Through-the-wall frequency stepped imaging system utilizing near field multiple antenna positions, clutter rejection and corrections for frequency dependent wall effects

• US 7,307,575 B2
• Filed: 06/20/2005
• Issued: 12/11/2007
• Est. Priority Date: 09/14/2004
• Status: Expired due to Fees

• Current Assignee: BAE Systems Information and Electronic Systems Integration Incorporated (BAE Systems Plc)
• Original Assignee: BAE Systems Information and Electronic Systems Integration Incorporated (BAE Systems Plc)
• Inventors: Zemany, Paul D.
• Primary Examiner(s): Tarcza, Thomas H.
• Assistant Examiner(s): Chen, Shelley
• Application Number:
  US11/156,948
• Publication Number:
  US 20060284757A1
• Time in Patent Office: 904 Days
• Field of Search: 342159-160, 342/162
• US Class Current: 342/22
• CPC Class Codes: G01S 13/32 using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated
• CPC Class Codes: G01S 13/56 for presence detection presence detection using near field arrangements G01V3/00, e.g. G01V3/08, G01V3/12; burglar, theft or intruder alarms with electrical actuation G08B13/22 - G08B13/26
• CPC Class Codes: G01S 13/87 Combinations of radar systems, e.g. primary radar and secondary radar
• CPC Class Codes: G01S 13/888 through wall detection
• CPC Class Codes: G01S 13/89 for mapping or imaging

Abstract

Lower resolution and clutter-prone two-tone CW radars can have the displayed images dramatically improved by three techniques involved in the subject invention. The three techniques involved are the stepping of each of the multiple radars for readings at multiple frequencies, weighting the results to compensate for wall-induced distortions and differential image processing. In one embodiment, weights for each frequency counteract the distortion produced by particular wall. For differential image processing, temporal snapshots of the images are subtracted one from the other such that the result is only due to moving objects, thus to provide a dramatic display of the presence and position of moving individuals behind a wall.

Claims

1. A method for improving the resolution and clutter resistance of two-tone CW through-the-wall radars employing signals at two frequencies, the radars detecting and plotting the position of moving objects behind a wall, comprising the steps of:

driving the through-the-wall radars at a multiplicity of frequencies in a stepped fashion to obtain multiple radar readings at the multiple frequencies, said frequencies being separated by amounts to ensure detection of movement;

detecting the phase difference between transmitted energy and returns to obtain phase differences; and

,

utilizing wall-related weighting and averaging of the phase differences for improving range accuracy, the wall-related weighting weighting the results at each frequency to compensate for wall-induced distortions, the weighting being derived from the wall material characteristics and refractive index thereof, the wall material characteristics and the refractive index thereof being determined empirically.

2. The method of claim 1, and further including the steps of:

generating an image of the returns for each radar at a first time and at a second time; and

,

subtracting one image from the other such that what remains is a result of objects moving behind the wall, with images due to stationary objects having been canceled, thus to provide clutter rejection.

3. A method for improving the resolution and clutter resistance of two-tone CW through-the-wall radars employing signals at two frequencies, the radars detecting and plotting the position of moving objects behind a wall, comprising the steps of:

driving the through-the-wall radars at a multiplicity of frequencies in a stepped fashion to obtain multiple radar readings at the multiple frequencies, said frequencies being separated by amounts to ensure detection of movement;

detecting the phase difference between transmitted energy and returns to obtain phase differences; and

, utilizing wall-related weighting and averaging of the chase differences for improving range accuracy, the wall-related weighting weighting the results at each frequency to compensate for wall-induced distortions, the weighting being derived from the wall material characteristics and refractive index thereof, the wall material characteristics and the refractive index thereof being estimated.

4. A method for improving the resolution and clutter resistance of two-tone CW through-the-wall radars employing signals at two frequencies, the radars detecting and plotting the position of moving objects behind a wall, comprising the steps of:

driving the through-the-wall radars at a multiplicity of frequencies in a stepped fashion to obtain multiple radar readings at the multiple frequencies, said frequencies being separated by amounts to ensure detection of movement;

detecting the phase difference between transmitted energy and returns to obtain phase differences; and

,

utilizing wall-related weighting and averaging of the phase differences for improving range accuracy, the wall-related weighting weighting the results at each frequency to compensate for wall-induced distortions, the weighting being derived from the wall material characteristics and refractive index thereof, the wall material characteristics and refractive index being derived from an auto-focus process, with the weights being determined when an object is in focus according to the auto-focus process.

5. The method of claim 4, wherein the achievement of the focus in the auto-focus process specifies the coefficient of refraction of the wall.

6. A method for improving the resolution and clutter rejection with respect to an image formed by the use of multiple multi-tone CW radars operating at multiple frequencies and positioned at different points about a building for the purpose of detecting the presence and location of a moving individual behind a building wall using through-the-wall multi-tone CW radars, comprising the steps of:

projecting multi-frequency energy from CW radars at different locations relative to the building;

generating for each radar an image using through-wall weighting and phase difference processing, the image depicting the position of a moving individual within the building at first and second times, the wall-related weighting weighting the results at each frequency to compensate for wall-induced distortions, the weighting being derived from the wall material characteristics and refractive index thereof, the wall material characteristics and the refractive index thereof being determined by one of empirical analysis, estimation and auto-focus; and

,

for each radar, subtracting one image from the other to remove the effect of returns from stationary objects within the building, thus to de-clutter a display of position of the moving individual.

