WO2010022408A1

WO2010022408A1 - Monitoring device

Info

Publication number: WO2010022408A1
Application number: PCT/ZA2009/000064
Authority: WO
Inventors: Oliver Darvall; Simon Germishuizen; Derek John House; Jan Henderik Schoeman
Original assignee: Oliver Darvall; Simon Germishuizen; Derek John House; Jan Henderik Schoeman
Priority date: 2008-08-19
Filing date: 2009-07-14
Publication date: 2010-02-25

Abstract

The invention provides a monitoring device (10) and a method of operating a monitoring device (10) which includes a first positioning system capable of providing absolute position data by utilizing a GPS receiver (120), a second positioning system capable of providing dead reckoning position data (110), a processing means (130)in communication with the first positioning system and the second positioning system, a power supply (400) for energizing the first positioning system, the second positioning system and the processing means, and a mounting means for mounting the monitoring device on an object or person to be monitored.

Description

TITLE OF THE INVENTION: MONITORING DEVICE

This invention relates to a monitoring device and in particular to a monitoring device for monitoring the positions of individuals.

BACKGROUND OF THE INVENTION

Monitoring devices for use in a personal tracking device are known in the art. These systems form part of an electronic monitoring system for the correctional services for non-custodial sentencing. The purpose of the electronic monitoring system is to provide an alternative sentence to low risk offenders. The principle of tracking offenders with GPS is well established, as is the principle of reporting position via GSM. Conventional systems rely on regularly recharging batteries as both GSM and GPS require a lot of power, especially if continuous monitoring is required.

OBJECTS OF THE INVENTION

However, several drawbacks are associated to the above mentioned prior art devices. For example, electronic monitoring systems may not work properly specifically in large countries where the infrastructure is limited in rural areas.

This is merely due to the fact that electrical supplies are not available at every home, telephones are not generally available and cell phone coverage is concentrated around main cities and main routes.

Accordingly, there is a need in the art to provide an even more effective monitoring device that overcomes, at least partly, the disadvantages associated with the state of the art devices. More specifically, an improved monitoring device should provide possible disabling of offenders if they violate prohibited regions. Furthermore, an improved monitoring device should be capable of operating for an extended period using power from its own internal battery cells, so as to last approximately a year in operation. During this entire period recharge process, connection to external power sources, or connection to a base unit, or the like should not be necessary.

It is also an object of the present invention to provide a new monitoring device involving an inventive step.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a monitoring device comprising:

- a first positioning system capable of providing absolute position data;

- a second positioning system capable of providing dead reckoning position data;

- a processing means in communication with the first positioning system and the second positioning system;

- a power supply for energizing the first positioning system, the second positioning system and the processing means; and

- a mounting means for mounting the monitoring device on an object to be monitored.

The first positioning system may include a receiver for receiving data of a Global Position System.

The second positioning system may include an accelerometer.

The second positioning system may include an electronic compass.

The processing means may be capable to calculate the initial position from the absolute position data and to calculate the location of the monitoring device from the dead reckoning position data with the initial position being provided by the absolute position data.

The monitoring device may include a transmitter for transmitting the location of the monitoring device to a control station.

The transmitter may transmit data via a system for mobile communications.

The transmitter may be mounted on or in the mounting means.

The monitoring device may include a further transmitter for transmitting the location of the monitoring device to a control station, the further transmitter transmits data to the control station in the event that there is no coverage of the networking service employed by the transmitter.

The transmitter may transmit the data via a geo-static satellite relay which in turn permits transmission of the message to the control station.

The monitoring device may include a timer which is mounted on or in the mounting means.

The timer may be arranged in communication with the GPS for activating the GPS at pre- determinable, alternatively random, times during the day and/or in the event of an unauthorized act.

The power supply may be in the form of a battery, preferable a primary cell or rechargeable lithium battery.

