System and method for locating the center of a beacon equipping guided vehicle routes

Abstract

The present invention concerns a system (1) and a method for locating the center of a beacon (4) installed along a guided vehicle route, the system (1) comprising: - an emitter (11) configured for remotely powering the beacon; - a receiver comprising a first receiving loop (12) and a second receiving loop (13) for picking up an electromagnetic signal produced by the beacon (4) and delivering to a processing unit (14) respectively a first signal (S1) and a second signal (S2); - the processing unit (14) capable of processing the first and second signals (S1, S2); the system (1) according to the invention being characterized in that the first receiving loop (12) and the second receiving loop (13) are configured for delivering signals (S1), (S2) which are temporally shifted when receiving the electromagnetic signal produced by the beacon (4)and the processing unit (14) is configured for calculating the time T at which the receiver passes the center of the beacon (4) from the amplitude of the first signal (S1) and the amplitude of the second signal (S2).

Description

[0001] The present invention concerns a system and a method for locating the center of a beacon installed at points along a route followed by a guided vehicle. The present invention further concerns a guided vehicle comprising the system

[0002] The present invention is directed to the localization of a guided vehicle along a route. It relates more particularly to a system and method for locating a guided vehicle relative to a beacon or balise installed at points along the route followed by said guided vehicle. By determining the center of a beacon, the system and method according to the invention allow a precise determination of the position / location of the guided vehicle. From a general point of view the present invention deals with beacons or balises installed on the route or way taken by the guided vehicle and which are configured for exchanging data with the guided vehicle by means of an electromagnetic signal when the guided vehicle passes near, for example above / over, said balise or beacon. In particular, said balise is an Eurobalise, i.e. a balise which complies with the European Train Control System, and is installed between rails of a railway followed by the guided vehicle. "Guided vehicle" according to the present invention refers to public transport means such as buses, trolleybuses, street-cars, subways, trains or train units, etc., as well as load transporting means such as, for example, overhead traveling cranes, for which safety is a very important factor and which are guided along a route or railway by at least one rail, in particular by two rails between which beacons / balises are placed.

[0003] Systems and methods for determining the position of a guided vehicle relative to a beacon are known in the art. For example, EP1227024B1 describes an embedded system for generating a signal for locating a railway vehicle, wherein said embedded system comprises an antenna having a first and second loop characterized by different radiation patterns and which generate respectively a first induced current and a second induced current when said antenna passes above a beacon located at a known position on the path of the railway vehicle. Since the first and second loops have different shapes, changes in the phase of the first induced current is different from changes in the phase of the second induced current, and this difference in the phases is then used by a processing system for determining the position of the railway vehicle relative to the beacon.

[0004] Generally, the dynamic of the transmission of electromagnetic signals between the beacon and the on-board system for locating a railway vehicle makes it difficult to estimate the center of the beacon and thus an accurate position of the railway vehicle. In particular, vital accuracy is limited by the presence of "side lobes" (see Fig. 4 of EP1227024B1) that are due to the return path of magnetic flux and which could increase the length of the zone where the beacon is received by the on-board system antenna. Even in the case of an "8-shape" loop as described in EP1227024B1, vital detection of the beacon center remains challenging because the induced signals could be very weak and a strong gain must be ensured in the receiver. Finally, known prior art uses phase detection for determining the center of the beacon, which also might not be reliable with weak signals.

[0005] An objective of the present invention is therefore to propose a system and a method for improving the determination of the location of a guided vehicle relative to a beacon or balise, guaranteeing a secure determination of the position of a beacon or balise, and which are simple, economical to implement, reduce the risks of false detection (notably due to the presence of side lobes), and improve therefore the reliability of the determination of the location of the guided vehicle relative to the beacon. Preferentially, the system and method according to the invention are free of any phase analysis of the electromagnetic signal transmitted by the beacon.

