EP1178169A1 - Anti-fraud remote control method for motor vehicle and system for carrying out this method - Google Patents

Abstract

Overlapping signals (S1,S2) are exchanged between a vehicle unit (1) and a user identification unit (I). The user signal (S2) includes an identifier (Si) and a phase image signal (34). The phase image signal received at the vehicle (42) is compared with a reference signal (41) derived from the carrier (27) which may be retarded by a fixed amount and car lock operation (38,O) is only permitted if the identification unit is within an acceptable distance. An Independent claim is also included for equipment which uses the signaling method.

Description

The present invention relates to a control method using distance from a member of a motor vehicle, in particular a member of blocking access to said vehicle and / or a starting device, and also a control system for the implementation of this process. A such control system is for example used to allow access said hands-free to a vehicle, and / or the start said hands-free to said vehicle; "Hands free" means that it is not necessary to use a key for these operations.

As shown in Figure 1, such a system includes generally an identification device I intended to be worn by a user U and able to establish a bidirectional dialogue remotely and without wire with a central control unit 1 on board vehicle V, to authenticate the user and order means of locking / unlocking of the door locks when the user has been recognized as authentic. The initialization of the communication can be activated by operating the external handle of door, for hands-free access, or by pressing a button startup, in hands-free startup mode. Alternatively, this initialization can be triggered by pressing a control button of the identification device I.

A commonly proposed system is to use low frequency carrier waves, of the order of 125 kHz for the communication from the vehicle to the identification device I, and ultra high frequency carrier waves, for example of the order of 434 or 868 MHz, for the Europe zone, and 315 or 902 MHz for the USA zone, for communication from the identification device I to vehicle V. The identification device I must include a battery to power its own electronic circuits. To minimize the power consumption, one can predict, for example, that the identification device is dormant for 9 ms and awake 1 ms, for periods of 10 ms.

As visible in FIG. 1, the bidirectional dialogue comprises a first communication of a first signal S ₁ emitted by a first transmitter 2 of the central unit 1 to a first receiver 3 of said identification device I and a second communication of a second signal S ₂ emitted by a second transmitter 4 from said identification device I to a second receiver 5 of the central unit 1. Here, the term signal is used to designate a sequence, continuous or interrupted, of data.

The power of transmitters 2 and 4 and the efficiency of receivers 3 and 5 is such that the first and second communications can only take place when the identification device I is at a distance less than or equal to a predetermined communication distance d _c of vehicle V, generally of the order of a few meters, to avoid, on the one hand, interference with other sources of environmental signals, and, on the other hand, to avoid the functioning of the system at a distance such that the user U is too far from the vehicle V to be aware of the operations carried out by said system.

For user authentication purposes, said first signal S ₁ comprises data characteristic of said vehicle to be recognized by said identification device and / or the second signal S ₂ comprises data characteristic of said identification device to be recognized by said vehicle. The bidirectional dialogue between the vehicle and the identification device can be encrypted, in order to avoid any untimely operation of the system and to secure it from criminals.

However, despite the possible encryption, there is a way to hack the system without knowing the said characteristic data. This hacking method is represented in FIG. 2. According to this method, it is assumed that the user U who is wearing the identification device I is located at a distance d _ℓ from the vehicle V greater than the authorized communication distance d _c , for example from 10 to 100 m from the vehicle. In this case, a pirate equipped with a first relay box 6 can approach the vehicle V at a distance d _c 'sufficient to communicate with it, for example at a distance of the order of 1 to 5 m . This pirate activates the start of the communication, for example by pulling the exterior door handle. This triggers the emission of said first signal S ₁ by the vehicle V to the relay box 6. This signal S ₁ sent by the vehicle is received by the relay box 6, which comprises a receiver 8 at 125 kHz. This receiver 8 is connected to a high frequency transmitter 9, of the order of several MHz. The transmitter 9 transmits, as shown by the arrow 10, to a second relay box 11, which is carried by another pirate who follows the user U at a distance d _u of the order of a few meters. The exchange of information between the two relay boxes 6 and 11 being carried out at very high frequency, it is possible to carry out this communication at a very large distance d _p , of the order of several tens or hundreds of meters for example.

The second relay box 11 includes a receiver 12 at the same frequency as the transmitter 9 of the first relay box 6. The signal 10 thus received is retransmitted at the frequency of 125 kHz by a transmitter 13 in order to send a signal 14 to the identification device I which conforms to the signal S ₁ emitted by the vehicle. The signal 14 being the repetition of the authentic signal S ₁ of the vehicle, the identification device I will recognize it and in turn send its response signal S ₂ , said response signal S ₂ being sent at high frequency, for example to 434 MHz and received by a receiver 15 of the second relay box 11, which will convert the signal at 434 MHz into a signal at a different frequency, for example at 315 MHz. The converted signal 17 is then sent by a transmitter 16 to the first relay box 6, this frequency difference being necessary so that the different signals do not interfere with each other. Of course, the frequency of the signal 17 is different both from the frequency of the signal 10 and of the signal S ₂ . This signal 17 is picked up by the first relay box 6, which comprises a receiver 18 of the same frequency as the transmitter 16. The receiver 18 is connected to a transmitter 19 which transforms the signal at 315 MHz into a signal S ₂ ' at 434 MHz, conforming to the signal S ₂ emitted by the identification device I, which is sent to the vehicle V.

To detect such hacking and interrupt the communication, one solution could be to use ultra-fast microwave communications. However, such ultra-fast system using microwaves would have an additional cost important. There is an important economic interest in keeping the existing systems using frequencies of the order of 125 kHz.

The risks of piracy by the method described above stem from the fact that it can be implemented using electronic components commonly available commercially, in video equipment for example. A relay box designed with such components would have a limited bandwidth, for example less than 1 MHz. The processing time required between the reception by the receiver 8 of the relay box 6 of a data carried by the signal S ₁ and the transmission by the transmitter 9 of this same data in the signal 10 would then be of the order of a microsecond (µs).

Thus, the signal S ₂ ′, received by the receiver 5 in the case of the hacking attempt represented in FIG. 2, would present, compared to the signal S ₂ which would have been received in the case of normal use of said system of control, as shown in Figure 1, a transmission delay Δt of the order of 4 to 5 µs. The calculation of the transmission delay Δt generated by the hacking process described above will be detailed below. The transmission delay Δt is negligible in comparison with the time constants necessary for the normal authorized transmission. For example, the total communication can be of the order of 20 to 40 milliseconds (ms), and the total duration of the operation of the system to initiate unlocking or locking of electric locks can be of the order of 100 ms. On the other hand, the transmission delay Δt is of the order of half of the period T _BF of the carrier wave at low frequency at 125 kHz.

