Method for locating a concealed object

• US 6,501,414 B2
• Filed: 04/02/2001
• Issued: 12/31/2002
• Est. Priority Date: 04/02/2001
• Status: Expired due to Fees

• Current Assignee: US Government As Represented By The Director National Security Agency
• Original Assignee: United States Administrator of National Aeronautics and Space Administration
• Inventors: Stolarczyk, Larry G., Byerly, Kent A., Arndt, G. Dickey, Ngo, Phong H., Carl, James R.
• Primary Examiner(s): Tarcza, Thomas H.
• Assistant Examiner(s): Andrea, Brian K
• Application Number:
  US09/826,402
• Publication Number:
  US 20020175849A1
• Time in Patent Office: 638 Days
• Field of Search: 342/22, 342/27, 342/194, 342/191, 342/196
• US Class Current: 342/22
• CPC Class Codes: F41H 11/12 Means for clearing land minefields; Systems specially adapted for detection of landmines
• CPC Class Codes: G01S 13/0209 Systems with very large relative bandwidth, i.e. larger than 10 %, e.g. baseband, pulse, carrier-free, ultrawideband
• CPC Class Codes: G01S 13/885 for ground probing prospecting or detecting using electromagnetic waves G01V3/12
• CPC Class Codes: G01V 3/12 operating with electromagnetic waves operating with millimetre waves G01V8/005

Abstract

Apparatus and methods are disclosed for detecting anomalies in microwave penetrable material that may be used for locating plastic mines or pipes underneath the ground. A transmitter is positioned at a plurality of different positions above the ground. A microwave signal is transmitted that is stepped over a plurality of frequencies. At each position, a plurality of reflections are received corresponding to each of the plurality of frequencies that were transmitted. A complex target vector may be produced at each position that contains complex values corresponding to magnitude, phase, and time delay for each of the plurality of reflections received at that location. A complex reference data vector may be produced, either based on predetermined values or based on data from the received plurality of reflections. A comparison is made between the complex target vector and the complex reference data vector to produce a channel vector. In one embodiment, an operator may be applied to the channel vector such as a complex filter matrix or to add a complex conjugate. A response signal is produced and anomalies are detected by variations in the response signal with respect to the plurality of positions.

Claims

1. A method of detecting anomalies in a microwave penetrable material, comprising:

transmitting a microwave signal that is stepped over a plurality of frequencies, said microwave signal being transmitted at each of a plurality of positions in said microwave penetrable material;

receiving a plurality of reflections for said plurality of frequencies at each of said plurality of positions, each of said plurality of reflections having a magnitude and a phase and a time delay;

producing a complex target data vector from said plurality of reflections at each of said plurality of positions;

producing a complex reference data vector which is representative of an anticipated complex target data vector of said microwave penetrable material when said anomalies to be detected are not present within said microwave penetrable material;

comparing said complex target data vector with said complex reference data vector for producing a response signal at each of said plurality of positions; and

detecting said anomalies in said microwave penetrable material from changes in said response signal for each of said plurality of positions.

2. The method of claim 1, further comprising:

utilizing accelerometers to determine said plurality of positions in said microwave penetrable material.

3. The method of claim 1, further comprising:

comparing said complex reference data vector with respect to each said complex target data vector to produce a complex channel vector for said plurality of frequencies at each of said plurality of positions.

4. The method of claim 3, further comprising:

producing a complex filter matrix, said complex filter matrix being a least squares operator, and operating on said complex channel vector with said complex filter matrix to obtain said response signal for each of said plurality of positions.

5. The method of claim 1, wherein each of said plurality of reflections have a complex impedance with a real part and an imaginary part.

6. The method of claim 5, further comprising:

producing a complex impedance vector from said real part and said imaginary part for each of said plurality of reflections at each of said plurality of positions;

7. The method of claim 1, further comprising:

comparing said complex reference data vector with respect to each said complex target data vector to produce a complex channel vector for said plurality of frequencies at each of said plurality of positions;

adding a complex conjugate of said complex channel vector to said complex channel vector for each of said plurality of frequencies to form a conjugate symmetric complex channel vector; and

taking an inverse Fourier transform of said conjugate symmetric complex channel vector to produce an impulse function to thereby indicate said anomalies in said microwave penetrable material.

8. The method of claim 1, wherein said step of producing a complex reference data vector for each of said plurality of positions further comprises:

providing a theoretical complex reference data vector.

21. The method of claim 1, further comprising:

producing a display for said response signal.

9. A method of detecting anomalies in a microwave penetrable material, comprising the steps of:

transmitting a microwave signal that is stepped over a plurality of frequencies, said microwave signal being transmitted at each of a plurality of positions in said microwave penetrable material;

receiving a plurality of reflections for said plurality of frequencies at each of said plurality of positions, each of said plurality of reflections having a magnitude and a phase and a time delay;

producing a complex target data vector for each of said plurality of positions;

producing a complex reference data vector which is representative of said complex target data vector which is anticipated when said anomalies to be detected are not present within said microwave penetrable material;

comparing said complex reference data vector with respect to each said complex target data vector to produce a complex channel vector for said plurality of frequencies at each of said plurality of positions;

producing a complex filter matrix, said complex filter matrix being a least squares operator;

operating on said complex channel vector with said complex filter matrix to obtain a response signal for each of said plurality of positions; and

detecting said anomalies in said microwave penetrable material from changes in said response signal for each of said plurality of positions.