[0006] The invention provides a system for locating the center of a beacon, defined by the features of claim 1. It is described a system being configured for being mounted on-board a guided vehicle, and capable of locating the center of the beacon when said guided vehicle passes the beacon, the system comprising: an emitter configured for remotely powering the beacon, in particular by means of radiant energy. Said emitter comprising for example an antenna comprising an emitting loop for radiating energy, in particular radio frequency energy, the beacon being then powered by said radiated energy and able to transmit, in return, an electromagnetic signal; a receiver comprising an antenna incorporating two receiving loops for picking up the electromagnetic signal produced by the beacon, more precisely the electromagnetic signal produced by the transmitting loop of the beacon, the receiver comprising thus said two receiving loops, respectively a first receiving loop and a second receiving loop, said loops being preferentially characterized by identical radiation patterns, for example the first and second receiving loops being identical in order to have the same radiation pattern, the first and second loops being configured for picking up the electromagnetic signal sent by the beacon in response to its powering by the emitter and delivering respectively a first signal and a second signal to a processing unit, said first and second signals being currents induced by the electromagnetic signal in said first and second receiving loops respectively, the first receiving loop and second receiving loop being thus preferentially configured or shaped for delivering identical signals when receiving the electromagnetic signal produced by the beacon. An advantage of working with a first and second receiving loops characterized by identical radiation patters is that the current signals generated by each loop in response to the electromagnetic signal of the beacon are identical, only shifted within the time, which then simplifies and makes more efficient the determination of the failures that may occur. Said first and second receiving loops being preferentially simple loops. Each current provides a measure of the amplitude of the electromagnetic signal in function of the position of the first, respectively second, receiving loop compared to the beacon, more precisely compared to the center of the transmitting loop, the first receiving loop being in particular connected to a first demodulator and the second receiving loop being in particular connected to a second demodulator. The first receiving loop and the second receiving loop are in particular placed / positioned so that the first signal is temporally shifted compared to the second signal when a guided vehicle equipped with a system according to the invention passes the beacon. In the context of the present invention, the center of the beacon refers to the center of the transmitting loop, e.g. its geometric center; a processing unit capable of processing the first and second signals delivered by respectively the first receiving loop and the second receiving loop, wherein the processing unit is configured for determining the time T at which the receiver passes the center of the beacon from the amplitude of the first signal and the amplitude of the second signal, wherein T is in particular given by: T = T 1 + T 2 / 2 wherein preferentially, T1 is the time at which the amplitude of the first signal reaches its maximum when the first receiving loop passes (e.g. passes above / over) said beacon and T2 is the time at which the amplitude of the second signal reaches its maximum when the second receiving loop passes (e.g. passes above / over) said beacon.

[0007] Preferentially, the time T is determined by the processing unit by calculating at least a third signal that is a time dependent function of the first and second signal, e.g. a function of the amplitudes of the first and second signals, like the sum and / or difference of the amplitudes / intensities of the first signal and the second signal in function of time, the processing unit being configured for determining an extremum of said third signal, the time at which said extremum appears coinciding with the time T at which the receiver passes the center of the beacon. The processing unit according to the invention is thus capable of determining the moments at which the center of the beacon and the centroid of the system formed by the first and second receiving loops are the closest, and thus the time / moment at which the position of the centroid is the closest to the position of the beacon, said moment corresponding to the time at which the third signal comprises an extremum, e.g. the time at which a maximum appears when the third signal is the sum of the amplitudes / intensities of the first and second signals, or the time at which a minimum appears when said third signal equals to the difference of the amplitudes / intensities of the first and second signals. Indeed, the first, or respectively second signal, reaches its maximum at the moment at which the first, or respectively second, receiving loop is in a plane parallel to the plane comprising the transmitting loop and aligned with the transmitting loop of the beacon, ideally, at the moment at which the center of the first, or respectively second, receiving loop and the center of the transmitting loop of the beacon are on a same straight line perpendicular to both the plane comprising the transmitting loop and the plane comprising the first, or respectively second, receiving loop.