The object of the present invention is to eliminate the disadvantages mentioned above and to propose a method and a system for motor vehicle remote control to prevent hacking of the system, in particular by means of relay boxes, taking into account the signal propagation and processing time between the vehicle and the identification device. The system of proposed order can be made from existing systems operating at a frequency of 125 kHz.

(i) le déclenchement de ladite première communication, ledit premier signal étant porté par une première onde porteuse ;

(ii) le déclenchement de ladite seconde communication, ladite seconde communication se déroulant au moins partiellement pendant que ladite première communication se poursuit, ledit second signal comportant un signal d'identification dudit dispositif d'identification et un signal de phase dit image, représentant la phase de ladite première onde porteuse reçue par ledit dispositif d'identification,

(iii) l'émission d'un ordre d'activation pour activer ledit organe après réception dudit signal d'identification par ladite unité centrale ;
ledit procédé comportant également, simultanément avec lesdites première et seconde communications, une étape de comparaisons successives entre la valeur actuelle de la phase représentée par ledit signal de phase image reçu par ledit second récepteur, et la valeur actuelle de la phase d'un signal de référence, conforme à l'onde porteuse en cours d'émission par ledit premier émetteur retardée d'un retard de référence prédéterminé, nul ou non nul ; la phase représentée par ledit signal de phase image présentant, lors de ladite étape de comparaisons, un retard de transmission par rapport à la phase de ladite onde porteuse en cours d'émission par ledit premier émetteur, ladite émission d'ordre d'activation étant interdite dès que le résultat desdites comparaisons successives remplit un critère d'annulation prédéterminé.

For this, the invention provides a remote control method for controlling a member of a motor vehicle, in particular a means of blocking access to said vehicle and / or a means of starting said vehicle, consisting in establishing a bidirectional dialogue by radio link between a central unit carried by said vehicle and a portable identification device intended to be worn by a user, said dialogue comprising at least a first communication, continuous or interrupted, of a first signal emitted from a first transmitter of said central unit to a first receiver of said identification device and a second communication, continuous or interrupted, of a second signal transmitted from a second transmitter of said identification device to a second receiver of said central unit, said dialogue being unable to '' establish that if the distance separating said central unit from said identification device is sensitive ment less than a predetermined communication distance, said method being characterized in that it comprises the steps consisting in:

(i) the triggering of said first communication, said first signal being carried by a first carrier wave;

(ii) the triggering of said second communication, said second communication taking place at least partially while said first communication continues, said second signal comprising an identification signal from said identification device and a so-called image phase signal, representing the phase of said first carrier wave received by said identification device,

(iii) issuing an activation order to activate said member after receipt of said identification signal by said central unit;
said method also comprising, simultaneously with said first and second communications, a step of successive comparisons between the current value of the phase represented by said image phase signal received by said second receiver, and the current value of the phase of a signal of reference, conforming to the carrier wave being transmitted by said first transmitter delayed by a predetermined reference delay, zero or non-zero; the phase represented by said image phase signal exhibiting, during said comparison step, a transmission delay with respect to the phase of said carrier wave being transmitted by said first transmitter, said activation order transmission being prohibited as soon as the result of said successive comparisons meets a predetermined cancellation criterion.

Preferably, the phase of said first carrier wave undergoes random modulation for the duration of its transmission.

In this case, said random modulation comprises advantageously the introduction of phase jumps in said first carrier wave at random times succeeding each other at intervals irregular greater than or equal to a modulation period predetermined.

Advantageously, said modulation period is substantially greater than the period of said first carrier wave; preferably of the order of 5 to 10 times the period of said first wave carrier.

Preferably, the amplitude of said phase jumps is substantially fixed.

Preferably in this case, the ratio between said amplitude and said modulation period is much lower than the fundamental frequency of said first carrier wave, preferably less than 1/5 ^th of this fundamental frequency.

Preferably, the phase represented by said phase signal image presents, with respect to the phase of said carrier wave, alterations introduced by the identification device in function said phase jumps according to a predetermined algorithm known to said central unit.

According to another characteristic of the invention, said delay of transmission being dependent on the time required for the journey of said first signal between said central unit and said identification device and time required for said second signal to travel between said device said central unit, said reference delay is chosen substantially equal to a predetermined normal delay, said normal delay being the substantially fixed value of said transmission delay when the distance separating said central unit from said identification device is significantly less than said communication distance predetermined.

In a particular embodiment of the invention, said cancellation criterion is validated if the difference between said two values of phase compared exceeds a predetermined maximum level.

In another particular embodiment of the invention, the difference between said two compared phase values includes the sum of a so-called continuous component, substantially constant over a duration greater than said modulation period, and of a component fluctuating over the duration of said modulation period, said criterion cancellation being validated as soon as the absolute value of said component continuous exceeds a first predetermined threshold.

Preferably in this case, said cancellation criterion is validated as soon as the integral of the amplitude of said fluctuating component over a predetermined integration period exceeds a second threshold predetermined.

The present invention also provides a system for implementation of the method described above, comprising said unit central unit and said portable identification device, characterized by the fact that said central unit comprises a first microcontroller linked to said organ for issuing said activation order, said first transmitter radio frequency linked to said first microcontroller to transmit said first carrier wave and said first signal, said second receiver radio frequency to receive said second signal, a module phase comparator linked to said second receiver and to said first transmitter for performing said successive comparisons, a module for decision related to said phase comparator module to receive the result said comparisons and linked to said first microcontroller for him issue a signal to cancel said activation order transmission as soon as said cancellation criterion is validated; said device identification comprising said first radio frequency receiver for receive said first signal, a second microcontroller to generate said identification signal, said second radio frequency transmitter for transmitting said second signal, said second transmitter being linked to said first receiver for receiving said image phase signal and said second microcontroller for receiving said identification signal.

Advantageously, said central unit comprises a signal random sequence generator linked to said first transmitter to modulate the phase of said first carrier wave during its program.

Preferably, said random sequence generator of signals is suitable for periodically delivering to said first transmitter a random binary signal, said first transmitter being able to introduce a phase jump in said first carrier wave each time that said received binary signal takes a predetermined value.

According to another characteristic of the invention, said identification device includes a demodulator connected to the first receiver for receiving said first carrier wave and at the second microcontroller to deliver said demodulated random binary signal to it of said first carrier wave; said second microcontroller being able to modify said image phase signal as a function of said signal random binary.

Preferably, said decision module includes a filter low pass whose input is connected to the output of said comparator module phase and whose output is connected to a signal comparator for compare the difference between said two phase values compared with said predetermined maximum level.

According to yet another characteristic of the invention, said decision module includes a high-pass filter whose input is connected at the output of said phase comparator module to extract said phase fluctuating component of said comparison result and the output of which is connected to a second comparator, separate or not from the comparator of signal, to compare said fluctuating component to said second threshold predetermined.

Advantageously, said first transmitter is capable of transmitting said first signal by amplitude or phase modulation of said first carrier wave.