The power supply may be in the form of a first battery and a second battery, the first battery being capable of energizing the first positioning system, the second positioning system and the processing means and the second battery being capable of energizing the transmitter and/or the further transmitter

The second battery may be charged by the first battery.

The monitoring device may include an alarm means which may be mounted on or in the mounting means and which is provided to alert the wearer and/or control station of any event such as low or near depletion battery conditions or the wearer committing an unauthorized act like going outside a permitted area.

The monitoring device may include an anti-tamper device, to sense removal of the monitoring device from the wrist or ankle of the wearer.

The anti-tamper device may be in the form of a fibre-optic cable which is arranged in and extends the length of a strap.

The monitoring device may include a co-ordinate storage means for storing co-ordinates which may define a zone or region into which the monitoring device may not proceed.

The monitoring device may include mounting means, the mounting means being in the form of a non-allergenic strap.

The monitoring device may include a sensing means for monitoring the integrity of the fibre-optic cable and for sending a signal to the alarm means in the event of a breach of such integrity.

According to a first aspect of the present invention, there is provided a method of operating a monitoring device, the method comprising:

- providing a dead reckoning tracker for continuous monitoring;

- providing a positioning system capable of determining absolute position data; - processing means for storing data of the dead reckoning tracker as long the position of the monitoring device is inside a permitted zone; and

- transmitting means for transmitting an alarm is transmitted when the position of the monitoring device is outside the permitted zone.

The dead reckoning tracker may be calibrated form the absolute position data of positioning system.

The continuous monitoring of the dead reckoning tracker may be confirmed by the absolute position data of positioning system when the position of the monitoring device is outside the permitted zone.

The transmitting means may transmit stored tracks, so as to reduce the number of scheduled updates to a command centre.

The number of scheduled updates from the monitoring device to a command centre may be chosen such that a power supply for energizing the monitoring device operates for up to one year without recharging.

The update may be scheduled several times a day, preferably at random intervals.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described, by way of non-limiting example and drawings. The different embodiments of the invention are now further illustrated with reference to the following drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows in a partially exploded view a perspective drawing of a monitoring device according to an embodiment of the present invention;

Figure 2 shows a perspective drawing of a detail of the monitoring device according to Figure 1 ;

Figure 3 schematically shows the monitoring device in a functional block diagram according to an embodiment of the present invention; and

Figure 4 schematically shows operation of the monitoring device in a flow chart.

In all drawings, like reference numerals refer to like parts, unless otherwise indicated.

Making now reference to Figure 1, an embodiment of the invention is described. Figure 1 schematically shows an exploded three dimensional view of a monitoring device 10. When in use, the monitoring device 10 is locked or mounted in position on a wrist or ankle of a person in order to monitor his whereabouts. The monitoring device 10 can be used basically for anyone that requires monitoring or tracking outside of a facility. This includes "low risk" prisoners or persons on parolees who are being released after already serving a long sentence for a serious crime, or people that have not yet been actually convicted of anything but are awaiting trial.

Before providing a detailed description of the functions of the monitoring device 10, a short overview on the components of the monitoring device 10 is given in the following.

As depicted in Figure 1, the monitoring device 10 includes a housing which is comprised by two parts forming a top housing 12 and a bottom housing 14. Inside the top housing 12 and the bottom housing 14 a main printed circuit board 16 is located. The main printed circuit board 16 includes various electronic components which will be described below. The electronic components on the main printed circuit board 16 are powered by a main power source 18 and a pulsed power source 20. An auxiliary printed circuit board 22 is located below the main printed circuit board 16. The auxiliary printed circuit board 22 carries connectors and a light source (not explicitly shown in Figure 1), such as a surface mounted LED for example, which is capable of providing light to a fibre optic cable. The auxiliary printed circuit board 22 can be attached, inter alia, to the main printed circuit board 16.

Further components being mounted inside the housing include a vibration motor 24, a plastic optical fibre 26, spacers 28, a strap 30, a webbing 32 at least partially covering the strap 30, a locking pin 34 suitable for locking the strap 30 and a retaining ring 36 suitable for retaining the strap 30.