[0008] The present invention concerns also a guided vehicle as defined by the features claim 10.

[0009] The present invention concerns also a method for locating the center of a beacon, which is defined b the features of claim 11.

[0010] The beacon can be installed at a point along a route followed by a guided vehicle on board of which a system for locating the center of the beacon is mounted, said system comprising an emitter, a receiver comprising a first receiving loop and a second receiving loop, and a processing unit. It is described a method comprising the following steps: remotely powering the beacon or balise by means of the emitter, wherein the beacon is adapted for transmitting an electromagnetic signal to the system; transmitting the electromagnetic signal to the receiver, wherein said electromagnetic signal is produced by the beacon in response to its powering; picking up the electromagnetic signal by means of the first receiving loop and delivering a first signal to the processing unit; picking up the electromagnetic signal by means of the second receiving loop and delivering a second signal to the processing unit, wherein the second signal is temporally shifted compared to the first signal, and preferentially identical to the first signal: the first signal beginning at a time t1B at which the first receiving loop receives for the first time the electromagnetic signal and ending at a time t1E at which the first receiving loop ceases to receive the electromagnetic signal, the same applying to the second receiving loop, i.e. the second signal beginning at a time t2B and ending at a time t2E, wherein preferentially t1B<t2B≤t1E<t2E and preferentially t1E-t1B = t2E-t2B; determining by means of the processing unit the time T at which the receiver passes the center of the beacon from the amplitude / intensity of the first signal and the amplitude / intensity of the second signal, wherein T is in particular given by T = (T1 + T2) / 2, with T1, T2 preferentially as previously defined. Preferentially, the method comprises calculating a third signal being the difference or the sum of the amplitudes of the first signal and the second signal in function of the time and determining an extremum of the third signal, the time at which said extremum appears coinciding with the time at which the receiver passes the center of the beacon. The skilled man will understand that the time T determined by the present invention is the time at which the center of the receiver is the closest to the center of the beacon when said receiver passes said beacon, the center of the receiver being the centroid of the system formed by the first and second receiving loops (i.e. the median point on the straight line segment bounding the geometric center of the first receiving loop and the geometric enter of the second receiving loop) and the center of the beacon being the geometric center of the transmitting loop.

[0011] Further aspects of the present invention will be better understood through the following drawings, wherein like numerals are used for like and corresponding parts: Figure 1schematic representation of a first preferred embodiment of a system according to the invention mounted on board a guided vehicle. Figure 2schematic representation of a second preferred embodiment of a system according to the invention. Figure 3schematic representations of the signals processed by the processing unit according to the invention.

[0012] Figure 1 shows a system 1 according to the invention mounted on-board a guided vehicle 2 which is configured to follow a route defined by a pair of rails 3. A beacon 4 or balise is installed on the route or rail track followed by the guided vehicle 2, for example between the rails 3. The rail track may comprise several beacons 4 forming a system of beacons 4, each beacon being configured to exchange information with the guided vehicle 2 when the latter passes at proximity, for instance above / over, said beacon 4. The beacon 4 and the system 1 exchange information by means of electromagnetic signals transmitted from the beacon 4, respectively system 1, to the system 1, respectively beacon 4.

[0013] The system 1 according to the invention is configured for being mounted on-board the guided vehicle 2 and is able to locate the center of the beacon 4. Therefore, the system 1 enables to locate the guided vehicle 2 relative to the beacon 4, and to determine efficiently the position of the vehicle on a network equipped with a system of beacons 4 wherein the position of each beacon 4 is known.

[0014] The system 1 for locating the center of the beacon 4 installed along a route followed by the guided vehicle 2 on board of which the system 1 is configured to be mounted comprises at least an emitter 11, a receiver and a processing unit 14.