• la figure 1 est un schéma synoptique de fonctionnement d'un procédé de commande à distance selon l'invention en fonctionnement normal ;
• la figure 2 est un schéma synoptique de fonctionnement d'un procédé de commande à distance selon l'invention lors d'une tentative de piratage ;
• la figure 3 est un schéma synoptique d'un système pour la mise en oeuvre du procédé selon l'invention ;
• la figure 4 est un chronogramme représentant schématiquement les signaux comparés dans le système de la figure 3 lors de l'utilisation normale du système et d'une tentative de piratage ;
• la figure 5 est un chronogramme représentant schématiquement les signaux comparés dans le système de la figure 3 lors d'une tentative de piratage après insertion d'un saut de phase dans la première onde porteuse ;
• la figure 6 est un vue détaillée et agrandie du module de décision du système défini par le cadre VI de la figure 3 dans un premier mode de réalisation ;
• la figure 7 est un chronogramme représentant un signal de phase reçu par le module de décision de la figure 6 pour différentes valeurs du retard de transmission entre les signaux comparés ;
• la figure 8 est un chronogramme représentant le signal de phase reçu par le module de décision de la figure 6 pour des valeurs particulières du retard de transmission ;
• la figure 9 est un vue détaillée et agrandie du module de décision de la figure 6 dans un second mode de réalisation ;
• la figure 10 est un chronogramme représentant la partie fluctuante du signal de phase reçu par le module de décision de la figure 9 lors de l'utilisation normale du système ;
• la figure 11 est un chronogramme représentant la même la partie fluctuante de signal de phase qu'à la figure 10 lors d'une tentative de piratage.
• la figure 12 un schéma synoptique d'une variante d'une partie du système de la figure 3 délimitée par le cadre XII ;
• la figure 13 est un chronogramme représentant le signal de phase reçu par le module de décision de la figure 6 lorsque le module de remise en forme de la figure 12 est utilisé .

The invention will be better understood, and other objects, details, characteristics and advantages thereof will appear more clearly during the following description of a particular embodiment of the invention, given solely by way of illustration and not limiting, with reference to the attached drawing. On this drawing :

• FIG. 1 is a block diagram of the operation of a remote control method according to the invention in normal operation;
• Figure 2 is a block diagram of a remote control method according to the invention during a hacking attempt;
• Figure 3 is a block diagram of a system for implementing the method according to the invention;
• Figure 4 is a timing diagram schematically showing the signals compared in the system of Figure 3 during normal use of the system and a hacking attempt;
• Figure 5 is a timing diagram schematically representing the signals compared in the system of Figure 3 during a hacking attempt after insertion of a phase jump in the first carrier wave;
• Figure 6 is a detailed and enlarged view of the decision module of the system defined by frame VI of Figure 3 in a first embodiment;
• FIG. 7 is a timing diagram representing a phase signal received by the decision module of FIG. 6 for different values of the transmission delay between the compared signals;
• FIG. 8 is a timing diagram representing the phase signal received by the decision module of FIG. 6 for particular values of the transmission delay;
• Figure 9 is a detailed and enlarged view of the decision module of Figure 6 in a second embodiment;
• FIG. 10 is a timing diagram representing the fluctuating part of the phase signal received by the decision module of FIG. 9 during normal use of the system;
• FIG. 11 is a timing diagram showing the same fluctuating phase signal part as in FIG. 10 during a hacking attempt.
• Figure 12 is a block diagram of a variant of a part of the system of Figure 3 delimited by the frame XII;
• Figure 13 is a timing diagram representing the phase signal received by the decision module of Figure 6 when the fitness module of Figure 12 is used.

The control method according to the invention uses a bidirectional dialogue over the air between a central unit 1 carried by a vehicle V to control one of its visible members O in FIG. 3, and a portable identification device I, intended to be worn by a user U of said vehicle. The control system for the implementation of the method according to the invention comprises the central unit 1 and the portable identification device I, which will now be described with reference to Figure 3.

The central unit 1 comprises a first microcontroller 20 linked to said member O, which can in particular be a contact switch or a door lock, by a communication network 21. The microcontroller 20 is generally in a state of semi-sleep or waiting for an alarm clock. When the user actuates the exterior door handle, not shown, an activation signal is sent to the microcontroller 20, as indicated by the arrow 22. In response, the microcontroller sends a general power signal, to supply the various components electronics of the central unit 1. Then, the microcontroller 20 generates the signal S ₁ to communicate to the identification device I and sends it as input to a low frequency generator 24 to modulate the amplitude or phase of the wave low frequency carrier 27 which it generates. The low frequency generator 24 has a fundamental frequency f ₀ for example of the order of 125 kHz. The output of the low frequency generator 24 is connected to an amplifier device 25 in order to amplify the carrier wave 27 carrying the signal S ₁ . The first transmitter 2 comprises said low-frequency generator 24 and said amplifier device 25. The output of the amplifier device 25 is connected to antennas 26 for transmitting to the identification device I the carrier wave 27 carrying the signal S ₁ . The antennas 26 comprise several identical antennas, for example three or more, located at different points of the vehicle V to allow the emission of the wave 27 in several directions around the vehicle V. Preferably, the antennas 26 comprise an antenna 26g on the driver's door, an antenna 26d on the front passenger door, and an antenna 26c on the boot of the vehicle V. Conventionally, each antenna 26 has a capacity connected in series to a coil with a core connected to ground.

Wave 27 is received by antennas 28 of the device identification I with attenuation depending on the distance from spread. The antennas 28 each comprise a winding mounted in parallel with a capacity of which one terminal is connected to ground, the respective windings of the three antennas 28x, 28y and 28z having axes mutually orthogonal so that the carrier wave 27 can be detected whatever its polarization and whatever the orientation of the identification device I. The three antennas 28 are connected respectively at three inputs 3x, 3y and 3z of the low frequency receiver 3. The one of the antennas 28 which is the best oriented is thus able to produce an amplitude of 2 mV.

The identification device I comprises a second microcontroller 30, the second transmitter 4 and a battery (not shown) for supplying its various elements, the low frequency receiver 3 preferably being of low consumption. The receiver 3 has a first output connected to a fitness module 31 and a second output linked to the microcontroller 30 to deliver the first signal S ₁ obtained by demodulation of the carrier wave 27. The receiver 3 sends to the delivery module in form 31 a signal in slots 64 reproducing, in a clipped form, the carrier wave 27 as it receives it, without demodulation, so that the fitness module 31 eliminates the parasites generated during the first communication.

In an alternative embodiment of the invention which will be explained below, the fitness module 31 is also able to reduce the frequency of the signal in slots 34 to a fraction of the fundamental frequency f ₀ , in order to make it suitable for be transported by the second transmitter 4 on a carrier wave 35. The square-wave signal 34 may also include alterations with respect to the transitions of the low frequency carrier wave 27, introduced according to a predetermined algorithm.

