There is a growing need for the tracking and recovery of objects to combat increasingly high levels of crime. There are various methods of electronically tagging objects and thereby causing a warning alarm to occur when the object is removed from its proper location. However, this technique only operates over a relatively short range and if the object is not recovered at the time of removal, further tracking of it is no longer possible.

It would therefore be desirable to have an object tracking system which addresses this limitation by providing for longer range tracking over a wide area.

SUMMARY OF THE INVENTION According to the invention, there is provided an object tracking system comprising: a security device located on an object to be tracked, the security device including a transmitter for transmitting an object tracking signal ; a plurality of vehicle mounted mobile beacon units, each beacon unit forming part of a secondary vehicle-based object tracking network and including a beacon receiver for receiving the object tracking signal from the security device's transmitter; and a primary vehicle tracking network for tracking the vehicles making up the secondary network, so that the position of the vehicle that has received the object tracking signal will provide the approximate position of the object.

Conveniently, the system includes sensing means for sensing an unauthorised or unscheduled act being carried out on or by the object, the sensing means being arranged to trigger the transmission of the object tracking signal.

Typically, the sensing means includes a receiver located on the security device for receiving location, proximity or activation signals.

Preferably, the primary vehicle-based tracking network is of the type disclosed in South African patent 94/4150, and includes a strategically located array of signpost stations. In addition, the primary tracking network may also be defined by a conventional GSM network including a plurality of GSM sites, as described in international patent application no. PCT/IB00/01597.

Conveniently, the object tracking signal includes an object identification signal for identifying the object.

In one version, the signpost station transmits a signpost location signal for identifying its location, the signpost location signal being received by the beacon receiver, with each beacon unit including a beacon transmitter for transmitting the object identification signal together with the signpost location signal to a central station, either directly or via a dedicated receiving station. In another version, the position of the vehicle is determined using a GPS receiver.

Alternatively, each beacon unit is provided with a beacon identification signal and includes a beacon transmitter for transmitting its own identification signal as well as the object identification signal to the central station.

Advantageously, the object may be either an animate or an inanimate object.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic diagram of an object tracking system according to the invention; and Figure 2 shows a schematic diagram of a primary vehicle tracking network used in the object tracking system shown in Figure 1.

DESCRIPTION OF EMBODIMENTS An object tracking system 10 comprises an object 12 having a security device 14 mounted thereon. The security device 14 comprises a 403.9 MHz radio transmitter 16, a 915 MHz radio receiver 18, a processor 20 and a sensor 22.

The security device 14 has a unique identification code associated with it for identifying the object 12 to which it is mounted, with the transmitter 16 being capable of transmitting a corresponding object identification signal. The receiver 18 is arranged to receive location, proximity and/or activation signals, as will be discussed in more detail further on in the specification. The security device 14 may include an optional 915 MHz radio transmitter 23 for allowing the device 14 to interact directly with a primary vehicle tracking and recovery network 24, such as described in South African Patent number 94/4150, for long-range tracking.

A mobile beacon unit 26 mounted within a vehicle 28 comprises a 403.9 MHz radio receiver 30, a processor 32, a GSM transceiver unit 34, and, optionally, a GPS unit 36. Each beacon unit 26 also includes a 915MHz transmitter 37 which allows the beacon unit 26 to form part of the primary vehicle tracking and recovery network 24. Each beacon unit 26 is also provided with a unique beacon identification code, with the beacon transmitter 37 being arranged to transmit a corresponding beacon identification signal.

The primary network 24 will now be described in more detail with reference to Figure 2. The network 24 includes a strategically located array of signpost stations 38 which are small uniquely coded radio transmitters with a long life battery that are mounted at road intersections, bridges, lamp posts and the like. A beacon unit 26 passing a signpost station 38 would receive messages from these signpost stations 38 and would therefore know its position by virtue of the signpost station's location signal. Such a network 24 typically comprises many tens of thousands of vehicles 28 operating within the coverage area of the network 24 which could act as receiver sites for tracking objects 12, as will become clearer further on in the specification.

The primary vehicle tracking and recovery network may also be defined by the GSM transceiver unit 34 which utilises a conventional GSM network 40 comprising a plurality of GSM sites 42 for determining the approximate position of the vehicle 28. This aspect is described in more detail in International patent application no. PCT/IB00/01597.

As mentioned above, the beacon unit 26 may optionally include a GPS unit 36 for determining the position of the vehicle 28. Although the GSM network 40 provides good communications, generally, it is unable to provide the exact location of the vehicle. In cases where the exact position of the vehicle is required, it is envisaged that the GPS receiver would be required.

The system 10 includes sensing means 22 for sensing an unauthorised or unscheduled act being carried out on or by the object 12. The sensing means 22 triggers the transmitter 16 of the security device 14 into sending an object tracking signal, the object tracking signal including the object identification signal for identifying the object 12.

