used to transmit and/or receive the main electromagnetic signal.
  7. 7. A device comprising a proximity sensor according to any preceding claim.
  8. 8. A device according to claim 7, further comprising a primary antenna for transmitting and/or receiving the main electromagnetic signal.
  9. 9. A device according to claim 8, wherein the antenna of the proximity sensor is located adjacent to the primary antenna.
  10. 10. A device according to any of claims 7 to 9, being a monitoring tag.
  11. 11. A device according to any of claims 7 to 9, being a portable tracking device.
  12. 12. A proximity sensor substantially as herein described and with reference to the drawings.
GB1002476.8A 2009-09-01 2010-02-15 Proximity sensors Expired - Fee Related GB2477917B (en)

Priority Applications (8)

Application Number Priority Date Filing Date Title
GB1002476.8A GB2477917B (en) 2010-02-15 2010-02-15 Proximity sensors
PT100087477T PT2290391T (en) 2009-09-01 2010-08-23 Proximity sensors
EP10008747.7A EP2290391B1 (en) 2009-09-01 2010-08-23 Proximity sensors
ES10008747T ES2859780T3 (en) 2009-09-01 2010-08-23 Proximity sensors
NZ587538A NZ587538A (en) 2009-09-01 2010-08-24 Proximity sensor for a transmitter or receiver to detect blocking of a signal
US12/862,761 US8629771B2 (en) 2009-09-01 2010-08-25 Proximity sensors
IL207866A IL207866A0 (en) 2009-09-01 2010-08-30 Proximity sensors
AU2010214710A AU2010214710B2 (en) 2009-09-01 2010-08-30 Proximity sensors

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB1002476.8A GB2477917B (en) 2010-02-15 2010-02-15 Proximity sensors

Publications (3)

Publication Number Publication Date
GB201002476D0 GB201002476D0 (en) 2010-03-31
GB2477917A true GB2477917A (en) 2011-08-24
GB2477917B GB2477917B (en) 2012-02-22

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Application Number Title Priority Date Filing Date
GB1002476.8A Expired - Fee Related GB2477917B (en) 2009-09-01 2010-02-15 Proximity sensors

Country Status (1)

Country Link
GB (1) GB2477917B (en)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2588371A (en) * 2019-09-30 2021-04-28 G4S Monitoring Tech Limited Electronic tag

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5959533A (en) * 1997-05-27 1999-09-28 Pro Tech Monitoring, Inc. Tamper detection for body worn transmitter
US20090160708A1 (en) * 2006-01-25 2009-06-25 Fujitsu Ten Limited Radar apparatus and interference detection method

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2473067B (en) * 2009-09-01 2011-08-17 Guidance Monitoring Ltd Proximity sensors

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5959533A (en) * 1997-05-27 1999-09-28 Pro Tech Monitoring, Inc. Tamper detection for body worn transmitter
US20090160708A1 (en) * 2006-01-25 2009-06-25 Fujitsu Ten Limited Radar apparatus and interference detection method

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2588371A (en) * 2019-09-30 2021-04-28 G4S Monitoring Tech Limited Electronic tag

Also Published As

Publication number Publication date
GB2477917B (en) 2012-02-22
GB201002476D0 (en) 2010-03-31

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