The niche signal 34 at the output of the fitness module 31 reproduces the transitions of the low frequency carrier wave 27 as emitted by the transmitter 2, possibly at a reduced frequency, possibly with desired alterations and without disturbances parasites. A rocker switch with two inputs 32 is controlled by the second microcontroller 30 by a line 33 to generate the second signal S ₂ . The rocker switch 32 has a first input linked to the fitness module 31 to receive the signal 34 and a second input linked to the second microcontroller 30 to receive an identification signal S _i , generated by the second microcontroller 30. According to the position of said flip-flop, the second signal S ₂ output from said flip-flop switch 32 to the second transmitter 4 is composed alternately of sequences of data of the identification signal S _i and of sequences of data reproducing the carrier wave at low frequency 27.

The second transmitter 4 is at ultra high frequency, for transmitting the second signal S ₂ by frequency modulation of a carrier wave 35 of fundamental frequency for example of the order of 434 MHz. The carrier wave 35 is transmitted via an antenna 36 of the second transmitter 4 to an antenna 37 connected to the second receiver 5 of the central unit 1. The ultra high frequency receiver 5 is capable of demodulating the carrier wave 35 that it receives to transmit at its output the signal S ₂ to the first microcontroller 20. The microcontroller 20 is able to authenticate the identification signal S _i contained in the signal S _{2 which} it receives, and to issue an activation order 38 to the organ O after the authenticity of the identification signal Si has been recognized.

As a variant, the second signal S ₂ can be transmitted by amplitude modulation of the carrier wave 35. In this case, the rocker switch 32 is replaced by a summing module and the signal S ₂ is formed by modulation d amplitude of the signal 34 within said summing module (not shown). The signal S ₂ then comprises simultaneously, and no longer alternately, the identification signal S _i and the signal 34 reproducing the transitions of the low frequency carrier wave 27.

Preferably, the second microcontroller 30 is linked to a permanent memory 56 in which identification and encryption data characteristic of said identification device I are stored, in order to use these identification data in the authentication of the first signal S ₁ and / or the generation of the identification signal S _i . The first microcontroller 20 is also linked to a permanent memory 49 where identification and encryption data characteristic of said vehicle V are stored, in order to use these identification data in the generation of the first signal S ₁ and / or the authentication of the identification signal S _i .

The remote control method according to the invention provides an anti-piracy function, by detecting the transmission delay Δt generated by an hacking attempt by the previously described process. To detect the presence of the delay Δt transmission, the central unit 1 includes a comparator module for phase 40, the two inputs of which are linked respectively to the generator low frequency 24 and to the second receiver 5, and the output of which is connected to a decision module 43 for delivering a phase signal Φ to it. The decision module 43 is connected to the microcontroller 20 to deliver it a cancellation signal An when the phase signal Φ satisfies a criterion predetermined cancellation. The phase signal Φ is a voltage signal which is between a minimum value, for example equal to -0.75 V, taken when the phase shift measured by the phase comparator module 40 is substantially zero (modulo 2π) and a maximum value, for example equal to + 0.75V, taken when the phase shift measured by the phase comparator module 40 is substantially π (modulo 2π).

The calculation of the transmission delay generated by the hacking method described above will now be carried out with reference to Figures 1 and 2. During normal use of the control system, as shown in Figure 1, assume that a given transition of the low frequency wave 27 carrying the signal S ₁ is emitted at the instant t ₁ by the transmitter 2. This transition is received by the receiver 3 at the instant t ₂ , separated from the instant t ₁ by the propagation time of the low frequency carrier wave 27 over the distance separating the vehicle V from the portable identification device I, ie substantially d _c / c, where c is the speed of light in the air , substantially equal to 3.0.10 ⁸ m / s. This same transition is represented by a part of the signal 34 included in the signal S ₂ . The second ultra-high frequency transmitter 4 transmits this part of the signal 34 at time t ₃ , separated from the reception time t ₂ by a response delay R, due to the signal processing time by the first receiver 3, the shaping module 31 and the second transmitter 4. The part of the signal S ₂ representative of this same transition of the low frequency carrier wave 27 is received by the second receiver 5 at the instant t ₄ ; then it is finally received by the phase comparator 40 at time t _B after a reception delay Q due to the reception and processing of the second signal S ₂ in the second receiver 5. Thus, t _B is substantially equal to: t _B = t ₁ + P + R + Q, where P = 2d _c / c is the total propagation time of the electromagnetic wave to go back and forth between the vehicle V and the identification device I, which is around 13.2 ns. The delays Q and R are for example of the order of 2 to 10 μs each. Thus P can be neglected before Q and R.

When using the hacking method described above, represented in FIG. 2, suppose that a given transition of the low frequency wave 27 carrying the signal S ₁ is emitted at said time t ₁ by the transmitter 2. This transition is received by the receiver 8 of the first relay box 6 at time t ₂ ', separated from time t ₁ by the propagation time of the carrier wave at low frequency over the distance d _c ' separating the vehicle V of the first relay box 6. The part of the signal 10 representative of this same transition is transmitted by the transmitter 9 of the first relay box 6 at time t ₃ ', separated from time t ₂ ' by time response Tr of the relay box 6, which is substantially of the order of 1 µs. This part of the signal 10 is received by the receiver 12 of the second relay box 11 at the instant t ₄ ', separated from the instant t ₃ ' by the propagation time of the wave over the distance d _p separating the two. relay boxes 6 and 11. This part of the signal 10 received is converted in the second relay box 11 into a part of the signal 14 representative of this same transition and sent by the transmitter 13 at time t ₅ ', separated from instant t ₄ ′ by the response time Tr of the second relay box 11, substantially equal to the response time of the first relay box 6.

The identification device I receives said part of the signal 14 at time t ₆ 'and responds to the part of signal S ₂ representative of this same transition at time t ₇ ', with the same response delay R as during normal use of the system. The propagation and processing times of the signals S ₂ , 17 and S ₂ 'constituting the communication from the identification device I to the vehicle V are identical to the times involved in the communication of the signal S ₁ . The part of signal S ₂ 'representative of said transition of the low frequency wave transmitted at time t ₁ is received by the second receiver 5 at time t ₁₂ ', and finally received by the phase comparator module 40 at the instant t _B ', substantially equal to: t _B ' = t ₁ + 2d _ℓ / c + R + Q + 4Tr. The total propagation time 2d _ℓ / c is of the order of 100 to 1000 ns for a distance d _ℓ between 15 m and 150 m. It has thus been shown that the signal representing the transition transmitted at t ₁ is finally received by the phase comparator module 40 with a transmission delay Δt = t _B '-t _B of the order of 4 to 5 µs in the case of 'an attempt to hack with relay boxes compared to the normal use of the system. The major part of said transmission delay Δt is due to the response time Tr of the relay boxes.

