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Secure distance ranging by electronic means

  • US 5,126,746 A
  • Filed: 07/08/1991
  • Issued: 06/30/1992
  • Est. Priority Date: 07/08/1991
  • Status: Expired due to Fees
First Claim
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1. A secure distance ranging system including a reader and a tag in communication with each other, to prevent an adversary to provide a false ranging response, said system comprising:

  • means in said reader for sending a binary bit number to said tag,means in said tag for encrypting said number in accordance with preselected encrypted keys,means in said tag for sending a first tag signal to said reader after said encryption,means in said reader responsive to the arrival of said first tag signal at said reader for generating a start signal for transmission to said tag,means in said tag responsive to the arrival of said start signal at said tag for sending a second tag signal to said reader at a time T following the arrival of said start signal at said tag, the time T being one of a number of different times dependent upon the value of the encrypted number,means in said reader for measuring the total length of time from the beginning of said start signal until the time said second tag signal reaches said reader, andmeans in said reader for subtracting the time T from said total time.

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