The operation of the object tracking system 10 will now be explained. Under normal circumstances, the security device 14 is in a semi-passive state, consuming minimal electrical power. When an unauthorised or unscheduled act is sensed, or the receiver 18 receives an activation signal, the device 14 begins transmitting a short pulsed object tracking signal at regular intervals, for example once per second. If the device 14 comes within range of a vehicle 26 forming part of the primary network 24, the vehicle's beacon unit 26 will receive and decode the object tracking signal being sent by the security device 14. A secondary vehicle-based object tracking network is thus effectively defined comprising the beacon unit 26 and the object's security device 14. If fitted with the optional transmitter 23, the device 14 can be tracked directly by the primary network 24. The beacon unit 26 then relays the object tracking signal to a control room 44 either directly or via one of a plurality of dedicated receiving stations 46. The position of the vehicle 28, and therefore that of the object 12, can then be determined by either the primary vehicle tracking network 24 and its associated signpost stations 38 or the GSM transceiver unit 34. Clearly, the GPS unit 36, if fitted, could also be used to determine the position of the vehicle 28.

Recovery forces can thus be dispatched to the last known location of the object 12 and, using a portable 403.9 MHz receiver, locate the object 12.

It is clear that the sensing means may take various forms, and depends primarily on the nature of the object being monitored. A number of examples are described below.

Consider first a package in the cargo area of a vehicle or on a trailer on the vehicle. Periodically the security device 14 emits a very low power RF pulse, also known as a proximity signal, that is received by the receiver 30 on the vehicle beacon unit 26. This in turn sends back a responding low power pulse.

Clearly, the beacon unit 26 could initiate and the security device 14 respond.

As long as the proximity loop is closed the security device 14 remains passive.

However, if the loop is broken by the security device 14 getting out of range, the beacon unit 26 immediately sends a message to the control room 44 who can then initiate a response. The security device 14 would simultaneously also be activated into transmitting the object tracking signal at normal power which can then be received by other vehicles in the vicinity, forming part of the primary vehicle tracking network. Recovery forces equipped with tracking receivers may also track the tracking signal. As the object 12 moves through the network and/or as other vehicles pass in its vicinity, its position gets updated in order to assist in recovery operations. In this application, it is envisaged that the optional 915MHz transmitter 23 may be added to the security device 14 to allow for longer range tracking of the object 12. Recovery forces can then use the same tracking equipment as for vehicle recovery in order to locate the object 12. Tracking range in this mode can be from 1 to 20 kilometers or more, and depends on various factors such as terrain and elevation.

While Figure 1 depicts an object located on a vehicle such as a parcel, it could just as well be a household television set, a probationary prisoner, a computer in an office or virtually any other type of person, animal or asset. The only substantial variation in dealing with these types of objects will be the manner in which the security device is activated and housed. Some examples of the activation means includes a simple trip wire arrangement for objects such as solar panels, a disturbance in a magnetic field for objects which can be magnetically connected to a support surface and a low frequency receiver such as a wire frame in a door way.

A second application of the present invention would be that of an electronic manacle for probationary prisoners. In this case a sensor would be mounted at the premises to which the prisoner is restricted, and the prisoner would wear the security device 14. Again, failure of the loop constituted by the back and forth signal transmissions between the sensor and the security device 14 would cause the sensor to send an alarm and for the security device 14 to transmit an object tracking signal. This same approach could easily be modified for keeping track of elderly people or Alzheimer's patients in nursing homes.

A third application would be useful for home appliances such as television sets. These are normally connected to a mains power source and if the source is disconnected, the security device would be activated and again tracked as described above. A number of residents in an area could share a common sensor that would provide immediate notification of the activation of the security device. Local security companies could also equip their patrol vehicles with beacon devices to allow for rapid response and tracking. It is also possible for the security device 14 to be triggered by means of suitable activation signals being transmitted by either the signpost stations 38 or the GSM sites 42.

An additional application of the present invention includes the use thereof in an electronic seal or padlock that, upon tampering therewith, would transmit the object tracking signal to notify beacon units of the unauthorised act. Further applications include the protection of solar panels, equipment at remote radio sites, livestock, pets, cable drums and buried telephone or power cables. The only variation required in dealing with these various types of objects, as mentioned above, is the manner in which the security device is activated.

Although the principle of radio transmitter tracking of people, animals and objects is widely known, current systems have severe limitations in terms of tracking and recovery range, power consumption and therefore size and cost.

The present invention addresses these limitations by using a tracking receiver network that is widely deployed for other reasons, thereby making the whole concept economically viable.

A further advantage of the present invention, which is related to its versatility, is the fact that it comprises modular units which can be conveniently connected together, depending on the nature of the object being monitored, as well as the associated accuracy required in determining the object's location.

It should be clear that the specific frequencies used for the transmitters and receivers in this specification are merely exemplary, with the present invention not being limited to these frequencies.