Passive intrusion detection system

• US 5,705,984 A
• Filed: 05/10/1996
• Issued: 01/06/1998
• Est. Priority Date: 05/10/1996
• Status: Expired due to Term

• Current Assignee: the united states of america as represented by the secretary of the navy
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Wilson, Douglas H.
• Primary Examiner(s): Hofsass, Jeffrey
• Assistant Examiner(s): Huang, Sihong
• Application Number:
  US08/649,862
• Time in Patent Office: 606 Days
• Field of Search: 340/561, 340/562, 340/552, 340/566
• US Class Current: 340/561
• CPC Class Codes: G08B 13/169 using cable transducer means
• CPC Class Codes: G08B 13/2497 using transmission lines, e.g. cable

Abstract

An intrusion detection system. An RF energy source transmits energy into aransmission cable that has a characteristic impedance subject to change by deformation of the cable. The cable is buried along a path that corresponds to a perimeter to be monitored. When an intruder places weight in the vicinity of the cable, the cable deforms and changes its characteristic impedance. Consequently a portion of energy transmitted into the cable reflects back where time-domain or frequency-domain reflectometry apparatus uses the reflected energy to identify the location of the intrusion along the cable and provide other characteristics of the intruder.

Claims

1. A system for detecting any intrusion past a perimeter of an area, said system comprising:

RF transmission cable means at the perimeter, said RF transmission cable means having a conductor structure consisting of first and second conductors and having a dielectric material separating said first and second conductors, said RF transmission cable means having a characteristic impedance that is normally constant throughout its length and that, at any point, can change in response to a physical condition that represents an intrusion;

RF generating means attached to a first end of said RF transmission cable means for energizing said cable means with RF energy, a portion of the RF energy being reflected from any point along the length of said RF transmission cable means at which the impedance differs from the normal characteristic impedance; and

means for monitoring the energy reflections thereby to produce an indication that an intrusion has occurred and the location of that intrusion.

2. An intrusion detection system as recited in claim 1:

wherein said RF transmission cable means terminates at a second end remote from said RF generating means in a corresponding termination impedance; and

wherein said monitoring means includes reflectometry means for determining the location of source of a reflection of the RF energy.

3. An intrusion detection system as recited in claim 2:

wherein said RF generating means produces periodic RF pulses for energizing said RF transmission cable means; and

wherein said reflectometry means comprises means for determining the position of each reflection by time-domain reflectometry.

4. An intrusion detection system as recited in claim 2:

wherein said RF generating means produces a variable RF frequency signal in an iterative fashion; and

wherein said reflectometry means includes means for determining the position of each reflection by frequency-domain reflectometry.

5. An intrusion detection system as recited in claim 2 wherein said monitoring means additionally comprises time display means connected to said reflectometry means for displaying representations of the reflected RF energy over an expanded time interval.

6. An intrusion detection system as recited in claim 2 wherein the area comprises earth at the perimeter and said RF transmission cable means comprises a compressible RF transmission cable buried at the perimeter whereby weight applied to the surface of the earth at the perimeter deforms said transmission cable thereby to alter the impedance at a corresponding location.

7. An intrusion detection system as recited in claim 2 additionally comprising discrete sensor means for monitoring a condition, said sensor means having the characteristic impedance and being attached to the second end of the RF transmission cable means and said monitoring means additionally includes means for monitoring signals from said sensor means.

8. An intrusion detection system as recited in claim 2 additionally comprising means for altering the characteristic impedance of said RF transmission cable means in response to an external event.

9. An intrusion detection system as recited in claim 2 additionally comprising RF energy control means for controlling the characteristics of the RF energy applied to said RF transmission cable means.

10. An intrusion detection system as recited in claim 1 wherein said RF transmission cable means includes a first RF transmission cable, at least one additional RF transmission cable and connector means for connecting each additional RF transmission cable in parallel with at least a portion of the first RF transmission cable without changing the characteristic impedance.

11. An intrusion detection system as recited in claim 1 wherein said RF transmission cable means comprises a single RF transmission cable with portions thereof being positioned in a serpentine pattern whereby at least one portion lies physically essentially parallel to another portion.

12. An intrusion detection system as recited in claim 11 having at least one additional RF transmission cable and connector means for connecting each additional RF transmission cable in parallel with at least a portion of the single RF transmission cable without changing the characteristic impedance.

13. An intrusion detection system for detecting any intrusion past a perimeter of an area, said system comprising:

an RF transmission cable having a conductor structure consisting of first and second conductors arranged symmetrically about a central axis and having a dielective material that spaces said first and second conductors thereby to form a cable with a characteristic impedance throughout its length that, at any point, can change in response to a change in the spacing of the conductors, said RF transmission cable being buried at the perimeter at a depth that enables the spacing change to occur in response to weight applied proximate the buried cable and having first and second ends;

an RF energy source for directing RF energy into the first end of the transmission cable whereby a portion of the electrical energy in a pulse is reflected from any point in said RF transmission cable that has an impedance that differs from the characteristic impedance; and

a reflectometer connected to the first end of the RF transmission cable for producing an indication that an intrusion has occurred and the location of that intrusion in response to reflected energy.

14. An intrusion detection system as recited in claim 13 wherein portions of said RF transmission cable extend in a serpentine fashion such that portions of the cable lie along essentially parallel paths.

15. An intrusion detection system as recited in claim 13 wherein said reflectometer uses time-domain reflectometry for determining the position of any intrusion along the cable.

16. An intrusion detection system as recited in claim 13 wherein said reflectometer uses frequency-domain reflectometry for determining the position of any intrusion along the cable.

17. An intrusion detection system as recited in claim 13 additionally comprising:

discrete sensor means for monitoring a condition, said sensor means having the characteristic impedance and being attached to the second end of the RF transmission cable; and

means for monitoring signals from said sensor means.

18. An intrusion detection system as recited in claim 13 wherein said transmission cable comprises coaxial cable.

19. An intrusion detection system as recited in claim 13 additionally comprising time display means connected to said reflectometer for displaying representations of the reflected energy over a time interval.

20. An intrusion detection system as recited in claim 13 additionally comprising means for altering the characteristic impedance of said RF transmission cable in response to an external event.

21. An intrusion detector system as recited in claim 13 additionally comprising an RF energy control unit for varying the characteristics of said RF energy source during the operation of said intrusion detection system.

22. An intrusion detection system as recited in claim 13 wherein said RF transmission cable extends around the area and includes at least one additional RF transmission cable connected electrically in parallel thereto.