Method and device for protecting a motor vehicle

Device for protecting a motor vehicle (V), adapted for authorizing the starting of this vehicle only after a prior identification of a specific electronic tag (B) fitted with a transmitter/receiver circuit (6), situated inside the cabin of the said motor vehicle, the said protection device comprising a central unit (UC) mounted on the vehicle (V) and comprising a transmitter/receiver circuit associated with antennas (INT1-INT3, EXT1-EXT3) adapted for covering the entire volume of the cabin, the said central unit and the electronic tag (B) being programmed in such a way as to implement an authorization procedure according to which the central unit (UC) instructs the transmission of an identification request signal on receipt of which the electronic tag (B) is adapted for transmitting in response an identification signal identifiable by the said central unit, and the protection device being characterized in that:

• the antennas (INT1-INT3, EXT1-EXT3) associated with the central unit (UC) are disposed and adapted so that each zone of the cabin is covered by at least two antennas,
• the electronic tag (B) comprises means of three-dimensional measurement (A1-A3) of the orientation of the magnetic fields received by the said electronic tag,
• and the central unit (UC) and the electronic tag (B) are programmed to implement an authorization procedure consisting:
• in determining in a random manner a sequence of emissions which consists of a succession of magnetic fields to be emitted one after the other in a given order from the antennas (INT1-INT3, EXT1-EXT3) of the vehicle (V),
• in sending to the electronic tag (B) a signal representative of the randomly determined sequence of emissions,
• in instructing the successive emission by the antennas (INT1-INT3, EXT1-EXT3) of the vehicle (V) of the magnetic fields forming the sequence of emissions,
• upon receipt of the successive magnetic fields by the electronic tag (B), in measuring, for each of the said magnetic fields, a value representative of the resulting orientation of the said electronic tag with respect to the source of emission of this magnetic field, in such a way as to establish a series of values each representative of the measured resulting orientation,
• and in comparing the randomly determined sequence of emissions with the series of measured values, and in validating the authorization procedure upon a correlation between the said sequence of emissions and series of values.