Vehicle access system

1. A method of operating an access system for a vehicle, the method comprising:

scanning for a first communication signal from a first communications device using a first communication channel of a first communications system; and

initiating a vehicle access process based on detecting the first communication signal from the first communications device;

wherein the vehicle access process comprises:

comparing a received signal strength indication, RSSI, of the received first communication signal to a predetermined threshold signal strength value;

sending a challenge signal for a second communications device using a second communication channel of a second communications system in the event that the RSSI of the received first communication signal exceeds the threshold signal strength value; and

controlling the vehicle access system based on a response signal received from the second communications device, the response signal having been sent in response to the challenge signal.

2. A method as claimed in claim 1 , wherein the second communications device is configured to allow a user to gain access to a vehicle.

3. A method as claimed in claim 1 , further comprising identifying a device identifier for the first communications device from the first communication signal and checking if the device identifier corresponds with that of an approved device.

4. A method as claimed in claim 3 , further comprising turning on a transmitter for transmitting the challenge signal via the second communication channel in the event that the first communications device is an approved device.

5. A method as claimed in claim 4 , wherein the transmitter comprises a Radio Frequency, RF, transmitter and the second communications channel comprises an RF channel.

6. A method as claimed in claim 5 , wherein the transmitter comprises a Low Frequency, LF, transmitter and the second communications channel comprises an LF channel.

7. A method as claimed in claim 1 , further comprising sending a signal strength request signal to the first communications device over the first communication channel and receiving a signal strength confirmation communication signal from the first communications device over the first communications channel, the signal strength confirmation communication signal comprising a signal strength value of the signal strength request signal as received at the first communications device.

8. A method as claimed in claim 7 , further comprising comparing the signal strength value to a predetermined threshold signal strength value.

9. A method as claimed in claim 8 , further comprising sending the challenge signal for the second communications device in the event that the signal strength value exceeds the threshold signal strength value.

10. A method as claimed in claim 1 , wherein the vehicle is provided with multiple antennas for receiving communication signals sent over the first communication channel, the method further comprising comparing the signal strength of signals received from the first communications device at two or more of the antennas to determine an approach vector for the first communications device.

11. A method as claimed in claim 10 , further comprising turning on a transmitter for transmitting the challenge signal on the side of the vehicle that the first communications device is approaching.

12. A method as claimed in claim 1 , comprising cancelling the vehicle access process in the event that no response signal is received from the second communications device.

13. A method as claimed in claim 1 , comprising determining that the first communications device is moving away from the vehicle in dependence on one or more changes in the signal strength of signals received over the first communications channel from the first communications device.

14. A method as claimed in claim 13 , further comprising turning on an RF transmitter in order to determine if the second communications device is moving away from the vehicle.

15. A method as claimed in claim 14 , further comprising locking the vehicle in the event that either or both of the first and second communications devices have left the vicinity of the vehicle.

16. A control unit arranged to control an access system for a vehicle, the control unit comprising:

one or more inputs for receiving a first communication signal from a first communications device and a second communication signal from a second communications device;

a processor arranged to manage a vehicle access process; and

an output arranged to output vehicle access control signals;

wherein the processor is arranged to:

scan for a first communication signal from a first communications device using a first communication channel of a first communications system;

initiate a vehicle access process based on detecting the first communication signal from the first communications device, the vehicle access process comprising comparing a received signal strength indication, RSSI, of the received first communication signal to a predetermined threshold signal strength value and, in the event that the RSSI of the received first communication signal exceeds the threshold signal value, sending from the output a challenge signal for a second communications device using a second communication channel of a second communications system; and

wherein the output is arranged to output a vehicle access control signal to control the vehicle access system based on a response signal received from the second communications device, the response signal having been sent in response to the challenge signal.

17. An access system for a vehicle comprising a control unit as claimed in claim 16 , a first antenna arranged to operate over the first communication channel and a second antenna arranged to operate over the second communication channel.

18. A vehicle comprising a control unit as claimed in claim 16 .

19. A non-transitory computer-readable medium having stored thereon a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of claim 1 .