Securing method, interrogation unit and securing system for implementing the securing method

摘要

In a method for securing an object against unauthorized use, an interrogation unit ( 1 ) transmits an interrogation signal (s) and tests whether a response unit ( 2 ), which is to be carried by an authorized user, responds to the interrogation signal with a response signal (r) that contains as information a code authorized for desecuring a security device ( 3 ). If so, the security device is desecured. The securing method shall prevent unauthorized persons from establishing a radio link via relay stations between the interrogation unit ( 1 ) and the response unit ( 2 ) for providing the authorized code via this radio link without being noticed by the authorized user. The securing method prevents the desecuring of the security device ( 3 ) if the signal transit time of a signal transmitted as the interrogation signal (s) and received as the response signal (r) by the interrogation unit ( 1 ) during a monitoring time interval (t 4 -t 5 ) is greater than a prescribed value. The method is carried out in a keyless security system for securing motor vehicles against unauthorized use.