Method of strip-map synthetic aperture radar auto-focus processing

• US 8,102,299 B2
• Filed: 10/26/2009
• Issued: 01/24/2012
• Est. Priority Date: 10/24/2008
• Status: Active Grant

• Current Assignee: Thales Holdings UK Plc (Thales SA)
• Original Assignee: Thales Holdings UK Plc (Thales SA)
• Inventors: Young, Stuart, Stevens, Malcom
• Primary Examiner(s): Brainard, Timothy A
• Application Number:
  US12/605,428
• Publication Number:
  US 20100103028A1
• Time in Patent Office: 820 Days
• Field of Search: 342/25
• US Class Current: 342/25.A
• CPC Class Codes: G01S 13/9019 Auto-focussing of the SAR signals
• CPC Class Codes: G01S 13/904 SAR modes
• CPC Class Codes: G01S 13/9054 Stripmap mode

Abstract

A strip-map Synthetic Aperture Radar (SAR) auto-focus image generation process is provided. Batches of raw radar return data are processed in order to form batch images which each have a valid region between invalid regions. The process determines an estimate of the first derivative of a phase error at an end of the valid region, determines a time-shift corresponding to that estimate and uses that information in determining a starting point for the next batch of raw radar return data.

Claims

1. A method of strip-map synthetic aperture radar auto-focus image generation, comprising the steps of:

forming, by an image processing circuit, one or more batch images from batches of radar return data, said radar return data and batches of radar return data having a chronological order, wherein at least one batch image has a valid region abutted to a valid region of another batch image and an overlap region overlapped with the valid region of another batch image, the valid region of the one or another batch image being the portion of the batch image that is used to form a final strip-map image;

for one or more batches of radar return data, performing the steps of;

determining, by an image analysis circuit, an estimated first derivative of a phase error in a batch image, substantially at an end of the valid region;

determining, by a time analysis circuit, a chronological shift corresponding to said first derivative at an end of the valid region; and

determining, by a triggering circuit, a start of a next batch of radar return data dependent on said chronological shift.

2. The method according to claim 1, wherein the step of determining a start of a next batch of radar return data comprises:

determining the start of the next batch of radar return data corresponding to an end of a previous batch of raw radar return data; and

adjusting said start of the next batch dependent on said chronological shift.

3. The method according to claim 2, further comprising determining the start of the next batch of data such that valid regions of connected batch images form a contiguous image.

4. The method according to claim 1, further comprising applying a Fourier transform to the batch image and determining a chronological shift corresponding to a linear rate of change in a Fourier transformed domain according to a time-shift process.

5. The method according to claim 1, further comprising subtracting the estimated first derivative of the phase error at the end of a valid region of a predetermined batch image from an estimated first derivative of phase errors for a subsequent batch image.

6. The method according to claim 1, further comprising subtracting a phase error at a start of a valid region of a first batch image from phase errors for a rest of said valid region.

7. The method according to claim 1, wherein the overlap regions correspond to invalid regions generated by a matched filter process.

8. The method according to claim 1, further comprising determining an estimated second derivative of the phase error and integrating said estimated second derivative to find said estimated first derivative.

9. The method according to claim 1, wherein said processing comprises auto-focus processing to provide an auto-focus batch image from a non-auto-focused batch image.

10. The method according to claim 9, further comprising comparing an auto-focused batch image with a corresponding non-auto-focused batch image, and rejecting said auto-focused batch image if said comparison reveals an improvement in the auto-focused batch image below a predetermined threshold.

11. The method according to claim 10, further comprising abutting a non-auto-focused batch image in place of the corresponding auto-focused batch image if said auto-focused batch images is rejected.

12. The method according to claim 9, further comprising applying multiple iterations of autofocus processing while accumulating said estimated first derivative.

13. The method according to claim 1, further comprising applying an integrity evaluation for the second derivative, said integrity evaluation based on energy in raw data contributing to said second derivative.

14. The method as claimed in claim 1, further comprising applying one or more acceptability tests to batches of raw radar return data prior to processing said batches.

15. The method according to claim 14, wherein the acceptability test comprises comparing a contrast value for pixels in the raw radar return data with a threshold.

16. The method according to claim 1 wherein the step of determining a start of a next batch of radar return data comprises determining the start of the next batch of radar return data to be the end of the overlap region of the previous batch of raw radar return data, minus a normal overlap equal to a length needed to ensure that the valid data is contiguous, plus the determined chronological shift.

17. A storage medium encoded with computer readable instructions, wherein said instructions when executed configure a computer to perform the steps of:

forming one or more batch images from batches of radar return data, said radar return data and batches of radar return data having a chronological order, wherein at least one batch image has a valid region abutted to a valid region of another batch image and an overlap region overlapped with an overlap region of another batch image;

for one or more batches of radar return data, performing the steps of;

determining an estimated first derivative of a phase error in a batch image, substantially at an end of the valid region;

determining a chronological shift corresponding to said first derivative at an end of the valid region; and

determining a start of a next batch of radar return data dependent on said chronological shift.

18. A radar apparatus to process batches of radar return data, said radar return data and batches of radar return data having a chronological order, the radar apparatus comprising:

an image processing circuit to form one or more batch images from the batches of radar return data, wherein at least one batch image has a valid region abutted to a valid region of another batch image and an overlap region overlapped with an overlap region of another batch image;

an image analysis circuit to determine an estimated first derivative of a phase error in a batch image, substantially at an end of the valid region;

a time analysis circuit to determine a chronological shift corresponding to said first derivative at an end of the valid region; and

a triggering circuit to determine a start of a next batch of radar return data dependent on said chronological shift.

19. A computer apparatus operable to perform strip-map synthetic aperture radar auto-focus image generation, comprising:

a processor coupled to a memory,

wherein the processor is programmed to generate an auto-focused strip-map synthetic aperture radar image from batches of radar return data, said radar return data and batches of radar return data having a chronological order, by performing the steps of;

forming one or more batch images from the batches of radar return data, wherein at least one batch image has a valid region abutted to a valid region of another batch image and an overlap region overlapped with an overlap region of another batch image;

for one or more batches of radar return data, performing the steps of;

determining an estimated first derivative of a phase error in a batch image, substantially at an end of the valid region;

determining a chronological shift corresponding to said first derivative at an end of the valid region; and

determining a start of a next batch of radar return data dependent on said chronological shift.