Decreasing range resolution of a SAR image to permit correction of motion measurement errors beyond the SAR range resolution

• US 7,760,128 B1
• Filed: 05/14/2008
• Issued: 07/20/2010
• Est. Priority Date: 03/25/2005
• Status: Active Grant

• Current Assignee: National Technology & Engineering Solutions Of Sandia LLC (Honeywell International Inc.)
• Original Assignee: Sandia Corporation (Martin Marietta Corporation)
• Inventors: Doerry, Armin W., Cordaro, J. Thomas, Heard, Freddie E.
• Primary Examiner(s): Tarcza; Thomas H
• Assistant Examiner(s): Barker; Matthew M
• Application Number:
  US12/120,265
• Time in Patent Office: 797 Days
• Field of Search: 342/25.F
• US Class Current: 342/25.00F
• CPC Class Codes: G01S 13/9019 Auto-focussing of the SAR signals
• CPC Class Codes: G01S 13/904 SAR modes
• CPC Class Codes: G01S 7/295 Means for transforming co-ordinates or for evaluating data, e.g. using computers

Abstract

Motion measurement errors that extend beyond the range resolution of a synthetic aperture radar (SAR) can be corrected by effectively decreasing the range resolution of the SAR in order to permit measurement of the error. Range profiles can be compared across the slow-time dimension of the input data in order to estimate the error. Once the error has been determined, appropriate frequency and phase correction can be applied to the uncompressed input data, after which range and azimuth compression can be performed to produce a desired SAR image.

Claims

1. A method of producing a SAR image, comprising:

providing input data;

forming a SAR image based on the input data;

decreasing a range resolution of the SAR image to produce a further SAR image;

applying a focusing algorithm to the further SAR image to obtain information indicative of a motion measurement error associated with collection of the input data;

using the motion measurement error information to modify the input data; and

forming a still further SAR image based on the modified input data.

2. The method of claim 1, wherein said decreasing includes performing a filtering operation on the SAR image.

3. The method of claim of claim 2, wherein the filtering operation blurs the SAR image with respect to a range dimension of the SAR image.

4. The method of claim 2, wherein the motion measurment error information includes a phase error.

5. The method of claim 4, wherein the motion measurement error information includes a phase and frequency correction calculated based on the phase error, and wherein said using includes applying the phase and frequency correction to the input data.

6. The method of claim 1, wherein the focusing algorithm includes a Phase Gradient Autofocus algorithm.

7. The method of claim 1, wherein the motion measurment error information includes a phase error.

8. The method of claim 7, wherein the motion measurement error information includes a phase and frequency correction calculated based on the phase error, and wherein said using includes applying the phase and frequency correction to the input data.

9. The method of claim 1, including applying a focusing algorithm to said still further SAR image.

10. The method of claim 1, wherein said input data includes resampled phase history data.

16. The apparatus of claim 1, wherein the focusing algorithm includes a Phase Gradient Autofocus algorithm.

11. An apparatus for producing a SAR image, comprising:

an input for receiving input data;

a SAR image former coupled to said input for forming a SAR image based on the input data and for decreasing a range resolution of the SAR image to produce a further SAR image;

an error extractor coupled to said SAR image former for applying a focusing algorithm to the further SAR image to obtain information indicative of a motion measurement error associated with collection of the input data;

a corrector coupled to said input and said error extractor for modifying the input data based on the motion measurement information; and

a SAR image former coupled to said corrector for forming a still further SAR image based on the modified input data.

12. The apparatus of claim 11, wherein said first-mentioned SAR image former performs a filtering operation on the SAR image to decrease the range resolution of the SAR image.

13. The apparatus of claim 12, wherein the filtering operation blurs the SAR image with respect to a range dimension of the SAR image.

14. The apparatus of claim 12, wherein the motion measurment error information includes a phase error.

15. The apparatus of claim 14, wherein the motion measurement information includes a phase and frequency correction calculated based on the phase error, and wherein said corrector applies the phase and frequency correction to the input data.

17. The apparatus of claim 11, wherein the motion measurment error information includes a phase error.

18. The apparatus of claim 17, wherein the motion measurement information includes a phase and frequency correction calculated based on the phase error, and wherein said corrector applies the phase and frequency correction to the input data.

19. The apparatus of claim 11, including a focusing apparatus coupled to said last-mentioned SAR image former for applying a focusing algorithm to the still further SAR image.

20. The apparatus of claim 12, wherein said input data includes resampled phase history data.