[0015] The emitter 11 is configured for remotely powering the beacon 4. In particular, the emitter 11 comprises an antenna for emitting radiant energy capable of powering the beacon 4 when the guided vehicle 2, or more precisely the emitter 11 of the system 1 according to the invention, is moving in the proximity of, e.g. above, the beacon 4. Said emitter 11 comprises for example an antenna comprising an emitting loop capable of radiating energy, in particular radio frequency energy, wherein said radiated energy is able to power the beacon 4, i.e. to supply said beacon 4 in energy so that the latter becomes able to transmit, in return, an electromagnetic signal to the system 1. The beacon 4 is of a type known in the art, comprising an antenna circuit capable of picking up the energy radiated by the emitter 11 and of using said energy for transmitting information back to the system 11 by means of a transmitter comprising a transmitting loop for sending the electromagnetic signal to the system 1. Beacons 4 are installed at known positions / locations along the route followed by the guided vehicle 2.

[0016] The receiver comprises two loops, respectively a first receiving loop 12 and a second receiving loop 13, said loops being preferentially characterized by identical radiation patterns in order to facilitate the determination of failures. "Identical radiation patterns" means that the two loops provide an identical response when submitted to an identical electromagnetic signal. Preferentially the first and second receiving loops are identical. The first receiving loop 12 and second receiving loop 12 deliver respectively a first signal S1 and a second signal S2 when receiving the electromagnetic signal emitted by the beacon 4 in response to its powering by the emitter 11. The first receiving loop 12 and second receiving loop 13 are aligned in a same plane with one another, e.g. in a horizontal plane, and disposed one after another compared to the displacement direction of the guided vehicle 2, with or without overlapping of a part of the loop, so that the first signal S1 and the second signal S2 are temporally shifted when the receiver passes the beacon 4. They are for example rectangular loops disposed side by side in a same plane (see Fig. 1), or having an overlapping side as shown in Figure 2, wherein according to a second preferred embodiment, the first receiving loop 12 and the second receiving loop 13 slightly overlap. In particular, the first receiving loop 12 and the second receiving loop 13 are in a same plane parallel to the plane comprising the transmitting loop of the beacon 4 when said system is mounted on board the guided vehicle 2. Preferentially, the external size of the first receiving loop 12 and of the second receiving loop 13 is substantially equal to the external size of the transmitting loop of the beacon 4. When submitted to the electromagnetic signal transmitted by the beacon 4, the first receiving loop 12 delivers a first signal S1 and the second receiving loop 13 delivers a second signal S2, wherein said first and second signals are current induced by the electromagnetic signal in said first and second receiving loops respectively.

[0017] The processing unit 14 is configured for processing the first and second signals delivered by respectively the first receiving loop 12 and the second receiving loop 13. Figure 3 shows schematically the intensity of the first signal S1 and the second signal S2 in function of the position or location of the system 1 relative to the beacon 4 when the guided vehicle 2, and consequently the system 1, moves in the direction of displacement indicated by the arrow and passes above the beacon 4 (see e.g. Fig. 1 or 2). During said displacement, the first receiving loop 12 will first sense the electromagnetic signal transmitted by the transmitting loop of the beacon 4 and deliver a first signal S1 that will, in function of the displacement of the system along the direction of displacement, first increase while approaching the beacon 4, then reach a maximum when the first receiving loop is aligned above the beacon 4, and then decrease when the distance from the beacon 4 increases. The second receiving loop 13 will sense the electromagnetic signal after the first receiving loop 12, because it is installed after the first receiving loop 12 compared to the direction of displacement of the guided vehicle or system 1 when moving in direction and then over the beacon 4. Preferentially, the first receiving loop 12 and the second receiving loop 13 have the same radiation pattern, and consequently the second signal S2 will be identical to the first signal S1, but delivered by said second receiving loop with a temporal shift compared to the first signal S1, said shift depending on the displacement speed of the system 1 (or guided vehicle 2) when passing above the beacon and on the distance separating the first receiving loop 12 from the second receiving loop 13, more precisely the distance D separating the part of the first receiving loop 12 entering first in the magnetic field produced by the transmitting loop 41 of the beacon 4 from the part of the receiving loop 13 entering first in said magnetic field produced by the transmitting loop 41 when the system 1 (or guided vehicle 2) moves in the direction of displacement and passes above the beacon 4 (see Fig. 2). The first and second receiving loops are preferentially configured for delivering an identical signal when passing a beacon, but wherein said signals are temporally shifted. The processing unit 14 according to the invention receives two signals, first the first signal S1 and second the second signal S2, said signals being temporarily shifted, preferentially identical, and extending over a period of time ET1 for the first signal S1 and ET2 for the second signal S2 (see Fig. 3), with preferentially ET1 = ET2, ET1 respectively ET2 being the period of time during which the first, respectively second, receiving loop is under the influence of the magnetic field produced by the transmitting loop of the beacon 4, said periods ET1 and ET2 being temporally shifted compared to one another, i.e. ET1 begins at a time t1B and ends at a time t1E, and ET2 begins at a time t2B and ends at a time t2E, with t1B<t2B≤t1E<t2E and preferentially t1E-t1B = t2E-t2B. The period of time ET1 and ET2 preferentially overlap. In particular, T1=t1B +(t1E-t1B) / 2 and T2=t2B+(t2E-t2B) / 2.

