WO2018113997A1

WO2018113997A1 - Method for obtaining relatively accurate timings for location measurement

Info

Publication number: WO2018113997A1
Application number: PCT/EP2016/082579
Authority: WO
Inventors: Christopher Kevan Lowe; Robert William Young
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2016-12-23
Filing date: 2016-12-23
Publication date: 2018-06-28

Abstract

A base station (4) comprising: a local clock (16); at least one model of a clock of a neighbouring base station (14, 15); a transceiver (5); and a controller configured to cause the transceiver to transmit to a network entity time information derived from the local clock and time information derived from the model.

Description

METHOD FOR OBTAINING RELATIVELY ACCURATE TIMINGS FOR LOCATION MEASUREMENT

BACKGROUND

This invention relates to a location system.

Known location systems are based on the difference in timing between the reception of a location signal sent by an uplink user equipment (UE) device by multiple base stations. Each base station will mark the time of arrival of the signal and a central location server will calculate the position of the UE based on the differences in time at which the signal is received at each base station, the locations of which are known to the central location server.

To achieve an accurate location fix, typical prior art approaches require that the clocks on the base stations must be synchronized, or the offset between them known, to a suitable level of accuracy. Some systems use atomic clocks located at the base stations to provide a precise ongoing timing reference, whereas some may use a central time source and synchronize the base station clocks to that time source from time to time using a network time protocol over a link between the central time source and each base station. However, these are not ideal for a mobile network. Atomic clocks are expensive, and synchronizing the base station clocks to a suitable level of accuracy is difficult.

The GPS satellite positioning system may provide relatively accurate times, but in a typical base station any GPS capability is normally provided through a separate subsystem to the main base station timing system. As a result, the accuracies available through this system may be around 100 ns. 'Matrix' style systems may estimate the clocks across a network, however the accuracies in these systems are lower.

It is desirable to use a mechanism to provide the effect of synchronized clocks across the network where no master clock source and no accurate clock at each base station are required.

SUMMARY OF THE INVENTION According to one aspect there is provided a base station (the first base station) comprising: a local clock; at least one model of a clock of a neighbouring base station; a transceiver; and a controller configured to cause the transceiver to transmit to a network entity time information derived from the local clock and time information derived from the model.

The controller may be configured to: receive from the neighbouring base station a notification comprising time information indicative of a time in a local clock of the neighbouring base station; and configure the model in dependence on the received time information. In this way, the model may reflect an estimated offset between the local clock of the first base station and the local clock of the neighbouring base station. The controller may be configured to be triggered to cause the transceiver to transmit the time information derived from the local clock and time information derived from the model in response to the first base station receiving a communication of a user equipment device with the base station. This may allow the first base station to participate in locating the user equipment device by means of that communication.

The controller may be configured to estimate by means of the model a time in the local clock of the neighbouring base station at which the communication of the user equipment device was received by the claimed base station. The said time information derived from the model may represent that estimated time. This may allow differences between the local clocks of the base stations to be cancelled out.

According to a second aspect there is provided a network entity comprising: a receiver, configured to receive from at least one base station ("first base station") time information based on a local clock of the base station and on a model of a local clock of another base station.

The network entity may comprise a processor configured to estimate a location of a communication device based on the received time information. Thus the time information can be used to assist in locating the communication device. The communication device may be a user equipment device.

The network entity may be configured to: receive from the first base station: (i) an estimate of an offset between the local clock of that base station and the local clock of the other base station and (ii) an estimate of the time in the local clock of the other base station at which a signal transmitted by the communication device was received by the said at least one base station; and to estimate the location of the communication device in dependence on those estimates. This may help to allow differences between the local clocks of the base stations to be cancelled out. The network entity may be configured to: receive from the other base station: (i) an estimate of an offset between the local clock of that other base station and the local clock of the first base station and (ii) an estimate of the time in the local clock of the first base station at which a signal transmitted by the communication device was received by the other base station; and estimate the location of the communication device in dependence on those estimates. This may help to allow differences between the local clocks of the base stations to be cancelled out. The network entity may comprise a memory storing data defining the locations of the base stations. These locations may assist in determining the location of the device.