The plastic optical fibre 26 is mounted so as to provide an anti-tamper function. Accordingly, the fibre-optic cable 26 is arranged in and extends the length of the strap 30. By monitoring the integrity of the fibre-optic cable 26 and sending a signal to an alarm means in the event of a breach of such integrity, the anti-tamper function is achieved. It is envisaged that in an operative condition, the fibre-optic cable can be illuminated.

Mounting of the plastic optical fibre 26 is further illustrated making reference to Figure 2 now. In Figure 2, the bottom housing 14 with the attached plastic optical fibre 26 is shown. Illumination of the plastic optical fibre 26 can be provided by a suitable light source mounted on the auxiliary printed circuit board 22. The light source can be located in a light source pocket formed within the bottom housing 14, as indicated by reference numeral 38 in Figure 2.

The strap 30 or any other suitable mounting means is typically in the form of a non-allergenic strap on which the various components as described above are able to be mounted and/or received. The strap 30 can be sized according to the dimensions of the wearer's ankle or wrist. Thέ monitoring device 10 is then mounted on the wearer's wrist or ankle and the respective ends of the fibre optic cable 26 and strap 30 are then joined or spliced together thereby to retain the mounting device 10 in position on the wearer and to permit communication between the various components. The locking pin 34 and the retaining ring 34 provide a locking arrangement to inhibit removal by the wearer.

This present embodiment of the invention is focused at reducing battery consumption to a level which would allow a unit to operate for up to one year without recharging, using existing battery technologies. The principle is to reduce the number of updates from the monitoring device 10 to a command centre to around eight updates per day. To still achieve the required continuous tracking requirements of the application this requires the monitoring device 10 to have a tracking and monitoring capability onboard rather than relying on the command centre to check for geographical violations. To meet the requirement for continuous monitoring without excessive power consumption a dead reckoning tracker is realized in the monitoring device 10 using a low power 3 -axis accelerometer and electronic compass. If the wearer has gone outside the zone, then a GPS confirmation is performed. If the wearer is still within his permitted zone the update is stored, if not then an immediate alarm is transmitted via GSM. In normal operation an update is scheduled around eight times a day, at random intervals and the stored tracks are transmitted in a text message via GSM.

To limit the amount of communication required with the command centre the wearer's permitted zones are programmed into the monitoring device 10. This allows the unit to check whether the wearer is within his permitted zone without having to communicate via GSM (GSM is the largest consumer of power).

If the unit is operated outside of GSM coverage, then a low frequency relay is provided. This will receive alarms from the monitoring device 10 and relay it to a command centre via satellite communications. Communications are ir/tiated by the monitoring device 10 if there is a scheduled update, position violation, battery low indication or the device has been tampered with. If the wearer needs to report to the command centre, the centre will transmit a "come home" request to the monitoring device 10 during one of the regular updates and activate the alarm.

Making now reference to Figure 3, an overall description of the monitoring device 10 is provided according to a functional block diagram.

As depicted in Figure 3, the monitoring device 10 includes a navigation system 100. The navigation system 100 is the sub system responsible for determining the absolute position of the monitoring device 10. To achieve this task, the navigation system 100 is comprised of two complimentary systems: a dead reckoning system 110 and a receiver for Global Positioning System (GPS) signals, which is referred to as GPS receiver 120 in following.

The signals from the dead reckoning system 110 and the GPS receiver 120 are received and processed by a navigation processor 130.

It should be noted that the power consumption of the GPS receiver 120 is too high to use this facility on a continuous basis in a stand alone application. The GPS signal may also not be available for example when the signal is too weak inside a building.

The purpose of the dead reckoning system 110 is to keep track of the position of the monitoring device 10 when the information from the GPS receiver 120 is not current or not available. The dead reckoning system 110 continuously calculates the absolute position of the monitoring device 10 and the position is corrected with the GPS receiver 120 to negate measurement errors in the dead reckoning system 110.

