Key location system

1. A system for detecting the distance between a vehicle and a key associated with the vehicle, the system comprising:

a transceiver associated with the vehicle;

the key; and

wherein either the transceiver or the key transmits two data items to the other of the transceiver and the key;

wherein the first of the two data items comprises information relating to a first time interval indicating a time difference between transmission of two previously transmitted messages and the second of the two data items comprises information relating to a second time interval indicating a time difference between reception of the two previously transmitted messages; and

a processor configured to calculate an estimated signal transit time between the transceiver and the key based at least in part on a ratio of a length of the two time intervals;

wherein the processor estimates the signal transit time based at least in part on the determined times and transmits the estimated signal transit time to either the transceiver or the key; and

the processor calculates a distance between the vehicle and the key based on the estimated signal transit time.

2. The system as claimed in claim 1 , wherein the two data items are transmitted in two separate messages.

3. The system as claimed in claim 1 , wherein a first message includes an indication of the data items among additional data items in the first message.

4. The system as claimed in claim 3 , wherein the processor registers a timestamp indicating the respective reception time for each of the two data items or uses the respective reception times of these two data items to determine a ratio between the time differences to determine the estimated signal transit time.

5. The system as claimed in claim 1 , wherein the two time intervals include one time interval indicating a time difference between two messages transmitted in a first direction, and a second time interval indicating a time difference between two messages transmitted in opposite directions.

6. The system as claimed in claim 1 , wherein the two time intervals include a first time interval indicating a time difference between two messages transmitted in a first direction, and a second time interval indicating a time difference two messages transmitted in opposite directions.

7. The system as claimed in claim 1 , wherein the estimated signal transit time between a transceiver and a key is calculated based on:

TOF estimate=½ *[TR 3_ B−TS 1_ B −(1+ X )*( TR 3_ B−TR 2_ B )];

wherein:

TR 3 _B indicates a time a third message was received;

TS 1 _B indicates a time a first message was sent;

TR 3 _B indicates a time the third message was received;

TR 2 _B indicates a time a second message was received; and

X indicates the ratio of the two time intervals.

8. The system as claimed in claim 1 , wherein the estimated signal transit time is calculated using:

TOF estimate=½ *[TR 3_ B−TS 2_ B −( TS 2_ B−TS 1_ B )/ X]

wherein:

TR 3 _B indicates a time a third message was received;

TS 2 _B indicates a time a second message was sent;

TS 1 _B indicates a time a first message was sent; and

X indicates the ratio of the two time intervals.

9. The system as claimed in claim 1 , the estimated signal transit time is calculated using:

TOF estimate=½*(½ *[TR 3_ B−TS 1_ B −(1+ X )*( TR 3_ B−TR 2_ B )]+½ *[TR 3_ B−TS 2_ B −( TS 2_ B−TS 1_ B )/ X ])

wherein:

TR 3 _B indicates a time a third message was received;

TS 1 _B indicates a time a first message was sent;

X indicates the ratio of the two time intervals;

TR 2 _B indicates a time a second message was received; and

TS 2 _B indicates a time the second message was sent.

10. The system as claimed in claim 1 , wherein the transceiver transmits precisely one message to the key, and after which the key transmits two messages to the transceiver.

11. The system as claimed in claim 1 , further comprising two transceivers, each transceiver transmitting a message to the key, after which the key transmits two messages to at least one transceiver.

12. The system as claimed in claim 1 , wherein the transceiver transmits two messages to the key, after which the key transmits a message to the transceiver.

13. The system as claimed in claim 1 , further comprising two transceivers, each transceiver transmitting two messages to the key, after which the key transmits a message to at least one transceiver.

14. The system as claimed in claim 1 , wherein the transceiver transmits a message with two specified data items to the key, after which the key transmits a message, with two further specified data items to the transceiver.

15. The system as claimed in claim 1 , wherein the distance of the key from the transceiver is determined based at least in part on a product of signal propagation speed of electromagnetic signals in air and the estimated signal transit time TOFestimate between the key and the transceiver.

16. A method for determining the distance of a key from a transceiver in a motor vehicle on the basis of an estimated signal transit time between the transceiver and the key, the method comprising:

transmitting two data items in a first direction from one of the transceiver and the key to the other of the transceiver and the key;

receiving the two data items and determining reception times for each of the two data items;

using the respective reception times to determine the estimated signal transit time;

transmitting a message in return including an indication of the ratio between a first time interval and a second time interval;

the first time interval indicating a time difference between the transmission or reception of the two data items transmitted in the first direction,

and the second time interval indicating a time difference between two additional data items transmitted in a second direction opposite to the first direction;

wherein an estimated signal transit time of at least one user is determined from the indication, transmitted thereto, of the ratio of the lengths of two time intervals and times measured; and

calculating a distance between the vehicle and the key based on the estimated signal transit time.