One or more of the base stations may be configured to transmit a notification comprising a timing feature and an indication of the time in the local clock of that base station at which the timing feature was transmitted. One or more of the base stations may be configured to receive such a notification and estimate a difference between its local clock when the timing feature was received at that base station and the indicated time in the local clock of the transmitting base station at which the timing feature was transmitted. Each of those base stations may be configured to perform that process reciprocally. The notification or least some of the notifications may be conveyed by a continuous data transmission. This can avoid the need to send multiple messages. Alternatively, The timing feature or at least some of the timing features may be conveyed by a separate data transmission from a second data transmission conveying the indication of a time at which that timing feature was transmitted. This can permit the timing feature to be carried in a shorter message than would otherwise be required.

A network entity may be configured to estimate the relative offset(s) of the local clocks of the base stations by estimating, in dependence on the locations of the base stations, one or more relative offsets that best fit the time difference information referred to above. The network entity may receive information indicating the times in the local clocks of multiple ones of the base stations at which a single timing feature transmitted from one of the base stations has been transmitted. The network entity may be configured to estimate the relative offset(s) of the local clocks of the base stations by estimating, in dependence on the locations of the base stations, one or more relative offsets that best fit that further information together with the time difference information referred to above. This may provide redundant information that allows the estimates of clock offset to be made more accurately. The fit of the relative offset(s) to the data designated (i) and (ii) above and optionally the further information may be determined in accordance with a Kalman filter and/or an error minimisation algorithm such as a least squares method. The local clock(s) of one or more of the base stations may be set independently of any central time source. The local clock(s) of one or more of the base stations may be set with less relative precision than the resolution to which the relative offset(s) of the local clocks of the base stations is/are estimated. In these situations the estimation of the clock estimates in the manner set out herein can avoid the need for more precise setting of the clocks.

According to a third aspect there is provided a method for estimating the relative offsets of local clocks of a plurality of wireless transceivers, the method comprising: transmitting from a first one of the transceivers a notification comprising a timing feature and an indication of the time in the local clock of the first transceiver at which the timing feature was transmitted; receiving the notification at a second one of the transceivers, and determining the time in the local clock of the second transceiver at which the timing feature was received; and in dependence on (i) the time in the local clock of the second transceiver at which the timing feature was received and (ii) the indicated time at which that timing feature was transmitted in the local clock of the first transceiver, estimating the relative offset of the local clocks of the transceivers.

The method may comprises transmitting notifications from multiple transceivers, each notification comprising a timing feature and an indication of the time in the local clock of the transceiver that transmitted the respective notification at which the timing feature was transmitted; receiving each of the notifications at the second transceiver and determining (i) the time in the local clock of the second transceiver at which each of the respective timing features was received and (ii) the indicated times at which those timing features were transmitted in the local clock of the respective transceivers that transmitted them; and in dependence on that information estimate the relative offsets of the local clocks of the second transceiver and the other transceivers. In this way, the clock offsets between multiple transceivers can be determined. This may also permit the offsets to be estimated more accurately, for example by using information relating to the locations or relative locations of the transceivers. The step of estimating the relative offsets of the local clocks of the transceivers may comprise estimating, in dependence on the locations of the transceivers, a set of relative offsets that best fits the information designated (i) and (ii) above. This may provide additional accuracy in estimating the clock offsets.

The local clock(s) of one or more of the wireless transceivers may be set independently of any central time source. The local clock(s) of one or more of the wireless transceivers may be set with less relative precision than the resolution to which the relative offset(s) of the local clocks of the transceivers is/are estimated. In these situations the estimation of the clock estimates in the manner set out herein can avoid the need for more precise setting of the clocks. According to a fourth aspect there is provided a method for estimating the location of a device relative to a plurality of wireless transceivers, the method comprising: measuring time differences of arrival for signals between the device and the wireless transceivers; and estimating the location of the device relative to the wireless transceivers in dependence on the time differences of arrival and the relative offsets of the local clocks of the transceivers as determined in accordance with any methods set out herein.