10. The method of claim 9, further comprising producing said complex filter matrix of values for said plurality of frequencies and said time delays.

11. The method of claim 9, further comprising:

utilizing said magnitude and phase and time delay for each of said plurality of reflections to produce a respective said complex target data vector for each of said plurality of positions.

12. The method of claim 9, wherein said step of operating includes multiplying said complex filter matrix times said complex channel vector and dividing by the number of said plurality of frequencies to obtain a response signal for each of said plurality of positions.

13. The method of claim 9, wherein said complex filter matrix is constant for each of said plurality of positions.

14. The method of claim 9, further comprising:

utilizing said magnitude and phase and time delay for one or more of said plurality of reference reflections at one or more of said plurality of positions to produce said complex reference data vector.

15. The method of claim 9, further comprising:

utilizing accelerometers to determine said plurality of positions in said microwave penetrable material.

16. The method of claim 9, further comprising:

weighting values within a first said complex channel vector for a first of said plurality of positions with respect to corresponding values within a second said complex channel vector for a second of said plurality of positions.

17. The method of claim 16, wherein said corresponding values correspond to respective of said plurality of frequencies.

18. The method of claim 9, further comprising:

using a metal detector to distinguish metal objects buried within said microwave penetrable material.

19. The method of claim 9, wherein:

said complex reference data vector is constant for each of said plurality of positions.

20. The method of claim 9, wherein:

said complex reference data vector varies for at least two of said plurality of positions.

22. A method of detecting anomalies in microwave penetrable material, comprising:

transmitting a microwave signal that is stepped over a plurality of frequencies, said microwave signal being transmitted at each of a plurality of positions in said microwave penetrable material;

receiving a plurality of reflections for said plurality of frequencies at each of said plurality of positions, each of said plurality of reflections having a complex impedance with a real part and an imaginary part;

producing a complex impedance vector from said real part and said imaginary part of said plurality of reflections at each of said plurality of positions;

producing a complex expectation vector such that said complex expectation vector is representative of said complex impedance vector when said anomalies to be detected are not present in said microwave penetrable material;

comparing said complex expectation vector with respect to said complex impedance vector at each of said plurality of frequencies to obtain a signal for each of said plurality of positions; and

detecting said anomalies in said microwave penetrable material from changes in values of said signal at each of said plurality of positions.

23. The method of claim 22, further comprising:

producing said complex expectation vector at each frequency from said real part and said imaginary part for each of said plurality of reflections, a respective said complex expectation vector being produced for each of said plurality of positions.

24. The method of claim 22, wherein said step of comparing further comprises subtracting said complex expectation vector with respect to said complex impedance vector to obtain a plurality of resulting values and taking a summation of said resulting values.

25. The method of claim 22, further comprising:

windowing said complex impedance vector by applying a weighting function to a first complex impedance vector at a first position with respect to a second complex impedance vector at a second position for respective of said plurality of frequencies.

26. The method of claim 22, further comprising:

utilizing accelerometers to determine said plurality of positions with respect to said microwave penetrable material.

27. The method of claim 22, further comprising:

producing a display for said signal.

28. A method of detecting anomalies in a microwave penetrable material, comprising the steps of:

transmitting a microwave signal that is stepped over a plurality of frequencies, said microwave signal being transmitted at each of a plurality of positions in said microwave penetrable material;

receiving a plurality of reflections for said plurality of frequencies at each of said plurality of positions, each of said plurality of reflections having a magnitude and a phase and a time delay;

utilizing said plurality of reflections to produce a complex target data vector for each of said plurality of positions;

producing a complex reference data vector such that said complex reference data vector is representative of said complex target data vector when said anomalies to be detected are not present in said microwave penetrable material;

comparing said complex reference data vector with respect to each said complex target data vector to produce a complex channel vector with respect to said plurality of frequencies for each of said plurality of positions;

adding a complex conjugate of said complex channel vector to said complex channel vector for each of said plurality of frequencies to form a conjugate symmetric complex channel vector;

taking an inverse Fourier transform of said conjugate symmetric complex channel vector to produce an impulse function to indicate said anomalies in said microwave penetrable material.

29. The method of claim 28, further comprising:

receiving a plurality of reference reflections for said plurality of frequencies at one or more of said plurality of positions, each of said plurality of reference reflections having a magnitude and a phase and a time delay, and

utilizing said magnitude and phase and time delay for each of said plurality of reference reflections at said one or more of said plurality of positions to produce said complex reference data vector.

30. The method of claim 28, further comprising:

selecting said one or more of said plurality of positions for said producing of said complex reference data, said selecting being continually updated such that said one or more of said plurality of positions are selected within a predetermined distance from a moving transmitter.

