Communications: directive radio wave systems and devices (e.g. – Determining distance – With remote cooperating station
Patent
1991-07-08
1992-06-30
Sotomayor, John B.
Communications: directive radio wave systems and devices (e.g.,
Determining distance
With remote cooperating station
342 44, 342 47, 342 45, G01S 1310, G01S 1380
Patent
active
051267462
ABSTRACT:
A system for secure distance ranging between a reader 11 and a tag 12 wherein the distance between the two is determined by the time it takes to propagate a signal from the reader to the tag and for a responsive signal to return, and in which such time is random and unpredictable, except to the reader, even though the distance between the reader and tag remains the same. A random number (19) is sent from the reader and encrypted (26) by the tag into a number having 16 segments of 4 bits each (28). A first tag signal (31) is sent after such encryption. In response, a random width start pulse (13) is generated by the reader. When received in the tag, the width of the start pulse is measured (41) in the tag and a segment of the encrypted number is selected (42) in accordance with such width. A second tag pulse is generated at a time T after the start pulse arrives at the tag, the time T being dependent on the length of a variable time delay t.sub.v which is determined by the value of the bits in the selected segment of the encrypted number. At the reader, the total time from the beginning of the start pulse to the receipt of the second tag signal is measured (36, 37). The value of t.sub.v (21, 22, 23, 34) is known at the reader and the time T is subtracted (46) from the total time to find the actual propagation t.sub.p for signals to travel between the reader 11 and tag 12. The propagation time is then converted into distance (46).
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Gaither Roger S.
Moser William R.
Sotomayor John B.
The United States of America as represented by the United States
Valdes Miguel A.
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