According to a fifth aspect there is provided a method for estimating the location of a device relative to a plurality of wireless transceivers, the method comprising: transmitting a first signal from a first one of the transceivers and receiving the first signal at a second one of the transceivers; storing the time of transmission of the first signal in a local clock of the first one of the transceivers and the time of reception of the first signal at the second one of the transceivers; transmitting a second signal from the second one of the transceivers and receiving the second signal at the first one of the transceivers; storing the time of transmission of the second signal in a local clock of the second one of the transceivers and the time of reception of the second signal at the first one of the transceivers; transmitting a third signal from the device and receiving the third signal at the first and second ones of the transceivers; storing the time of reception of the third signal at the first one of the transceivers in the local clock of that transceiver and storing the time of reception of the third signal at the second one of the transceivers in the local clock of that transceiver; and estimating the location of the device in dependence on the stored information.

The step of estimating the location of the device may comprise: estimating a propagation delay between the first and second ones of the transceivers in dependence on the stored information; and estimating the difference in propagation delays between (i) the device and the first one of the transceivers and (ii) the device and the second one of the transceivers in dependence on the stored information. According to a sixth aspect there is provided a system comprising a plurality of base stations or other transceivers configured to perform any one or more of the steps set out above and a network entity as set out above. The network entity may be co-located with one of the transceivers.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will now be described by way of example with reference to the accompanying drawings.

In the drawings:

Figure 1 shows a communication network for linking a user equipment device to a location server.

Figures 2 and 3 illustrate the sending of timing signals between a plurality of neighbouring base stations.

Figure 4 illustrates a UE device sending a location signal to a plurality of base stations.

DETAILED DESCRIPTION OF THE INVENTION

The system to be described below may be used in a communications network, as illustrated in Figure 1.

A location server 1 is located in a core network 3. The core network is connected to a number of base stations 4, 14, 15. The internal components of the base stations are shown in detail for base station 4 but the other base stations are similar. Base station 4 comprises a wireless transceiver 5, a processor 6 and a memory 7, with two parts for storing code and messages respectively, and a clock 16. A wireless transceiver 8 in the base station communicates with a wireless transceiver 10 of a user equipment device 9. The user equipment device also comprises a processor 1 1 , a memory 12 with two parts for storing code and messages respectively, and a user interface 13 for presenting information or for sensing environmental data. The user interface 13 may comprise a mechanism for communicating or interacting with the device's environment or user, for example a display, touch screen, or one or more transducers. The base stations 14 and 15 also each comprise a local clock, 17 and 18 respectively. The UE 9 may transmit to the plurality of base stations either simultaneously or in quick succession. Base stations 4, 14 and 15 may each communicate with the core network 3 and a UE device 9. The base stations may also communicate wirelessly with one another (not shown in Figure 1 ). The spatial positions of base stations 4, 14 and 15 are known to the location server 1 .

For illustration, suppose that the local clocks of base stations 4, 14 and 15 (clocks 16, 17 and 18 respectively) have time offsets relative to a reference time of a, b and c seconds respectively (see Figure 2). At a reference time to + k, base station 4 transmits a timing signal to neighbouring base stations 14 and 15, shown at 19 and 20 respectively in Figures 2 and 3. According to the local clock of base station 4, the time that it transmits its timing signal to the other base stations is to + k + a. The timing signal sent from base station 4 is received by base station 14 at a local time of to + k + b + x1 , where x1 is the time taken (At) for the signal to travel from base station 4 to base station 14. Similarly, it is received by base station 15 at a local time of to + k + c + x3, where x3 is the time taken for the signal to travel from base station 4 to base station 15. Base stations 14 and 15 send out their own timing signals to the other base stations. The time taken for the timing signals sent from base station 14 to travel to base station 15 and vice versa is x2.

Upon receiving the timing signals from neighbouring base stations, each receiving base station determines the time in its own local clock at which the respective timing signals from each of the other base stations were received. Using that information, together with knowledge of when the timing signals were transmitted, a model for the timing offsets and optionally the drift of the clock in each base station may be modelled.