In a preferred embodiment of the invention, the low frequency generator 24 is capable of delaying the signal 41 transmitted to the phase comparator module 40 by an operational delay Dr relative to the carrier wave 27. The operational delay Dr is chosen substantially equal to the sum of the response delay R of the identification device I and the reception delay Q. The transition of the carrier wave 27 emitted at time t ₁ is thus received by the phase comparator module 40 in signal 41 at time t _A = t ₁ + Dr ≈ t _B. The delayed signal 41 serves as a reference signal in the phase comparator module 40. The signal 42 received by the phase comparator module 40 comprises the signal 34 representative of the carrier wave 27. This signal 42 for its part, at the reception by the phase comparator 40, a delay (t _B -t ₁ ) relative to the carrier wave 27 generated by the low frequency generator 24.

The operation of the detection of a hacking attempt by the system according to the invention will now be described with reference to Figures 4 to 13. In Figure 4, there is shown, over a given period T _BF , the signal 41 conforms to the low frequency carrier wave 27 and delayed by the operational delay Dr with respect to it. Curve 44 represents the phase of this signal varying over the interval [0, 2π [. The signal 41 is represented as a sinusoidal signal for the sake of clarity, but in practice, it can be clipped so as to be substantially a slot signal whose transitions correspond to the passages by the zero value of the sinusoidal signal represented.

In the event of normal use of the system according to the invention, at a given time t ₀ , the phase of the signal 41 at the input of the phase comparator 40 is represented by the point A, and the phase of the signal 42 is represented by the point B. The phase shift  _B between signals 41 and 42 compared by the phase comparator 40 is  _B = 2π (t _B -t _A ) / T _BF , which is of the order of 0.01 radians. In other words, the phase shift measured by the phase comparator 40 is substantially zero in this case. On the other hand, in the event of an attempt at hacking using relay boxes, the phase of the signal 41 is unchanged, but the phase of the signal 42 at the same instant t ₀ given is represented by the point B '. The measured phase shift is then  _B '= 2π (t _B ' -t _A ) / T _BF , or  _B '= 2πΔt / T _BF , which is significantly greater than π radians.

As the vehicle and the carrier of the identification device I cannot move a significant distance relative to the wavelength of wave 27 during said dialogue, the total duration of which is substantially a fraction of a second , the transmission delay Δt is substantially constant throughout this duration. Thus, the phase shift  _B 'proportional to the transmission delay Δt is substantially constant throughout the duration of said dialogue.

With the simple phase shift measurement described above, an attempt to hack using relay boxes could not always be detected. Indeed, in the case where the transmission delay Δt is equal to an integer N greater than or equal to 1 of periods T _BF , with reference again to FIG. 4, the phase of the signal 41 represented by the point A is unchanged , but the phase of signal 42 at the same given instant is represented in this case by point B ". The measured phase shift is then substantially sensiblement _B " = 2Nπ; therefore, the phase signal Φ is substantially minimal. In the example shown in FIG. 4, the integer N is equal to 1. To also detect a hacking attempt with such a well-chosen transmission delay Δt, the method and the system according to the invention use a random modulation of the phase of the carrier wave 27.

To this end, the control system according to the invention comprises, in the central unit 1, a generator of sequences of random signals 39 controlled by the microcontroller 20 and connected at output to the low frequency generator 24 to randomly modulate the phase of the carrier wave 27 generated. Preferably, the phase modulation of the carrier wave 27 is carried out as follows: the generator of random signal sequences 39 periodically delivers with a modulation period T _m , a modulation bit b _m equal to 0 or 1. L the transmission of the modulation bits b _m is controlled by clock signals delivered to the generator of sequences of random signals 39 by the microcontroller 20, which includes a clock (not shown). The modulation bits b _m are transmitted to the low frequency generator 24 by a voltage signal V _m which can take a high value h and a low value ℓ. The voltage signal V _m makes a transition between its two values h and ℓ each time a bit of value 1 is transmitted, and keeps a constant value between times.

If the value of the modulation bit b _m received by the low frequency generator 24 is 0, the carrier wave 27 generated is unchanged; if the value of the modulation bit b _m received by the low frequency generator 24 is 1, the low frequency generator 24 instantly causes a phase jump of amplitude δ to the carrier wave 27. The phase jumps generated by the generator 24 are alternately a phase advance and a phase delay. The signal 41 delivered by the generator 24 to the phase comparator 40 remains of course always in accordance with the carrier wave 27, by following its phase jumps.

Referring again to FIG. 4, suppose that the transmission delay Δt is such that the measured phase shift  _B "is substantially equal to 2π at said given time t _0. After one or more modulation periods T _{m have} elapsed , a first phase jump δ has necessarily been inserted in the carrier wave 27. In fact, the opposite case would indicate that the generator of random signals 39 generates only 0s, which is excluded. phase in the carrier wave 27 has the effect of suddenly transforming the curve 41 into the curve 41 'sketched in FIG. 4, as represented by the arrow 51. In this example, the first phase jump is a phase delay d 'amplitude δ equal to π / 2.

In Figure 5, there is shown the signal 41 'in which the phase jump has been reflected, as well as its phase 44', and the signal 42 in which, because of the delay substantially equal to an integer greater than or equal to 1 of periods T _{BF which} it presents with respect to signal 41, the phase jump has not yet been reflected, as well as its phase 50. Between the insertion of the first phase jump and the insertion of the following phase jump , whose date is by definition random, the phase comparator 40 compares the phase 44 to the delayed phase 50 of T _BF . Thus, the phase shift measured by the comparator module 40 becomes, as shown in FIG. 5, substantially equal to δ. After the expiration of an additional modulation period T _m , either the modulation bit generated will be a 0 and the carrier wave 27 will remain continuous, or the modulation bit b _m generated will be a 1 and the carrier wave 27 will undergo an amplitude phase advance equal to δ. Because of the unpredictable succession of phase jumps δ of the carrier 27, it is then no longer possible to choose the transmission delay Δt so as to produce a measured phase shift  _B "which remains permanently zero.

Because of the random phase modulation of the carrier wave 27 described above, the phase signal Φ at the output of the phase comparator 40 has, in superposition, a continuous component Φ _c and a fluctuating component Φ _f whose amplitude is that of phase jump δ. The consequence of the random phase modulation of the carrier wave 27 is to spread the signal S ₁ in frequency. More specifically, the carrier wave 27 of fundamental frequency f ₀ has, after frequency modulation, a spectrum spread substantially in the range [f ₀ -δ / (2πT _m ), f ₀ + δ / (2πT _m )] . The choice of the phase jump δ and of the modulation period T _m must therefore be made with the concern of respecting the regulations on the spectral width of the signals emitted in the environment. To reduce frequency spreading, the modulation period T _m can be lengthened or the amplitude of the phase jump δ can be reduced. The modulation period T _m is therefore chosen to be greater than or equal to the period of the low frequency signal T _BF . Preferably, the modulation period T _m is of the order of 5 to 10 times T _BF , or 40 to 80 µs. Thus, as in the example shown in FIG. 11, the modulation period T _m is preferably longer than the transmission delay Δt. The typical frequency f _f of the fluctuations of the fluctuating phase signal Φ _f is therefore of the order of 1 / Δt, ie for example a few tens of kilohertz (kHz).