7. The method of claim 6, and further including the step of frequency stepping each of the multi-tone radars.

8. The method of claim 7, and further including the step of averaging the results from projecting multiple frequencies through the wall.

9. The method of claim 7, and further including the step for each radar of applying a through-wall weight to returns at each of the frequencies, the weights being such as to compensate for wall-induced distortions.

10. A method for improving the resolution and clutter rejection with respect to an image formed by the use of multiple multi-tone CW radars operating at multiple frequencies separated by amounts to ensure detection of movement and positioned at different points about a building for the purpose of detecting the presence and location of a moving individual behind building walls using through-the-wall multi-frequency CW radars, comprising the steps of:

rendering from the outputs of all of said multi-tone radars a set of images derived at different times from returns from objects in the building in terms of the phase difference between transmitted energy and returns, weighting the phase difference results using through-wall weights and subtracting one image from another to eliminate representations on the generated image that are the result of returns from objects in the building that are not moving, such that what is depicted by the display is only due to a moving object, whereby the location of moving individuals within the building can be robustly determined, the wall-related weighting weighting the results at each frequency to compensate for wall-induced distortions, the weighting being derived from the wall material characteristics and refractive index thereof, the wall material characteristics and the refractive index thereof being determined by one of empirical analysis, estimation and auto-focus.

11. The method of claim 10, and further including the steps of frequency-stepping each of the multi-frequency radars and averaging the results such that the resolution of the range to a moving object is improved by averaging the results from projecting multiple frequencies through the wall and into the building.

12. The method of claim 11, and further including the step for each radar of applying a weight to returns at each of the frequencies, the weight being such as to compensate for wall-induced distortions, whereby resolution associated is increased due to the effective sharpening of returns from objects within the building due to compensation produced by the weighting.

13. A system for reliably determining the presence and location of a moving individual behind a wall, comprising:

a number of multi-tone CW through-the-wall radars employing signals at multiple frequencies separated by amounts to ensure detection of movement and located at different positions relative to the wall, said CW radars being driven at multiple frequencies and including processing for weighting the returns with through-wall weights, taking the phase difference between transmitted energy and returns at the different frequencies and averaging the results of the multiple frequencies returns to improve the range resolution of the radar; and

,

an auto-focusing system for determining when a moving object is in focus, said auto-focusing system providing an output used for determining said weights.

14. The system of claim 13, wherein the through-wall weighting includes weighting the returns at each of said frequencies to compensate for wall-induced distortions, whereby the sharpness of the radar returns is increased by removing distortions caused by said wall.

15. The system of claim 13, wherein said output of said auto-focusing system includes the index of refraction of said wall used to determine said weights.

16. The system of claim 13, and further including a visual image generator coupled to said radar for generating sequential images from returns from objects in said building, a differential image processor for subtracting one sequential image from the other and for outputting results only due to objects that have moved between the times associated with the different sequential images.

17. A method for improving the resolution and clutter resistance of two-tone CW through-the-wall radars that detect and plot the position of moving objects behind a wall, comprising the steps of:

driving the through-the-wall radars at a multiplicity of two-tone frequencies in a stepped fashion to obtain multiple radar readings at the multiple frequencies;

averaging the radar return results for improving range accuracy; and

,

weighting the results at each frequency to compensate for wall-induced distortions, the weighting being derived from the wall material characteristics and refractive index thereof, the wall material characteristics and the refractive index thereof being determined empirically.

18. A method for improving the resolution and clutter resistance of two-tone CW through-the-wall radars that detect and plot the position of moving objects behind a wall, comprising the steps of:

driving the through-the-wall radars at a multiplicity of two-tone frequencies in a stepped fashion to obtain multiple radar readings at the multiple frequencies;

averaging the radar return results for improving range accuracy; and

,

weighting the results at each frequency to compensate for wall-induced distortions, the weighting being derived from the wall material characteristics and refractive index thereof, the wall material characteristics and the refractive index thereof being estimated.

19. A method for improving the resolution and clutter resistance of two-tone CW through-the-wall radars that detect and plot the position of moving objects behind a wall, comprising the steps of:

driving the through-the-wall radars at a multiplicity of two-tone frequencies in a stepped fashion to obtain multiple radar readings at the multiple frequencies;

averaging the radar return results for improving range accuracy; and

,

weighting the results at each frequency to compensate for wall-induced distortions, the weighting being derived from the wall material characteristics and refractive index thereof, the wall material characteristics and refractive index being derived from an auto-focus process, with the weights being determined when an object is in focus according to the auto-focus process.

20. The method of claim 19, wherein the achievement of the focus in the auto-focus process specifies the coefficient of refraction of the wall.

21. A system for reliably determining the presence and location of a moving individual behind a wall, comprising:

a number of multi-tone CW through-the-wall radars located at different positions relative to the wall, said CW radars being driven at multiple frequencies and including processing for averaging the returns at the multiple frequencies to improve the range resolution of the radar; and

,

an auto-focusing system for determining when a moving object is in focus, said auto-focusing system providing an output used for determining said weights.

22. The system of claim 21, wherein said output of said auto-focusing system includes the index of refraction used to determine said weights.