Each base station transmits to the location server 1 information indicating the time in its local clock at which it transmitted a timing reference signal and the times in its local clock at which it received timing reference signals from the other base stations. The location server can then model the network to estimate the timing offsets between the base stations' clocks. This may be done in a number of ways. For example, the location server 1 may store information defining the physical location of each base station. Using that local clock information, together with the remote clock information contained within the time signal it can estimate the distance between each base station and hence the expected time for signals to pass between each base station and each other base station. That can be used as an estimate of x1 , x2, x3 etc. With those values estimated, the respective values can be subtracted from the measured times and the base station clock offsets estimated by a best fit to the available data, e.g. using a least squares or similar error minimisation method. In an alternative approach, if sufficient inter-base station timing data is available the location server can infer the x1 , x2, x3 etc. values from the timing data received from the base stations, with or without additional knowledge of the base stations' locations. In the absence of accurate position information for the base stations, this could be synthesised using a simulated annealing process or similar fitting method, based on the time differentials. Again, the clock offsets can be estimated by a best fit to the available data, e.g. using a least squares or similar error minimisation method. The outcome of the estimation process is that the location server forms an estimate of the offsets between the local clock of each base station and a reference timing, i.e. the values a, b, c etc.

In an alternative approach, once the clock offsets a, b, c etc. have been estimated, the location server may transmit a signal to each base station to inform it of the current offset between its clock and the reference timing. Each base station may then adjust its clock in dependence on the received signal to align its clock to the reference timing. With multiple estimates of the timing offset at successive times, the location server or the base station may form an estimate of the drift between the base station clock and the reference timing. The base station may then automatically adjust its clock in accordance with the estimated drift. Alternatively, the base station clocks need not actually be adjusted. The location server could store the base station clock offsets and take them into account when forming a location estimate. The estimation of clock offsets could be done at one or more base stations rather than in the core network.

Once each base station has modelled the times of the local clocks in its neighboring base stations, the location of a UE can be estimated. The location server can process timing information from the base stations to perform uplink differential time of arrival (U-DTOA) from the UE.

As illustrated in Figure 4, UE 9 transmits a location signal to the base stations 4, 14 and 15. The UE sends the location signal at a reference time to + j. This location signal is received by base station 4 at a local time of to + a + j + y1 , where y1 is the time taken (At) for the signal to travel from the UE to base station 4 and 'a' represents the inaccuracy of the clock in the base station. The location signal is received by base station 14 at a local time of to + b + j + y2, where y2 is the time taken for the signal to travel from the UE to base station 14. The signal is received by base station 15 at a local time of to + c + j + y3, where y3 is the time taken for the signal to travel from the UE to base station 15. When a base station has received the location signal from the UE, it sends the following information to the location server:

- The identity of the UE that transmitted the location signal. This information may be included in the location signal.

- The local time in its own clock at which it received the location signal from the UE.

- Optionally, the base station may also send estimates it has made or stored of the offsets of the neighbouring base stations' local clocks from its own local clock. Alternatively, that information may be stored at the location server. If each base station transmits its own time of arrival, along with its estimate of the adjacent base stations' clocks at that time of arrival, the location server can use this information to derive an estimate of time differences.

As an example, suppose a server is to estimate an offset between the clocks of two base stations A and B and subsequently to locate a device Q by means of signal timings as received at the base stations A and B. Suppose the local clock of B is 33μ8 in advance of the clock of A, and the signal propagation time between A and B is 1 .75μ8. Each beacon includes a field giving the value of the transmitting station's clock at the time of transmission. The method proceeds as follows:

1 . Base station B transmits a beacon at a time (TXB) 133μ8 in its local clock. Base station A receives the beacon from base station B (RXAB) at a time 101 .75μ8 in A's clock. The beacon indicates the transmission time in B's clock (TXB) of 133μ8. Base station A then forms a model of B's clock (MAB) as TX_B - RXAB = 31.25μ8.

2. Base station A transmits a beacon at a time (TXA) 100μδ in its local clock. Base station B receives the beacon from base station A (RXBA) at a time 134.75μ8 in B's clock. The beacon indicates the transmission time in A's clock (TXA) of 10Ομβ. Base station A then forms a model of B's clock (MBA) as TX_A - RXBA = -34.75μ8.

3. At a time 5555μδ in A's clock the device Q to be located transmits a beacon. Suppose the propagation time from Q to A is 8.125μ8 and the propagation time from Q to B is 4μ8. The beacon from Q arrives at A at a time (RXAQ) of 5563.125μ8. The beacon from Q arrives at B at a time (RXBQ) of 5592μ8.

4. Base station A can estimate that base station B's clock would have read RXAQ + MAB = 5594.375μ8 (EABQ) when the beacon from Q arrived at A.

