The present invention relates to a system for tracking an object. The system is based on the use of equipment capa- ble of transmitting and receiving radio-frequency waves. Such objects comprise, e.g., automotive vehicles. In the embodiment described below the system is adapted to the location of a certain vehicle, while the system may also be used for checking the identification data of said cer- tain vehicle.

Systems are known in the art for the location of a vehi¬ cle, whereby such systems are based on a network comprised of base stations equipped with a radio-fre- quency transmitter and receiver, and identification de¬ vices mounted onboard each vehicle. As the need for loca¬ tion arises, the transmitters of the base station network begin sending a coded signal, which is then recognized by the vehicle-mounted identification device, whereby the device activates its own transmitter.. The device sends a coded signal, which is received and identified by the closest base station, whereby the vehicle to be located can be tracked to within the vicinity of the identifying base station.

Such a system is functional in the location of a certain object to within the range of a known base station, and thus can be used to track, e.g., a stolen vehicle. How¬ ever, the system useability presumes that a tracking re- quest on the vehicle is issued before the vehicle leaves the operating area of the location system. Hence, this prior-art system is incapable of overcoming the chance that a stolen vehicle is transferred swiftly after the theft to the state border checkpoint with skillfully falsified identification data. Then, the possibilities of the checkpoint official to verify the credentials of the permissible passage of the vehicle through the checkpoint remain extremely limited.

It is an object of the present invention to achieve such a system which is capable of overcoming the above-de¬ scribed drawback. If a vehicle approaching a checkpoint such as a state border crossing point is provided with an identification device according to the present system, the identification data of the vehicle can be verified against the vehicle database available to the system and compared with the conventional identifying specifications of the vehicle such as the register plate, chassis number and others.

The above-described goal of the present system is achiev¬ ed by what is stated for the system in the characterizing part of claim 1.

The system according to the invention relates to the principles of prior-art systems in that the system pre¬ sumes the object being located to be equipped with an identification device having a receiver tuned to receive radio signals capable of activating a transmitter incor¬ porated in the device.

According to the main characterizing property of the in- vention, the identification device is arranged to respond to the presence of a least two different interrogating signals so that the device transmitter for sending the device identification code is activated in a manner de¬ termined by the respective interrogating signal. The identification device has two basic operating modes, namely, the device transmitter can be set to send the identification code for the duration of receiving a pre¬ determined local interrogation signal and the trans¬ mission of the device identification signal can be ir- revocably activated by another interrogation signal. Fur¬ thermore, the identification device is advantageously provided with a operating mode for deactivating the

transmission of the identification signal after the de¬ vice has received a respective device-specific signal.

This kind of arrangement makes it possible by sending a general interrogation signal to verify at a state border checkpoint, for instance, that the visible identification details of a vehicle passing the checkpoint are conform¬ ant to the identification data stored in the system data¬ base. Moreover, the system of course issues an alarm if a vehicle reported stolen approaches the checkpoint.

In the above-described situation, where a general inter¬ rogation signal of vehicle identification data is sent locally, it is possible that a plurality of vehicles equipped with the identification device may be at the checkpoint. Then, the device transmitters of the other vehicles queuing-in except that of the vehicle being checked can be temporarily deactivated using the operat¬ ing mode according to a further characterizing property of the invention, whereby the identification data of the vehicle being checked can be verified without interfer¬ ence. The deactivated device transmitters can then be reactivated each in turn. The locally sent general inter¬ rogation signal is transmitted at a low power level to limit its effect to a precisely confined local area. Cor¬ respondingly, this local interrogation signal controls the device transmitters responding to it to send their • own identification signals at a low power level, which similarly limits the range of the identification signal to the area of the interrogating station.'

According to another further characterizing property of the invention, a device-specific signal sent locally may be used to control the identification device when re- quired so as to, e.g., alter its characteristics.

The transmitters and receivers used in the system are

designed to operate in a narrow frequency band so that the system can be readily adapted to existing wide-area radio communications systems such a mobile telephone sys¬ tems (ARP, NMT, GSM) , broadcasting carrier subsignals including RDS, ERMES and the PORSAG paging network.

The near-range local interrogation transmissions of the system can be sent from transmitters located in the base stations of, e.g., customs stations, traffic checkpoints, etc. The identification signal sent by the device can be received at the same local base stations used for sending the local interrogation signal as well as at receiving points specifically located at traffic nodes. Mutual com¬ munications between the different parts of the system are arranged via the public telephone network, for instance.

The interrogation and/or receiving units of the system may also be located onboard, e.g., official vehicles to carry out local tracking operations. The identification devices of the system can be fabricated into a compact package of light weight, whereby the identification de¬ vice is easy to place in a concealed and nondisturbing manner in a number of different objects. Such tracked objects include shippings of valuables, patient tracking and others. Also the interrogation and receiving equip¬ ment of the system can be made lightweight making them suitable for, e.g., tracking in a rugged terrain.

The identification code transmissions can be implemented using, e.g., frequency modulation.