The phase signal Φ at the output of the comparator module of phase 40 is sent to a decision module 43 capable of detecting the presence of a significant phase shift between signals 41 and 42.

In a first embodiment, visible in FIG. 6, the decision module 43 includes a low-pass filter 65, with cut-off frequency f _c for example substantially equal to 100 kHz in order to suppress noise, connected in series to a signal comparator 66 with a predetermined maximum level E. The signal comparator 66 is capable of delivering to the microcontroller 20 the cancellation signal An when the value of the phase signal Φ is greater than the maximum level E.

FIGS. 7 and 8 show the evolution of the phase signal Φ received by the signal comparator 66 as a function of time, around the emission of a modulation bit b _m of value 1, materialized by the transition performed at time t ₁ = 100 μs by the voltage signal V _m , represented by curve 73. Each time a modulation bit b _m of value 1 is transmitted, the corresponding phase jump, of amplitude δ chosen equal to π in this example, is reflected on the reference signal 41 with the operational delay Dr and on the signal 42 with the delay Δt + Q + R. Thus, the phase signal Φ has from time t ₁ + Dr a slot of temporal width substantially equal to the transmission delay Δt. Because of the low-pass filter 65, this slot is rounded.

Curve 67 represents the phase signal Φ for a transmission delay Δt = 0.2 μs, that is to say typical of normal use of the system. It can be seen that the phase signal Φ is substantially constant, with a value substantially equal to -0.75V. An oscillation 68 marks the location of the slot corresponding to the phase jump introduced at t _1. The oscillation 68 is of very low amplitude because it is very attenuated by the low-pass filter 65. The curve 69 represents the phase signal Φ for a transmission delay Δt = 2 µs. In this case, the signals 41 and 42 compared are in quadrature. It can be seen that the phase signal Φ oscillates weakly around a value substantially equal to 0V. Oscillation 70 marks the phase jump introduced at t ₁ . Curve 71 represents the phase signal Φ for a transmission delay Δt = 4 µs. In this case, the signals 41 and 42 compared are in phase opposition. It can be seen that the phase signal Φ oscillates weakly around a value substantially equal to + 0.75V. The oscillation 72, of greater amplitude than the previous ones because less attenuated by the filter 65, marks the phase jump introduced at t ₁ . With these three examples, it can be seen that the choice of a threshold E substantially equal to -0.5V makes it possible to detect a transmission delay greater than or equal to 2 μs. In these examples, it is the average value of the phase signal Φ which allows the detection and the phase jumps are not used.

In FIG. 8, the phase signal Φ is shown when the transmission delay Δt is an integer number of periods T _BF . For curve 74, the transmission delay is Δt = 8µs = T _BF . In this case, it can be seen that the phase signal Φ is on average substantially equal to -0.75 V, but that it has a large oscillation corresponding to the phase jump introduced at t ₁ . This oscillation, the temporal width of which is 8 μs, is not attenuated by the filter 65, so that the phase signal Φ momentarily crosses the threshold E = -0.5V. The curves 75, 76 and 77 represent the phase signal Φ in the case of a transmission delay Δt equal to 2, 4 and 8 T _{BF respectively} . It can be seen that the oscillation corresponding to the phase jump introduced at t ₁ doubles the width from one curve to another and that its maximum value approaches 0.75V. Thus, the four examples in FIG. 8 show that the threshold E = -0.5V also makes it possible to detect any transmission delay Δt substantially equal to an integer N greater than or equal to 1 of periods T _BF .

In Figures 7 and 8, the phase signal Φ is shown as produced by an analog system, but it is of course that a digital processing system can be used to obtain a equivalent result.

In a second embodiment, visible in FIG. 9, the decision module 43 has two branches. A first branch comprises a low-pass filter 45 to smooth the phase signal Φ. The cut-off frequency f ₁ of the low-pass filter 45 is, for example, less than 1 kHz, so that the signal at the output of the low-pass filter 45 is a so-called continuous phase signal Φ _c , which does not fluctuate over a very long duration in front of the period T _BF of the low frequency carrier wave 27. The continuous phase signal Φ _c is sent to a first signal comparator 46 with a predetermined threshold e , called continuous threshold. The signal comparator element 46 is capable of delivering a characteristic signal 47 when the value of the continuous phase signal Φ _c is greater than or equal to the continuous threshold e , and of delivering no signal when the value of the continuous phase signal Φ _c is less than the continuous threshold e .

In the second embodiment, the decision module 43 comprises, in parallel with the first branch, a second branch with a high-pass filter 52 for detecting the fluctuating phase signal Φ _f . The cut-off frequency f ₂ of the high-pass filter 52 is, for example, less than 10 kHz, to select the fluctuating component Φ _f and eliminate the continuous component Φ _c . The high-pass filter 52 is connected at output to an amplifier 53 to amplify the fluctuating phase signal Φ _f , then to a second signal comparator element 54, capable of delivering a characteristic signal 57 when the absolute value of the fluctuating phase signal Φ _f is greater than or equal to a fluctuation threshold ε, and not to deliver any signal when the absolute value of the fluctuating phase signal Φ _f is less than the fluctuation threshold ε. To be sensitive simultaneously to the existence of the continuous phase signal Φ _c and the fluctuating phase signal Φ _f , the decision module 43 includes a non-exclusive OR logic gate 55, connected as an input to the two signal comparator elements 46 and 54 , and able to output the cancellation signal An to the microcontroller 20 when it receives the characteristic signal 47 and / or the characteristic signal 57.

In FIG. 10, the curve 60 represents the evolution of the absolute value of the fluctuating component Φ _f as a function of time over several modulation periods T _m , during normal use of the system. Each time a modulation bit b _m of value 1 is transmitted, the corresponding phase jump is reflected on the reference signal 41 with the operational delay Dr and on the signal 42 with the delay P + Q + R. Thus, the fluctuating phase signal Φ _f presents a succession of slots 61 of temporal width substantially equal to the propagation time P, ie a few nanoseconds.

In FIG. 11, the curve 62 represents the evolution of the absolute value of the fluctuating component Φ _f as a function of time over several modulation periods T _m , during an attempt at hacking by relay boxes. In this case, each phase jump is reflected on the signal 42 with the delay Δt + Q + R. Thus, the fluctuating phase signal Φ _f in this case presents a succession of slots 63 of temporal width substantially equal to the transmission delay Δt, that is to say of the order of a few microseconds.