[0018] The processing unit 14 is connected to the first and second receiving loops for receiving the first and second signals, and is configured for analyzing said first and second signals transmitted by the first and second receiving loops 12, 13. In particular, the processing unit calculates at least one third signal S3 which is a function f either of the amplitude difference or of amplitude sum of the first signal S1 and the second signal S2 in function of the time t: S3 = f(S1(t)-S2(t)) or S3 = f(S1(t)+S2(t)), e.g. S3 = S1 - S2 or S3 = S1 + S2. The processing unit 14 may calculate other signals as function f of (S1(t), S2(t)), e.g. any of the following signals: S3'= f(S1(t)+S2(t)) or S3'=f(S1(t)-S2(t)), S4=f(S1(t)), or S5=f(S2(t)). Preferentially, from at least the third signal S3, and in particular additionally from S3' in order to further improve the precision, the processing unit is able to determine the moment at which the receiver was centered over the beacon 4, and therefore to locate the position of the receiver relative to the transmitting loop, and consequently of the guided vehicle 2, relative to the beacon. Indeed, the moment at which the receiver is centered over the transmitting loop of the beacon 4 is the moment at which the amplitude / intensity of S3 and / or S3' reaches an extremum, respectively a maximum for S3' and a minimum for S3. Note that the center of the beacon 4 corresponds in Figure 3 to the origin of the coordinate system of the graph and refers to the geometric center of the transmitting loop. The receiver is centered on the center of the beacon when the centroid of the system formed by the first and second receiving loops is the closest to the center of the transmitting loop. Since the position of the beacon center is precisely known, then the position of the guided vehicle 2 might also be precisely known. In particular, the processing unit 14 comprises two demodulators, respectively a first demodulator and a second demodulator, the first receiving loop 12 being connected to the first demodulator and the second receiving loop 13 being connected to a second demodulator for extracting information from the beacon electromagnetic signal.

[0019] In particular, as soon as the beacon electromagnetic field sensed by the first receiving loop 12 overcomes a threshold, then the first receiving loop 12 begins to receive messages from the beacon 4, said messages being embedded in the electromagnetic signal transmitted by the beacon 4. The same applies to the second receiving loop that will get the same messages as the first receiving loop. The time at which each message is received by the second receiving loop being only temporally shifted compared to the time at which the first receiving loop received said message. Preferentially, the processing unit 14 is configured for calculating the third signal only if at least one valid message has been received by the first, respectively second, receiving loops, and another message is received by the second, respectively first, receiving loop either while messages are still available and received on the first, respectively second, receiving loop or while messages are not anymore received by the first, respectively second, receiving loop. Advantageously, imposing a condition of reception of valid messages for the calculation of the third signal secures the determination of the position of the guided vehicle compared to the beacon 4.