As shown in Figure 3, the dead reckoning system 110 includes an accelerometer 112 and a compass 114. The accelerometer 112 measures the acceleration of the monitoring device 10 in three dimensions and thereby indicates how the monitoring device 10 moves. The acceleration information is used to calculate the change in position of the monitoring device 10. The accelerometer 112 is monitored continuously.

The compass 114 measures the magnetic heading of the monitoring device 10 during movement. This is used to improve the accuracy of the position calculation based on the accelerometer 112 measurement.

The GPS receiver 120 receives the radio frequency signals transmitted from the GPS satellite network. The GPS receiver 120 interprets these signals and supplies the absolute position of the monitoring device 10. The GPS receiver 120 is also an accurate source of time and provides time correction for the monitoring device 10.

Furthermore, as depicted on Figure 3, a communication system 200 is present which is also referred to as Man Machine Interface System or MMI system.

The communications system 200 has several functions. The first is to establish communications to the centralised server to indicate the position of the monitoring device 10 (both current and historical position). The second function is control of the monitoring device 10 which includes power management, systems monitoring and alerting, for example

One subsystem within the communications system 200 is the GSM communication 210. The GSM communication 210 is the primary means of communication to the centralised server. The GSM communication 210 utilises the existing GSM/GPRS infrastructure of the cellular service providers to transmit data to and from the monitoring device 10 to the centralised server. The transmitted data includes the position-time information of the monitoring device 10 and the monitoring device 10 health status. A further subsystem within the communications system 200 is a secondary communication 220. The secondary communication 220 can be provided as a Zigbee communication system, although other modes of communication can be employed as well. The secondary communication 220 is a secondary means of communication for the monitoring device 10. The main purpose of the secondary communication 220 is to provide a method of deploying the monitoring device 10 in areas where there is no cellular network coverage, i.e. with the GSM communication 210 not operational. In this case, a repeater is deployed together with the monitoring device 10 to relay the monitoring device 10 communications either to a high-power long-distance GSM system or directly to a satellite based communication system.

In addition to the above, the secondary communication 220 is used during the deployment of the monitoring device 10 to set the operational parameters of the monitoring device 10 such as the user, deployment period and to check the health of the monitoring device 10 on installation (e.g. check that the monitoring device 10 has not been damaged or the batteries have not run down during storage).

The third function of the secondary communication 220 is to facilitate the factory testing of the monitoring device 10 after final assembly.

A further subsystem within the communications system 200 is the MMI processor 230. The MMI functionality of the monitoring device 10 controls all aspects of the operation of the monitoring device 10 in order to minimize the power consumption of the monitoring device 10. The MMI processor 230 controls the power to all the other systems in the monitoring device 10 through a power management system. This minimises the power consumption of the monitoring device 10.

An inter-processor communication system is used to transfer information between the navigation system or more specially the navigation processor 130 and the MMI processor 230 of the communications system 200. This inter-processor communication system is integrally tied to a t)ata Storage system (see below) to minimise the power consumption during operation.

A tamper detection system 240 within the communications system 200 is used to detect attempts at removing the monitoring device 10, damaging the monitoring device 10 and/or opening of the monitoring device 10.

The alerting function system 250 within the communications system 200 informs the wearer of monitoring device 10 that he/she has to return to the centre where the monitoring device 10 was applied.

A system monitoring function 260 within the communications system 200 checks the battery voltages of the monitoring device 10 and raises an alarm if the remaining capacity is low.

A system wakeup unit 270 is used only during the final application of the monitoring device 10 (monitoring device 10 take on). This is used to wake the monitoring device 10 from the storage mode and to place it into application mode. The storage mode is a special very low power mode used to preserve the available battery power during storage of the monitoring device 10 prior to application.

As already mentioned above, a data storage system 300 is used to store long term records of the monitoring device 10 position versus time in a data storage unit 310. The data storage system 300 is also used as the bulk data transfer mechanism between the navigation system 100 and the communications system 200. Data transfer is performed using the inter-processor communication system 320.

