Application of time reversal to synthetic aperture imaging

• US 20090033549A1
• Filed: 07/09/2008
• Published: 02/05/2009
• Est. Priority Date: 07/09/2007
• Status: Active Grant

• Current Assignee: Carnegie Mellon University
• Original Assignee: Carnegie Mellon University
• Inventors: Jin, Yuanwei, Moura, Jose M. F.
• Granted Patent: US 7,928,896 B2
• Time in Patent Office: Days
• US Class Current: 342/25.F
• CPC Class Codes: G01S 13/9004 SAR image acquisition techniques
• CPC Class Codes: G01S 13/904 SAR modes

Abstract

A method and apparatus for target focusing and ghost image removal in synthetic aperture radar (SAR) is disclosed. Conventional SAR is not designed for imaging targets in a rich scattering environment. In this case, ghost images due to secondary reflections appear in the SAR images. We demonstrate, how, from a rough estimate of the target location obtained from a conventional SAR image and using time reversal, time reversal techniques can be applied to SAR to focus on the target with improved resolution, and reduce or remove ghost images.

Claims

1. A method of producing focused target data, comprising:

generating target data;

determining a time delay to the target from said target data;

transmitting a time reversed signal focused by said determined time delay; and

receiving a return in response to said transmitting.

2. The method of claim 1 wherein said generating comprises:

physically transmitting a signal from a first location and receiving a return at a second location a plurality of times, at least one time without the target being present and at least one time with the target being present, and wherein one of said first and second locations is variable and the other location fixed.

3. The method of claim 1 wherein said generating comprises:

physically transmitting a signal from a first location and receiving a return at a second location a plurality of times, at least one time without the target being present and at least one time with the target being present, and wherein said first and second locations are the same.

4. The method of claim 1 wherein said determining a time delay to the target comprises:

averaging data from a plurality of transmission positions to provide an estimated target location;

calculating a distance to the target using the estimated target location; and

calculating said time delay using said calculated distance to the target and the speed of light.

5. The method of claim 4 wherein an estimated target location is given by ({circumflex over (x)}_n,ŷ

_n), and wherein said distance to the target is given by;

${\hat{R}}_n<br><br>\left(u\right)=\sqrt{{\left(X_c+{\hat{x}}_n\right)}^2+{\left(Y_c+{\hat{y}}_n-u\right)}^2}+\sqrt{{\left(X_c+{\hat{x}}_n-X_B\right)}^2+{\left(Y_c+{\hat{y}}_n-Y_B\right)}^2}.$

where (X_B, Y_B) is the location of a fixed antenna, and (X_c, Y_c) is the center of the target area.

6. The method of claim 6 wherein said transmitting a time reversed signal comprises transmitting the signal kP(ω

)H*(ω

, u) either physically or mathematically, where k is an energy scaling factor, P(ω

) is a channel probing signal, and H*(ω

, u) represents a response of the channel at frequency ω and

aperture u.

7. An imaging method, comprising:

physically generating clutter data and combined target and clutter data;

using the clutter data to remove the clutter from the combined target and clutter data;

determining a time delay to said target from said target data;

transmitting a time reversed signal focused by using said estimated target location;

receiving a return in response to said transmitting; and

processing said return to produce an image.

8. The method of claim 7 wherein said physically generating comprises:

physically transmitting a signal from a first location and receiving a return at a second location a plurality of times, at least one time without the target being present to produce said clutter data and at least one time with the target being present to produce said combined target and clutter data, and wherein one of said first and second locations is variable and the other location fixed.

9. The method of claim 7 wherein said generating comprises:

physically transmitting a signal from a first location and receiving a return at a second location a plurality of times, at least one time without the target being present and at least one time with the target being present, and wherein said first and second locations are the same.

10. The method of claim 7 wherein said determining a time delay to the target comprises:

averaging data from a plurality of transmission positions to provide an estimated target location;

calculating a distance to the target using the estimated target location; and

calculating said time delay using said calculated distance to the target and the speed of light.

11. The method of claim 10 wherein an estimated target location is given by ({circumflex over (x)}_n,ŷ

_n), and wherein said distance to the target is given by;

${\hat{R}}_n<br><br>\left(u\right)=\sqrt{{\left(X_c+{\hat{x}}_n\right)}^2+{\left(Y_c+{\hat{y}}_n-u\right)}^2}+\sqrt{{\left(X_c+{\hat{x}}_n-X_B\right)}^2+{\left(Y_c+{\hat{y}}_n-Y_B\right)}^2}.$

where (X_B, Y_B) is the location of a fixed antenna, and (X_c, Y_c) is the center of the target area.

12. The method of claim 7 wherein said transmitting a time reversed signal comprises transmitting the signal kP(ω

)H*(ω

, u) either physically or mathematically, where k is an energy scaling factor, P(ω

) is a channel probing signal, and H*(ω

, u) represents a response of the channel at frequency ω and

aperture u.

13. A method of reducing ghost images from synthetic aperture images, comprising:

dividing the aperture of a synthetic aperture radar device into a plurality of sub-apertures;

collecting a signal from each of said sub-apertures;

producing sub-images from each signal from each sub-aperture; and

averaging the sub-images thereby reducing ghost images.

14. A system comprised of an antenna array and signal processing hardware configured to perform a method of producing focused target data, comprising:

generating target data;

determining a time delay to the target from said target data;

transmitting a time reversed signal focused by said determined time delay; and

receiving a return in response to said transmitting.

15. A system comprised of an antenna array and signal processing hardware configured to perform an imaging method, comprising:

physically generating clutter data and combined target and clutter data;

using the clutter data to remove the clutter from the combined target and clutter data;

determining a time delay to said target from said target data;

transmitting a time reversed signal focused by using said estimated target location;

receiving a return in response to said transmitting; and

processing said return to produce an image.

16. A system comprised of an antenna array and signal processing hardware configured to perform a method of reducing ghost images from synthetic aperture images, comprising:

dividing the aperture of a synthetic aperture radar device into a plurality of sub-apertures;

collecting a signal from each of said sub-apertures;

producing sub-images from each signal from each sub-aperture; and

averaging the sub-images thereby reducing ghost images.