[0020] Preferentially, the present invention proposes to determine the time T from the time at which said messages are received by the receiver. In particular, according to the present invention, the time T1 is given by T1 = t11+(t12-t11) / 2, and the time T2 is given by T2 = t21+(t22-t21) / 2, and the processing unit 14 is configured for measuring: the time t11 at which the first receiving loop 12 receives for the first time a message from the beacon 4, the time t12 at which the first receiving loop 12 receives for the last time a message from said beacon, the time t21 at which the second receiving loop 13 receives for the first time a message from the beacon 4, and the time t22 at which the second receiving loop 13 receives for the last time a message from said beacon, the processing unit being then configured for determining the time T from t11, t12, t21, t22, by means of electrical circuitry enabling to carry out Eq. 1, wherein t11<t21≤t12<t22. According to the present invention, the time interval [t11, t12] is comprised within the time interval [t1B, t1E] and the time interval [t21, t22] is comprised within the time interval [t2B, t2E]. In particular, the first receiving loop 12 and the second receiving loop 13 are configured for delivering identical signals S1, S2, so that t12-t11 = t22-t21. Finally, the processing unit 14 might be configured for comparing the time T obtained by means of the third signal S3 and the time T obtained by means of t11, t12, t21 and t22 in order to validate the value obtained for T and / or calculate an average value for T. Indeed, according to the present invention, the time T might be determined from the third signal S3, and / or from the times t11, t12, t21 and t22, and / or from the times t1E, t1B, t2E, t2B, and / or from a combination of the latter.

[0021] To summarize, the present invention proposes a simple system and method for determining the position of a guided vehicle relative to a beacon such an Eurobalise, wherein said position is determined by a system comprising an antenna with a first receiving loop and a second receiving loops generating shifted signals, the moment at which the guided vehicle is over the center of the beacon or balise, and thus its location, being determined from averaging the time at which each of said first and second receiving loops passes over the center of the balise, notably by calculating the sum and / or difference of intensities of the signals provided by the first and second receiving loop.

Claims

1. System (1) for locating the center of a beacon (4), the system (1) comprising: - an emitter (11) configured for remotely powering the beacon; - a receiver comprising a first receiving loop (12) and a second receiving loop (13) for picking up an electromagnetic signal produced by the beacon (4) in response to its powering and delivering to a processing unit (14) respectively a first signal (S1) and a second signal (S2) in response to the reception of said electromagnetic signal; - the processing unit (14) capable of processing the first and second signals (S1, S2); wherein - the first receiving loop (12) and the second receiving loop (13) are configured for delivering signals (S1), (S2) which are temporally shifted when receiving the electromagnetic signal produced by the beacon (4); - the processing unit (14) is configured for determining the time T at which the receiver passes the center of the beacon (4) from the amplitude of the first signal (S1) and the amplitude of the second signal (S2), wherein the first receiving loop (12) and the second receiving loop (13) are aligned in a same plane with one another and disposed one after another compared to a displacement direction of the system (1) when moving in direction and then over the beacon (4).

2. System (1) according to claim 1, wherein T is given by T = T 1 + T 2 / 2 wherein T1 is the time at which the amplitude of the first signal (S1) reaches its maximum when the first receiving loop (12) passes said beacon (4) and T2 is the time at which the amplitude of the second signal (S2) reaches its maximum when the second receiving loop (13) passes said beacon (4).

3. System according to claim 1, wherein the time T is determined by the processing unit (14) by calculating at least a third signal (S3) that is a function of the sum or difference of the amplitudes of the first signal (S1) and the second signal (S2) in function of time, the processing unit being configured for determining an extremum of said third signal (S3), the time at which said extremum appears coinciding with the time T at which the receiver passes the center of the beacon (4).