A power management system 400 is responsible for providing power at the correct voltage and current levels to all the monitoring device 10 sub systems. This also allows the various sub systems to be powered down to conserve power. The main battery 18 is the s#le source of power for the monitoring device 10. A long life high capacity battery is used in this application.

The voltage regulation system 410 consists of various voltage regulators and switches that allow the generation of multiple system voltages. The regulators are enabled and disabled to power the relevant peripherals when required.

As already outlined above, a problem associated with GSM communications system 210 is the very high peak current consumption. This high peak power consumption is however short in duration. In order to maintain a stable supply to the GSM communications system 210, a secondary rechargeable battery 20 with low internal impedance is used to supply the power to the GSM communications system 210. This secondary battery 20 is periodically recharged from main battery 18 to maintain sufficient charge in the secondary battery 20. The secondary battery 20 also serves as an emergency power supply for the whole monitoring device 10 should connection to the primary battery 18 fail.

The overall power consumption of the monitoring device 10 is as low as possible without compromising functionality and size. The allowed physical size of the monitoring device 10 determines the maximum battery 18 capacity available. This in turn limits the maximum current that can be drawn per device.

The processor selected for the monitoring device 10 is from the Texas Instruments MSP430 family which are optimised for low power consumption although other processors can be used as well. All other devices in for the monitoring device 10 are selected based on functionality as required by this application, power consumption, and device physical size.

Making now reference to Figure 4, which schematically shows operation of the monitoring device in a flow chart, operation of the monitoring device 10 is further outlined. The monitoring device 10 relies on strict energy management to achieve the requirement of operating for a long period of time without requiring regular charging. The largest consumer of energy in the monitoring device 10 is the GSM communication system 210. Even with the steady advance in the efficiency of electronic components this is always likely to be the limiting factor as the data has to be transmitted over relatively large distances and would therefore limit the minimum transmission power required. The monitoring device 10 has therefore been designed around the principle of limiting the number of transmissions to the command centre.

The second largest energy consumer is the GPS receiver 120. To meet the low energy consumption requirements it is necessary to reduce the number of GPS updates as much as possible. GPS technology relies on very high frequency RF transmissions, which propagate in a line of site manner, and are therefore prone to blanking by structures which come between the receiver and the GPS satellites. To an extent this is reduced by developing very high sensitivity devices, but there are still instances where blanking occurs and the GPS position cannot be relied on. To overcome this limitation of GPS technology the monitoring device 10 is provided with the dead reckoning navigation system.

The latest technology accelerometers 110 available on the market today have very low energy requirements and can therefore be operated continuously. To keep the energy requirements to a minimum the accelerometer 110 output is in the order of 20 times a second. Between recordings the navigation processor 130 is put into a sleep mode. The output from the 3 -axis accelerometer 110 is monitored continuously to see if movement can be detected. As the accelerometer 110 measures in 3 -axes, and under static conditions it will be subjected to the force of gravity, this can be used to determine the orientation of the monitoring device 10 and the state. The difference between gravity and the magnitude of the acceleration is used as an indication of whether the monitoring device 10 is accelerating or not. The orientation of the monitoring device 10 is determined from the 3 acceleration vectors and the assumption that gravity acts towards the earth (down). The monitoring device 10 provides for two types of movement viz. movement as a result of stepping or movement as a result of being in a vehicle of some sort. In the event of stepping being detected the distance travelled is approximated by summing the steps and multiplying it by a stride length. In the event of movement due to being in a vehicle, the acceleration in the forward axis is determined and integrated twice over time to first determine the velocity and then the displacement. To determine the acceleration in the forward axis the orientation of the monitoring device 10 is approximated by integrating the acceleration data over an extended period and using this data to determine the acceleration vector which is in the same direction as the compass 140 and at right angles to the gravity vector.