5. Base station B can estimate that base station A's clock would have read RXBQ + MBA = 5557.52μ8 (EBAQ) when the beacon from Q arrived at B. 6. The base stations provide the values RXAQ, RXBQ, EABQ, EBAQ to a location server. The location server could be sited in any suitable place. For instance, it could be part of a core network or it could be integrated with one of the base stations.

7. The location server can estimate the propagation delay between A and B as:

ABS (AVERAGE (EBAQ - RXBQ , EABQ - RXAQ) ) = 1 .75 S (SAB)

and can estimate the difference between the propagation delays from Q to A and from Q to B as:

AVERAGE (RXBQ - (E ABQ + SAB) , EBAQ + SAB - RXAQ) - -4.125μ8 (SQAB)

The values SAB and SQAB can then be used as inputs to a location algorithm.

It should be noted that the timing of the transmissions from A to B and from B to A and of the beacon from Q are not significant. They are cancelled out.

The value SAB can be used to help estimate the distance between base stations A and B. In a network of base stations, equivalent values can be received from multiple pairs of base stations. Then the location server can solve for the locations of the base stations so as to estimate the base stations' locations relative to each other.

The value SQAB can be used to help estimate the location of the device Q. If the locations of base stations A and B are known then knowledge of SQAB allows the location server to estimate the device as being located on a line where the difference in propagation delays would be expected to be equal to SQAB. With similar information from one, or more preferably more, other pairs of base stations the location of the device Q can be estimated. It should be noted that this can be done even though the offsets between the clocks of the base stations was not known a priori.

Any of the values MAB, MBA, EABQ and EBAQ can be calculated elsewhere than at the base stations. For example, base station A could send TXB, RXAB and RXAQ to the server, and the server could then calculate MAB and EABQ.

Once the location server has received the timing information from the base stations that have received the location signal it can convert those times to a common reference timing frame using the received or stored offsets between the respective base station local clocks and a reference timing frame, as described above. This step may be omitted if the location server has already caused the base stations to adjust their clocks to operate in a common reference frame, as also described above. Then, using the estimated positions of the base stations, which are stored at the location server, the location server can estimate the location of the UE based on the differences in the times at which each base station received the signal from the UE. This may be done by a known multilateration process. The multilateration process may use least squares or a Kalman filter to estimate the best fit of the timing data to a UE location. In this way, the location of the UE can be estimated to a reasonable degree of accuracy without the need for an accurate clock at each base station and without the need for a master clock source.

The base stations may send additional information to the location server, which may be used to assist in estimating the location of the UE. Examples of such information includes: an indication of the angle of arrival of the UE's location signal at the respective base station, an indication of the received signal strength of the UE's location at the respective base station, and an indication of a current power level setting for communication with the UE. The signals described with reference to Figures 2 and 3, which are sent between the base stations for the relative clock offsets of the base stations to be estimated, may be dedicated signals or may be used for additional purposes in the system.

If the location server has formed an estimate of the clock drift of multiple base stations, the process of estimating the location of the UE may be performed in dependence on that drift information. Greater weight may be placed on the timing information received from the base stations whose clocks drift less. This may be done through weightings applied in finding a preferred solution to the multilateration problem. In the system described above, only signals sent between the base stations are used to estimate the base stations' timing offsets. Alternatively, or in addition, the signals transmitted by the base stations could be received by other devices, such as UEs. They could send signals indicating the relative timing of receipt of signals from multiple base stations to the location server, and that information could also be used to help estimate the base stations' timing offsets.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1 . A base station comprising:

a local clock;

at least one model of a clock of a neighbouring base station;

a transceiver; and

a controller configured to cause the transceiver to transmit to a network entity time information derived from the local clock and time information derived from the model.

2. A base station as claimed in claim 1 , wherein the controller is configured to:

receive from the neighbouring base station a notification comprising time information indicative of a time in a local clock of the neighbouring base station; and

configure the model in dependence on the received time information.

3. A base station as claimed in claim 2, wherein the controller is configured to configure the model so as to represent an offset between the local clock of the claimed base station and the local clock of the neighbouring base station.

4. A base station as claimed in any preceding claim, wherein the controller is configured to be triggered to cause the transceiver to transmit the time information derived from the local clock and time information derived from the model in response to the claimed base station receiving a communication of a user equipment device with the base station.