4. System according to claim 1 or 2, wherein the time T1 is given by T1 = t11+(t12-t11) / 2, and the time T2 is given by T2 = t21+(t22-t21) / 2, and wherein the processing unit (14) is configured for measuring the time t11 at which the first receiving loop (12) receives for the first time a message from the beacon (4), the time t12 at which the first receiving loop (12) receives for the last time a message from said beacon, the time t21 at which the second receiving loop (13) receives for the first time a message from the beacon (4), and the time t22 at which the second receiving loop (13) receives for the last time a message from said beacon, and wherein the processing unit determines the time T from t11, t12, t21, t22, wherein t11<t21≤t12<t22.

5. System according to claims 3 and 4, wherein the processing unit (14) is configured for comparing the time T obtained by means of the third signal (S3) and the time T obtained by means of t11, t12, t21 and t22 in order to validate the value obtained for T and / or calculate an average value for T.

6. System according to one of the claims 1-5, wherein the first receiving loop (12) and the second receiving loop (13) are configured for delivering identical signals (S1), (S2).

7. System according to one of the claims 1-6, wherein the first receiving loop (12) and the second receiving loop (13) overlap.

8. System according to one of the claims 1-7, wherein the first receiving loop (12) is connected to a first demodulator and the second receiving loop (13) is connected to a second demodulator.

9. System according to one of the claims 1-8, wherein said first and second receiving loops (12, 13) are simple loops.

10. Guided vehicle (2) comprising the system (1) according to one of the claims 1-9.

11. Method for locating the center of a beacon (4) installed at a point along a route followed by a guided vehicle (2) on board of which a system (1) for locating the center of the beacon (4) is mounted, said system (1) comprising an emitter (11), a receiver comprising a first receiving loop (12) and a second receiving loop (13), and a processing unit (14), the method comprising the following steps: - remotely powering the beacon (4) by means of the emitter (11), wherein the beacon (4) is adapted for transmitting information to the system (1); - transmitting an electromagnetic signal to the receiver, wherein said electromagnetic signal is produced by the beacon (4) in response to its powering; - picking up the electromagnetic signal by means of the first receiving loop (12) and delivering a first signal (S1) to the processing unit (14); - picking up the electromagnetic signal by means of the second receiving loop (13) and delivering a second signal (S2) to the processing unit (14), wherein the second signal (S2) is temporally shifted compared to first signal (S1); - determining by means of the processing unit the time T at which the receiver passes the center of the beacon from the amplitude of the first signal and the amplitude of the second signal; wherein the first receiving loop (12) and the second receiving loop (13) are aligned in a same plane with one another and disposed one after another compared to a displacement direction of the system (1) when moving in direction and then over the beacon (4).

12. Method according to claim 11, wherein T is given by T = (T1 + T2) / 2, wherein T1 is the time at which the amplitude of the first signal reaches its maximum when the first receiving loop passes said beacon and T2 is the time at which the amplitude of the second signal reaches its maximum when the second receiving loop passes said beacon.

13. Method according to claim 11, comprising calculating a third signal (S3) being the difference or the sum of the amplitudes of the first signal (S1) and the second signal (S2) in function of the time and determining an extremum of the third signal (S3), the time at which said extremum appears coinciding with the time at which the receiver passes the center of the beacon (4).

14. Method according to claim 12, comprising measuring the time t11 at which the first receiving loop (12) receives for the first time a message from the beacon (4), the time t12 at which the first receiving loop (12) receives for the last time a message from said beacon, the time t21 at which the second receiving loop (13) receives for the first time a message from the beacon (4), and the time t22 at which the second receiving loop (13) receives for the last time a message from said beacon, and calculating T from T1 = t11+(t12-t11) / 2 and T2 = t21+(t22-t21) / 2.

15. Method according to one of the claims 11-14, wherein the first receiving loop (12) and the second receiving loop (13) are configured for delivering identical signals (S1, S2).

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