A Security System
This invention relates to security systems and in particular to a
security system for a vehicle.
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 SI at a first
frequency FI 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 SI.
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 Si 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 FI of the first signal component SI 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 FI and the second frequency F2.
When the remote transponder unit 14 has de-coded the first signal
component SI 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 SI 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 FI 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 SI 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.