The direction of the movement is determined from the compass 140, which is aligned with the forward axis. This direction together with the displacement, determined from either stepping or vehicular motion, is used to determine a new position. As the compass 120 has higher power requirements than the accelerometers a compass update is only requested if movement is detected and then at a maximum update rate of 1 per second.

The GPS receiver 120 is used to calibrate the monitoring device 10. For initial operation the GPS receiver 120 provides regular updates which are compared with the results of the dead reckoning system 110. By comparing the dead reckoning error with the GPS result errors in displacement and direction can be determined. The displacement errors are used to update the estimate for step length or the acceleration values (dependant on the cause of motion). The direction error is used to correct the compass readings. As the errors reduce, the number of GPS readings are reduced until an acceptable error is achieved. After this the minimum number of GPS readings would be a reading at each GSM update. If zone breaches are detected the result is always checked with the GPS. If the GPS cannot receive a signal then it will be assumed that the dead reckoning is correct and an alarm will be activated. The monitoring device 10 will attempt to get a GPS reading at regular intervals until the zone breach can be confirmed or the Monitoring device 10 has been returned to the command centre.

Exclusion zones are specified as central points with a given radius. Each time the monitoring device 10 calculates a new position from the dead reckoning algorithm the position is compared with all the exclusion zones. If there is a match then a position confirmation is requested from the GPS. This reduces the number of false alarms as a result of dead reckoning errors as well as reduces the energy requirement by not using the GPS excessively. If no movement is detected for an unusually long period, then a GPS position confirmation will be made and an alarm together with the position is sent to the command centre as this would indicate that either the monitoring device wearer is disabled or the monitoring device 10 has been removed without triggering the anti- tampering device.

The monitoring device 10 is provided with a number of circuits to monitor its serviceability. The battery voltage is continuously monitored, and the expected remaining capacity calculated. If the remaining capacity is reduced to such an extent that the monitoring device 10 may stop operating within a week, a "return home" alarm is activated on the monitoring device 10 to indicate to the user to return to the command centre.

The monitoring device 10 is supplied with anti-tampering devices. In the event that the anti-tampering device is triggered, indicating that the user is trying to remove the Monitoring device 10, an alarm together with the latest position is sent to the command centre so that the Monitoring device 10 can be recovered as soon as possible.

The monitoring device 10 continuously monitors its own processors and communications to ensure that it is operating correctly. If critical hardware errors are detected (such as GPS failure), an alarm is sent to the command centre and the "come home" alarm is activated. The Applicant believes that the monitoring device is advantageous in that it is able to provide a cost effective solution to the monitoring of persons in countries where the infrastructure is limited in rural areas. Furthermore, because the accelerometer and processor are low consumers of energy when compared to conventional active monitoring systems such as GPS/GSM, the monitoring device can operate for a longer period of time when compared with conventional GPS and GSM monitoring devices. Added to this is the reduced requirement of the need for constant monitoring by GPS/GSM, such task being performed by the accelerometer in conjunction with the processor as and when an unauthorized act is committed, the accelerometer and processor providing and/or permitting so called "intelligent monitoring".

Although certain embodiments only of the invention have been described herein, it will be understood by any person skilled in the art that other modifications, variations, and possibilities of the invention are possible. Such modifications, variations and possibilities are therefore to be considered as falling within the spirit and scope of the invention and hence forming part of the invention as herein described and/or exemplified.

This invention having been described in its preferred embodiment, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty. Accordingly, the scope of the invention is defined by the scope of the following claims.

Claims

1. A monitoring device comprising:

- a first positioning system capable of providing absolute position data;

2. The monitoring device according to claim 1, wherein the first positioning system includes a receiver for receiving data of a Global Position System.

3. The monitoring device according to claim 1 or 2, wherein the second positioning system includes an accelerometer.

4. The monitoring device according to any of claims 1 to 3, wherein the second positioning system includes an electronic compass.