As shown in FIG. 10, during normal use of the system according to the invention in the second embodiment, a non-zero fluctuating component Φ _f is also observed, but it has a very short characteristic time, of the order a few nanoseconds. The integral of the fluctuating component Φ _f over a modulation period T _m therefore has a value less by a factor of at least 100 in the case of normal use of the system compared to the case of a hacking attempt. To avoid the emission of the cancellation signal An in response to this fluctuation of very short characteristic time, and to limit the noise sensitivity of the system according to the invention, the second signal comparator element 54 is preferably able to integrate the fluctuating phase signal Φ _f over an integration duration T _i longer than the modulation period T _m and compare said fluctuating phase signal Φ _f with the fluctuation threshold ε after this integration. For example, the integration time T _i is of the order of 50 to 100 times T _m . The integration duration T _i is of course less than the total duration of the bidirectional dialogue.

• soit en détectant une composante continue Φ_c supérieure au seuil continu e, lorsque le retard de transmission Δt est sensiblement différent d'un nombre entier N supérieur ou égal à 1 de périodes T_BF ;
• soit en détectant une composante fluctuante Φ_f d'amplitude intégrée supérieure au seuil de fluctuation ε, notamment lorsque le retard de transmission Δt est sensiblement égal à un nombre entier N supérieur ou égal à 1 de périodes T_BF.

Thus, to recap, the decision module 43 of the second embodiment is capable of delivering the cancellation signal An in the event of an attempt at hacking using relay boxes, excluding the case of normal use of the system according to the invention, in two complementary ways:

• either by detecting a continuous component Φ _c greater than the continuous threshold e , when the transmission delay Δt is significantly different from an integer N greater than or equal to 1 of periods T _BF ;
• either by detecting a fluctuating component Φ _f of integrated amplitude greater than the fluctuation threshold ε, in particular when the transmission delay Δt is substantially equal to an integer N greater than or equal to 1 of periods T _BF .

As a variant, with the second embodiment of the decision module 43, the operational delay Dr can be chosen to be zero, to limit the cost of the system for example. In this case, the continuous phase signal Φ _c has a non-zero reference value  ₀ substantially equal to  ₀ = 2π (R + Q) / T _BF during normal use of the system. The continuous threshold e of the first signal comparator 46 is then chosen to be greater than the reference value  ₀ . In this case, the fluctuating phase signal Φ _f , during normal use and a hacking attempt, has a characteristic fluctuation time elongated substantially by R + Q. The second comparator element 54 is then designed to discriminate from one another the signal in slots of typical width R + Q + P, obtained in normal use, of the signal in slots of typical width R + Q + Δt, which would be obtained during a hacking attempt. In this variant, the modulation period T _m is chosen to be greater than the delay R + Q.

The microcontroller 20 is able to inactivate the transmission of the activation order 38 as soon as it receives the cancellation signal An. For example, the microcontroller 20 is able to interrupt the current communication protocol before the data of the signal S _i necessary for the transmission of the order 38 have not been received by the central unit 1 and / or to prohibit the transmission of the order 38 for a duration of prohibition determined from reception of the cancellation signal An.

In an alternative embodiment of the identification device I, the fitness module 31 is connected to the second microcontroller 30, as visible in FIG. 12. In this alternative, the signal in slots 64 is sent to a demodulator 87 in the fitness module 31 for retrieving the modulation bits b _m . The demodulator 87 includes a clock recovery module 79 for recovering a signal 83 conforming to the carrier wave 27 at 125 kHz and an exclusive OR gate 80. The signal 83 and the signal in slots 64 are sent at the input of the exclusive OR gate 80, to form at output a square wave signal 81 representing the sequence of modulation bits b _m having served to modulate the wave 27 at its emission.

The second microcontroller 30 is able to use the signal 81 to generate, according to a predetermined algorithm known to the central unit 1, an alteration signal 82 for altering the signal in slots 64. The alteration signal 82 is sent in input of a second exclusive OR gate 88 of the fitness module 31. At the second input of the second exclusive OR gate 88 is sent a slot signal 64 ′, which is produced by a frequency divider 78 of the fitness module in shape 31. The square-wave signal 64 'represents the transitions of the square-wave signal 64 at a frequency f ₀ ' equal to a fraction of the fundamental frequency f ₀ . The frequency divider 78 makes it possible to limit the bandwidth necessary for the transport of the signal 34 to the central unit 1. The phase signal 34 is finally obtained at the output of the second exclusive OR gate 88.

In this alternative embodiment, the signal 34 represents the carrier wave transitions 27 with its phase jumps introduced by the generator 24, at a possibly reduced frequency, and with the alterations introduced by the second microcontroller 30. For example, the second microcontroller 30 can, using the alteration signal 82, invert the phase of signal 34 with respect to a fixed duration to that of signal 64.

Thus, in FIG. 13, the phase signal Φ at the input of the comparator module 66 has been shown as a function of time for a duration of several modulation periods T _m . Curve 84 represents the voltage signal Vm corresponding to a random sequence of modulation bits b _m . During normal operation of the control system, the phase signal Φ conforms to that represented by curve 85 and has a low amplitude oscillation following each emission of a modulation bit of value 1, as previously explained. In the embodiment described above, the second microcontroller 30 can introduce a phase inversion in the signal 34 at time t _I , which causes a phase opposition between the signals 41 and 42 compared by the phase comparator 40. The phase signal Φ then takes a maximum value, as represented by curve 86, until the second microcontroller 30 introduces a second phase inversion which will return the phase signal Φ to its minimum value. In this variant, the decision module 43 is able to recognize the alterations introduced by the microcontroller 20 according to a predetermined algorithm and as a function of the bit sequence b _m received, so as not to trigger the cancellation signal An when a such alteration causes the momentary crossing of the threshold E by the phase signal Φ. The introduction by the second microcontroller 30 of alterations of the phase signal 34 makes it possible to defeat a possible hacking attempt using a simple signal repeater which would retransmit to the vehicle V a signal identical to the signal S ₁ sent by this one towards the identification device I.

Although the invention has been described in connection with several particular embodiments, it is obvious that it is not there in no way limited and that it includes all technical equivalents means described as well as their combinations, if these fall within the scope of the invention.