5. A base station as claimed in claim 4 as dependent on claim 2 or 3, wherein the controller is configured to estimate by means of the model a time in the local clock of the neighbouring base station at which the communication of the user equipment device was received by the claimed base station, and wherein the said time information derived from the model represents that estimated time.

6. A network entity comprising:

a receiver, configured to receive from at least one base station time information based on a local clock of the base station and on a model of a local clock of another base station.

7. A network entity as claimed in claim 6, the network entity comprising a processor configured to estimate a location of a communication device based on the received time information.

8. A network entity as claimed in claim 7, wherein the network entity is configured to:

receive from the said at least one base station: (i) an estimate of an offset between the local clock of that base station and the local clock of the other base station and (ii) an estimate of the time in the local clock of the other base station at which a signal transmitted by the communication device was received by the said at least one base station; and

estimate the location of the communication device in dependence on those estimates.

9. A network entity as claimed in claim 8, wherein the network entity is configured to:

receive from the other base station: (i) an estimate of an offset between the local clock of that other base station and the local clock of the said at least one base station and (ii) an estimate of the time in the local clock of the said at least one base station at which a signal transmitted by the communication device was received by the other base station; and

10. A network entity as claimed in any of claims 6 to 9, the network entity comprising a memory storing data defining the locations of the base stations.

1 1 . A method for estimating the relative offsets of local clocks of a plurality of wireless transceivers, the method comprising:

transmitting from a first one of the transceivers a notification comprising a timing feature and an indication of the time in the local clock of the first transceiver at which the timing feature was transmitted;

receiving the notification at a second one of the transceivers, and determining the time in the local clock of the second transceiver at which the timing feature was received; and in dependence on (i) the time in the local clock of the second transceiver at which the timing feature was received and (ii) the indicated time at which that timing feature was transmitted in the local clock of the first transceiver, estimating the relative offset of the local clocks of the transceivers.

12. A method as claimed in claim 1 1 , comprising:

transmitting notifications from multiple transceivers, each notification comprising a timing feature and an indication of the time in the local clock of the transceiver that transmitted the respective notification at which the timing feature was transmitted;

receiving each of the notifications at the second transceiver and determining (i) the time in the local clock of the second transceiver at which each of the respective timing features was received and (ii) the indicated times at which those timing features were transmitted in the local clock of the respective transceivers that transmitted them; and in dependence on that information estimate the relative offsets of the local clocks of the second transceiver and the other transceivers.

13. A method as claimed in claim 1 1 or 12, wherein (i) the local clock(s) of one or more of the wireless transceivers is/are set independently of any central time source and/or (ii) the local clock(s) of one or more of the wireless transceivers is/are set with less relative precision than the resolution to which the relative offset(s) of the local clocks of the transceivers is/are estimated.

14. A method for estimating the location of a device relative to a plurality of wireless transceivers, the method comprising:

measuring time differences of arrival for signals between the device and the wireless transceivers; and

estimating the location of the device relative to the wireless transceivers in dependence on the time differences of arrival and the relative offsets of the local clocks of the transceivers as determined in accordance with any of claims 1 1 to 13.

15. A method for estimating the location of a device relative to a plurality of wireless transceivers, the method comprising:

transmitting a first signal from a first one of the transceivers and receiving the first signal at a second one of the transceivers;

storing the time of transmission of the first signal in a local clock of the first one of the transceivers and the time of reception of the first signal at the second one of the transceivers; transmitting a second signal from the second one of the transceivers and receiving the second signal at the first one of the transceivers;

storing the time of transmission of the second signal in a local clock of the second one of the transceivers and the time of reception of the second signal at the first one of the transceivers;

transmitting a third signal from the device and receiving the third signal at the first and second ones of the transceivers;

storing the time of reception of the third signal at the first one of the transceivers in the local clock of that transceiver and storing the time of reception of the third signal at the second one of the transceivers in the local clock of that transceiver;

estimating the location of the device in dependence on the stored information.

16. A method as claimed in claim 15, wherein the step of estimating the location of the device comprises: estimating a propagation delay between the first and second ones of the transceivers in dependence on the stored information; and

estimating the difference in propagation delays between (i) the device and the first one of the transceivers and (ii) the device and the second one of the transceivers in dependence on the stored information.