31. The method of claim 28, further comprising:

utilizing accelerometers to determine said plurality of positions in said microwave penetrable material.

32. The method of claim 28, further comprising:

weighting values within a first said complex channel vector for a first of said plurality of positions with respect to corresponding values within a second said complex channel vector for a second of said plurality of positions.

33. The method of claim 32, wherein said corresponding values correspond to respective of said plurality of frequencies.

34. The method of claim 32, further comprising:

using a metal detector to distinguish metal objects buried within said microwave penetrable material.

35. The method of claim 28, wherein:

said complex reference data vector is constant for each of said plurality of positions.

36. The method of claim 28, wherein:

said complex reference data vector varies for at least two of said plurality of positions.

37. A method of detecting anomalies in a microwave penetrable material, comprising the steps of:

transmitting a microwave signal that is stepped over a plurality of frequencies, said microwave signal being transmitted at each of a plurality of positions in said microwave penetrable material;

receiving a plurality of reflections for said plurality of frequencies at each of said plurality of positions, each of said plurality of reflections having a magnitude and a phase and a time delay;

utilizing said plurality of reflections to produce a complex target data vector for each of said plurality of positions;

providing a theoretical reference signal such that said theoretical reference data vector is representative of said complex target data vector anticipated when said anomalies to be detected are not present in said microwave penetrable material;

comparing said theoretical reference signal to said complex target data vector for each of said plurality of frequencies at each of said plurality of positions to produce an error signal; and

detecting said anomalies in said microwave penetrable material from changes in values of said error signal at each of said plurality of positions.

38. The method of claim 37, further comprising:

determining said theoretical reference signal to be equal to a first quantity of a frequency dependent soil frequency minus an impedance of air divided by a second quantity of said frequency dependent soil frequency plus said impedance of air.

39. The method of claim 37, further comprising:

utilizing accelerometers to determine said plurality of positions in said microwave penetrable material.

40. The method of claim 37, further comprising:

using a metal detector to distinguish metal objects buried within said microwave penetrable material.

41. A method of detecting anomalies in microwave penetrable material, comprising:

transmitting a microwave signal that is stepped over a plurality of frequencies, said microwave signal being transmitted at each of a plurality of positions in said microwave penetrable material;

receiving a plurality of reflections for said plurality of frequencies at each of said plurality of positions,

determining a complex reference signal such that said complex reference signal is representative of said complex antenna impedance when said anomalies to be detected are not present in said microwave penetrable material;

determining a complex antenna impedance for each of said plurality of reflections at each of said plurality of positions;

comparing said complex reference signal with said complex antenna impedance for each of said plurality of reflections at each of said plurality of positions to produce a complex difference signal for each of said plurality of reflections at each of said plurality of positions;

amplifying said complex difference signal for respective of said plurality of reflections at each of said plurality of positions to produce an amplified complex difference signal;

taking a Fourier transform of said complex difference signal to thereby produce a response signal; and

detecting said anomalies in said microwave penetrable material from changes in said response signal with respect to each of said plurality of positions.

42. The method of claim 41, wherein said step of determining said complex reference signal further comprises determining an average complex antenna impedance for at least two of said plurality of reflections.

43. The method of claim 41, wherein said step of determining said complex reference signal further comprises determining a theoretical value for said complex reference signal.

44. The method of claim 41, further comprising:

utilizing accelerometers to determine said plurality of positions with respect to said microwave penetrable material.

45. The method of claim 41, further comprising:

mitigating sensor height variations by normalizing said complex channel vector with said complex reference data vector.

46. The method of claim 41, further comprising:

weighting values within a first said complex channel vector for a first of said plurality of positions with respect to corresponding values within a second said complex channel vector for a second of said plurality of positions.

47. The method of claim 46, wherein said corresponding values correspond to respective of said plurality of frequencies.

48. The method of claim 41, further comprising:

using a metal detector to distinguish metal objects buried within said microwave penetrable material.

49. The method of claim 41, wherein:

said complex reference data vector is constant for each of said plurality of positions.

50. The method of claim 41, wherein:

said complex reference data vector varies for at least two of said plurality of positions.

51. The method of claim 41, further comprising:

producing a display for said response signal.

52. A method of detecting anomalies in microwave penetrable material, comprising:

transmitting a microwave signal that is stepped over a plurality of frequencies, said microwave signal being transmitted at each of a plurality of positions into said microwave penetrable material;

utilizing one or more accelerometers to measure location information for each of said plurality of positions;

receiving a plurality of reflections for said plurality of frequencies at each of said plurality of positions;

producing a complex target data vector at each of said plurality of positions;

producing a complex reference data vector for each of said plurality of positions such that said complex reference data vector is representative of said complex target data vector when said anomalies to be detected are not present in said microwave penetrable material;

comparing said complex target data vector with said complex reference data vector for producing a response signal at each of said plurality of positions; and

detecting said anomalies in said microwave penetrable material from changes in said response signal for each of said plurality of positions.