5. The monitoring device according to any of claims 1 to 4, wherein the processing means is capable to calculate the initial position from the absolute position data and to calculate the location of the monitoring device from the dead reckoning position data with the initial position being provided by the absolute position data.

6. The monitoring device according to any of claims 1 to 5, wherein the monitoring device includes a transmitter for transmitting the location of the monitoring device to a control station.

7. The monitoring device according to claim 7, wherein the transmitter transmits data via a system for mobile communications.

8. The monitoring device according to claim 6 or 7, wherein the transmitter is mounted on or in the mounting means.

9. The monitoring device according to any of claims 6 to 8, wherein the monitoring device includes a further transmitter for transmitting the location of the monitoring device to a control station, the further transmitter transmits data to the control station in the event that there is no coverage of the networking service employed by the transmitter.

10. The monitoring device according to claim 9, wherein the transmitter transmits the data via a geo-static satellite relay which in turn permits transmission of the message to the control station.

11. The monitoring device according to any of the previous claims, wherein the monitoring device includes a timer which is mounted on or in the mounting means.

12. The monitoring device according to claim 11, wherein the timer is arranged in communication with the GPS for activating the GPS at pre-determinable, alternatively random, times during the day and/or in the event of an unauthorized act.

11. The monitoring device according to any of the previous claims, wherein the power supply is in the form of a battery, preferable a primary cell or rechargeable lithium battery.

12. The monitoring device according to claims 6 to 11, wherein the power supply is in the form of a first battery and a second battery, the first battery being capable of energizing the first positioning system, the second positioning system and the processing means and the second battery being capable of energizing the transmitter and/or the further transmitter

13. The monitoring device according to claim 12, wherein the second battery is charged by the first battery.

14. The monitoring device according to any of the previous claims, wherein the monitoring device includes an alarm means which may be mounted on or in the mounting means and which is provided to alert the wearer and/or control station of any event such as low or near depletion battery conditions or the wearer committing an unauthorized act like going outside a permitted area.

15. The monitoring device according to any of the previous claims, wherein the monitoring device includes an anti-tamper device, to sense removal of the monitoring device from the wrist or ankle of the wearer.

16. The monitoring device according to claim 15, wherein the anti-tamper device is in the form of a fibre-optic cable which is arranged in and extends the length of a strap.

17. The monitoring device according to any of the previous claims, wherein the monitoring device includes a co-ordinate storage means for storing co-ordinates which may define a zone or region into which the monitoring device may not proceed.

18. The monitoring device according to any of the previous claims, wherein the monitoring device includes mounting means, the mounting means being in the form of a non-allergenic strap.

19. The monitoring device according to any of claims 14 to 18, wherein the monitoring device includes a sensing means for monitoring the integrity of the fibre-optic cable and for sending a signal to the alarm means in the event of a breach of such integrity.

20. A monitoring device, substantially as described herein with reference to the accompanying drawings.

21. A method of operating a monitoring device, the method comprising:

- providing a dead reckoning tracker for continuous monitoring;

- providing a positioning system capable of determining absolute position data;

- processing means for storing data of the dead reckoning tracker as long the position of the monitoring device is inside a permitted zone; and

22. The method according to claim 21, wherein the dead reckoning tracker is calibrated form the absolute position data of positioning system.

23. The method according to claim 21 or 22, wherein the continuous monitoring of the dead reckoning tracker is confirmed by the absolute position data of positioning system when the position of the monitoring device is outside the permitted zone.

24. The method according to any of claims 21 to 23, wherein the transmitting means transmit stored tracks, so as to reduce the number of scheduled updates to a command centre.

25. The method according to claim 24, wherein the number of scheduled updates from the monitoring device to a command centre is chosen such that a power supply for energizing the monitoring device operates for up to one year without recharging.

26. The method according to claim 24 or 25, wherein the update is scheduled several times a day, preferably at random intervals.

27. A method of operating a monitoring device substantially as described herein with reference to the accompanying drawings.