Claims (18)

Remote control method for controlling a member (O) of a motor vehicle (V), in particular a means of blocking access to said vehicle and / or a means of starting said vehicle, consisting in establishing a bidirectional dialogue by channel wireless between a central unit (1) carried by said vehicle (V) and a portable identification device (I) intended to be worn by a user (U), said dialogue comprising at least a first communication, continuous or interrupted, d 'a first signal (S ₁ ) transmitted from a first transmitter (2.24) of said central unit (1) to a first receiver (3) of said identification device (I) and a second communication, continuous or interrupted, d a second signal (S ₂ ) transmitted from a second transmitter (4) of said identification device (I) to a second receiver (5) of said central unit (1), said dialogue being able to be established only if the distance separating said central unit (1) dud it identification device (I) is substantially less than a predetermined communication distance (d _c ), said method being characterized in that it comprises the steps consisting of: (i) the triggering of said first communication, said first signal (S ₁ ) being carried by a first carrier wave (27); (ii) the triggering of said second communication, said second communication taking place at least partially while said first communication continues, said second signal (S ₂ ) comprising an identification signal (S _i ) of said identification device and a so-called image phase signal (34), representing the phase of said first carrier wave (27) received by said identification device, (iii) issuing an activation order (38) to activate said member (O) after receipt of said identification signal (S;) by said central unit (1);
said method also comprising, simultaneously with said first and second communications, a step of successive comparisons between the current value of the phase represented by said image phase signal (42) received by said second receiver (5) and the current value of phase a reference signal (41), conforming to the carrier wave (27) being transmitted by said first transmitter (2,24) delayed by a predetermined reference delay (Dr), zero or non-zero, the phase represented by said image phase signal (42) exhibiting, during the comparison step, a transmission delay with respect to the phase of said carrier wave (27) during transmission by said first transmitter (2, 24), said issuance of an activation order (38) being prohibited as soon as the result of said successive comparisons (Φ) meets a predetermined cancellation criterion. Method according to claim 1, characterized in that the phase of said first carrier wave (27) undergoes random modulation for the duration of its emission. Method according to Claim 2, characterized in that the said random modulation comprises the introduction of phase jumps in the said first carrier wave (27) at random instants following one another at irregular intervals greater than or equal to a predetermined modulation period ( T _m ). Method according to Claim 3, characterized in that the said modulation period (T _m ) is substantially greater than the period (T _BF ) of the said first carrier wave (27); preferably of the order of 5 to 10 times the period (T _BF ) of said first carrier wave (27). Method according to one of claims 3 or 4, characterized in that the amplitude of said phase jump (δ) is substantially fixed. Method according to claim 5, characterized in that the ratio between said amplitude (δ) and said modulation period (T _m ) is significantly lower than the fundamental frequency (f ₀ ) of said first carrier wave (27), by preferably less than 1/5 ^th of this fundamental frequency (f ₀ ). Method according to one of claims 3 to 6, characterized in that the phase represented by said image phase signal (42) has, with respect to the phase of said carrier wave (27), alterations introduced by the device identification (I) as a function of said phase jumps according to a predetermined algorithm known to said central unit (1). Method according to one of claims 1 to 7, characterized in that , said transmission delay being dependent on the time necessary for the path of said first signal (S ₁ ) between said central unit (1) and said identification device (I ) and of the time necessary for the path of said second signal (S ₂ ) between said identification device (I) and said central unit (1), said reference delay (Dr) is chosen to be substantially equal to a predetermined normal delay (R + Q), said normal delay (R + Q) being substantially the fixed value of said transmission delay when the distance separating said central unit (1) from said identification device (I) is substantially less than said predetermined communication distance (d _c ). Method according to one of claims 1 to 8, characterized in that said cancellation criterion is validated if the difference between said two compared phase values (Φ) exceeds a predetermined maximum level (E). Method according to one of claims 2 to 9, characterized in that the difference between said two compared phase values (Φ) comprises the sum of a so-called continuous component (Φ _c ), substantially constant over a period greater than said modulation period (T _m ), and of a fluctuating component (Φ _f ) over the duration of said modulation period (T _m ), said cancellation criterion being validated as soon as the absolute value of said continuous component (Φ _c) exceeds a first predetermined threshold (e). Method according to Claim 10, characterized in that the said cancellation criterion is validated as soon as the integral of the amplitude of the said fluctuating component (Φ _f ) over a predetermined integration time (T _i ) exceeds a second threshold predetermined (ε). System for implementing the method according to claim 1, comprising said central unit (1) and said portable identification device (I), characterized in that said central unit (1) comprises a first linked microcontroller (20) to said member (0) for issuing said activation order (38), said first radio frequency transmitter (2,24) linked to said first microcontroller (20) for transmitting said first carrier wave (27) and said first signal (S ₁ ), said second radio frequency receiver (5) for receiving said second signal (S ₂ ), a phase comparator module (40) linked to said second receiver (5) and to said first transmitter (2,24) for carrying out said comparisons successive, a decision module (43) linked to said phase comparator module (40) to receive the result of said comparisons (Φ) and linked to said first microcontroller (20) to supply it with a cancellation signal (An) from said transmission of activation order (38) as soon as said cancellation criterion is validated; said identification device (I) comprising said first radio frequency receiver (3) for receiving said first signal (S ₁ ), a second microcontroller (30) for generating said identification signal (S _i ), said second radio transmitter -frequency (5) for transmitting said second signal (S ₂ ), said second transmitter (5) being linked to said first receiver (3) to receive said image phase signal (34) and to said second microcontroller (30) to receive said signal identification (S;). System according to claim 12, characterized in that the said system is also for implementing the method according to the claim 2, the said central unit (1) comprising a generator of random sequence of signals (39) linked to the said first transmitter (2 , 24) to modulate the phase of said first carrier wave (27) during its emission. System according to Claim 13, characterized in that the said system is also for implementing the method according to the Claim 3, the said generator of random sequence of signals (39) being capable of periodically delivering to the said first transmitter (2) a random binary signal (b _m ), said first transmitter (2,24) being able to introduce a phase jump in said first carrier wave (27) each time said received binary signal (b _m ) takes a predetermined value. System according to Claim 14, characterized in that the said system is also for implementing the method according to the Claim 7, the said identification device (I) comprising a demodulator (87) linked to the first receiver (3) for receiving said first carrier wave (27) and to the second microcontroller (30) for supplying it with said demodulated random binary signal (b _m ) from said first carrier wave (27); said second microcontroller (30) being able to modify said image phase signal (34) as a function of said random binary signal (b _m ). System according to one of claims 12 to 15, characterized in that said system is also for implementing the method according to claim 9, said decision module (43) comprising a low-pass filter (65), the l input is connected to the output of said phase comparator module (40) and the output of which is connected to a signal comparator (66) to compare the difference between said two compared phase values (Φ) at said predetermined maximum level (E ). System according to Claim 16, characterized in that the said system is also for implementing the method according to the Claim 11, the said decision module (43) comprising a high-pass filter (52) whose input is connected to the output of said phase comparator module (40) for extracting said fluctuating component (Φ _f ) from said result of the comparisons (Φ) and the output of which is connected to a second comparator (54), distinct or not from the signal comparator (66) , to compare said fluctuating component (Φ _f ) with said second predetermined threshold (ε). System according to one of claims 12 to 17, characterized in that said first transmitter (2,24) is capable of transmitting said first signal (S ₁ ) by amplitude or phase modulation of said first carrier wave (27 ).

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