It is known to provide a vehicle with a passive entry security system in which a security control unit in the vehicle sends a coded interrogation signal to a remote/portable transponder carried by the vehicle owner and one such prior art system is disclosed in EP 0218251. On receipt of the coded interrogation signal, the remote transponder sends a coded response signal back to the security control unit, for comparison with an expected response code.

It is a problem with some passive entry security systems that a relay unit can be used by a thief to capture the interrogation signal and relay it to the remote transponder and thereafter to relay the response signal back to the vehicle.

One solution to this problem is disclosed in DE 40 20 445, in which a specific time slot is provided for reception of the response signal and any signal received outside this time slot is ignored.

It is an object of this invention to provide an improved security system.

According to the invention there is provided a security system comprising a security control unit and a remote transponder unit arranged in use to communicate with each other using a coded security signal, wherein the security signal comprises first and second signal components which are transmitted at different frequencies and the frequency of transmission of the second signal component is encoded in the first signal component.

The unit which is arranged in use to receive the security signal may be arranged to decode the frequency of the second signal component from the first signal component.

The frequency of the second signal component may be generated in a variable manner by the unit arranged to transmit the security signal.

The variability of the generation of the frequency of transmission of the second signal component may be achieved by a substantially random selection, from within a predetermined range of frequencies, by the unit arranged to transmit the security signal.

The unit arranged to receive the security signal may be arranged to tune itself to receive the second signal component after it has decoded the frequency thereof from the first signal component.

The range of frequencies of transmission from which the frequency of transmission of the second signal component may be selected is such that the difference frequency between the frequency of the second signal component and the frequency of the first signal component is substantially the same as the frequency of the second signal component.

The unit arranged to receive the security signal may be arranged to determine the difference frequency as well as the frequency of transmission of the first and second signal components and thereby to determine whether or not a relay unit has been used to relay the security signal between the units.

The frequency of the first signal component may be in the order of 120 kHz. The frequency of the second signal component may be in the order of 40 kHz to 80 kHz. The difference frequency may be in the order of 40 kHz to 80 kHz.

The security system may comprise a security system of a vehicle.

The invention also provides a method of controlling a security system, the security system comprising a security control unit and a remote transponder unit arranged in use to communicate with each other using a coded security signal, the method including :

a) transmitting the security signal in first and second signal components, said components being transmitted at different frequencies ; and b) encoding the frequency of transmission of the second signal component in the first signal component.

The method may include decoding, in the unit which is arranged in use to receive the security signal, the frequency of the second signal component from the first signal component.

The method may include generating the frequency of the second signal component in a variable manner and may include achieving that variability by a substantially random selection from within a predetermined range of frequencies.

The method may include tuning the unit arranged to receive the security signal such that it receives the second signal component after it has decoded the frequency thereof from the first signal component.

The method may include selecting the frequency of transmission of the second signal component such that the difference frequency between the frequency of the second signal component and the frequency of the first

signal component is substantially the same as the frequency of the second signal component.

The method may include determining the difference frequency as well as the frequency of transmission of the first and second signal components and thereby determining whether or not a relay unit has been used to relay the security signal between the units.

The method may include transmitting the first signal component at a frequency in the order of 120 kHz and/or may include transmitting the second signal at a frequency in the order of 40 kHz to 80 kHz. The method may include transmitting the signal components at a difference frequency in the order of 40 kHz to 80 kHz.

The invention will now be described by way of example only and with reference to the accompanying drawings, in which : Figure 1 is a schematic diagram of a vehicle including a security system according to the invention ; and Figure 2 is a schematic diagram of the vehicle of Figure 1 while an attempt is being made to defeat the security system.

The security system of the present invention is a passive entry system and a general example of a known arrangement of this type is disclosed in EP 0218251. In this known system, a security control unit detects the approach of a user to the vehicle and transmits a coded command signal to a remote transponder which is carried by the user and which identifies itself by returning a response signal to the security control unit on receipt of the command signal. The security control unit opens the vehicle on receipt of a valid response signal.

Referring now to the Figures and in particular to Figure 1, a vehicle 10 includes a security system comprising a security control unit 12 fitted to the vehicle 10 and a portable/remote transponder unit 14 carried by a user. The units 12,14 are arranged in use to communicate with each other by passing coded security signals.

On detecting the approach towards the vehicle 10 of a remote transponder unit 14, the security control unit 12 transmits a coded security signal. This coded security signal is transmitted in the form of two signal components, comprising a first coded signal component Sl at a first frequency F1 and a second coded signal component S2 at a second frequency F2. The frequency F2 of transmission of the second signal component S2 is encoded in the information conveyed by the first signal component Sl.

The remote transponder unit 14 is capable of tuning itself to receive the range of frequencies sent by the security control unit 12. The remote transponder unit 14 is arranged to de-code the frequency F2 of the second signal component S2 from the first signal component Sl and is then arranged to tune itself to also receive the second signal component S2 at its respective frequency F2.

The frequency F2 of the second signal component S2 is generated internally by the security control unit 12 and is selected by the security control unit 12 such that its difference frequency with respect to the frequency F1 of the first signal component S1 is substantially the same as its generated frequency F2. The frequency F2 of the second signal component S2 may instead be selected substantially randomly from a number of second frequencies F2 which could be stored in the security control unit 12 and all of which would conform to the requirements laid out above for the difference frequency.

The remote transponder unit 14 is arranged to determine the difference frequency as well as the first frequency F1 and the second frequency F2.

When the remote transponder unit 14 has de-coded the first signal component S1 and from that the second signal component S2, it transmits a response signal S3 back to the security control unit 12 at a fixed frequency

F3 and upon receipt of the response of signal S3 the security control unit 12 disarms the alarm and opens the vehicle 10.

In this example, the frequency of the first signal component S1 is set to a nominal 120 kHz. The frequency F2 of the second signal component S2 is selected variably or randomly in the range of 40 kHz to 80 kHz such that it results in a difference frequency also in the order of 40 kHz to 80 kHz. The frequency of the response signal S3 is conveniently preset in the region of 433 MHz.

Referring now in particular to Figure 2, the vehicle 10 of Figure 1 is shown in a position in which the remote transponder unit 14 is out of range of the vehicle 10 and a pair of relay units 16,18 are interposed between them 10,14.

The relay units 16,18 are of the code grabber type known to car thieves. One of the relay units 16 is positioned close to the vehicle 10 and the second relay unit 18 is positioned close to the remote transponder unit 14. Such relay units 16,18 commonly operate with a large bandwidth and are used to try to trigger and capture the coded security signal from the security unit 12 and then to modulate it onto a higher frequency carrier wave F4, of for example 27 MHz, after which it is transmitted to the distant relay unit 18 as a captured signal S4.

The relay unit 18, which is in the vicinity of the remote transponder unit 14, demodulates this higher frequency signal S4 and generates sum and difference frequencies. The relay unit 18 then re-transmits the signal it has received from the first relay unit 16 to the remote transponder unit 14 at its original frequency F1 of 120 kHz as a relayed signal S5. This is done in an attempt to cause the remote transponder unit 14 to transmit its response signal S3, which would then be captured by the relay unit 18 and transmitted back to the relay unit 16 at, for example, 418 MHz. If this can be achieyed, the relay unit 16 can re-transmit the response signal S3 at its frequency of 433 MHz to the security control unit 12 and thereby defeat the security system.

Because the frequency F2 of the second signal component S2 is in the range of 40 kHz to 80 kHz and the difference frequency is also in the range of 40 kHz to 80 kHz, however, it is significantly more difficult to reject the difference signal without also rejecting what may be the genuine second frequency F2. This means that the relay units 16,18 have great difficulty in relaying the second signal component S2 to the remote transponder 14.

Thus the remote transponder 14 can, during normal communication with the security controller 12, receive the first signal component S1 and from that decode the frequency F2 of the second signal component S2 and determine the difference frequency.

If an attempt is made to relay the security signal, however, the remote transponder unit 14 cannot determine the frequency F2 of the second signal component S2 from the relayed signal S5. The remote transponder 14 cannot then receive the second signal component S2 and therefore does not transmit its response signal S3.

It can therefore be seen that it is possible for the security system of this invention to determine whether or not a relay transmitter arrangement 16, 18 has been used and for it then to take defensive action, i. e. by not transmitting its response signal S3. In this manner, it is made more difficult to make an effective code grabber 16,18 without incurring high cost and therefore the security of the vehicle 10 is improved.