Compensation of flight path deviation for spotlight SAR

• US 7,277,042 B1
• Filed: 05/12/2006
• Issued: 10/02/2007
• Est. Priority Date: 05/12/2006
• Status: Active Grant

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Raytheon Company (Rtx Corporation)
• Inventors: Cho, Kwang M., Hui, Leo H.
• Primary Examiner(s): Gregory; Bernarr E.
• Application Number:
  US11/433,707
• Time in Patent Office: 508 Days
• Field of Search: 342 25 R- 25 F, 342 82-103, 342/175, 342/176, 342/179, 342190-197, 342165-174
• US Class Current: 342/25.00F
• CPC Class Codes: G01S 13/9019 Auto-focussing of the SAR signals
• CPC Class Codes: G01S 13/9052 Spotlight mode
• CPC Class Codes: G01S 7/00 Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
• CPC Class Codes: G01S 7/40 Means for monitoring or calibrating

Abstract

A radar acquires a formed SAR image of radar scatterers in an area around a central reference point (CRP). Target(s) are within the area illuminated by the radar. The area covers terrain having a plurality of elevations. The radar is on a moving platform, where the moving platform is moving along an actual path. The actual path is displaced from an ideal SAR image acquisition path. The radar has a computer that divides the digital returns descriptive of the formed SAR image into multiple blocks, such as a first strip and an adjacent strip. The first strip is conveniently chosen, likely to generally align with a part of the area, at a first elevation. An adjacent strip covers a second part of the area at a second elevation. The first strip is overlapping the adjacent strip over an overlap portion. The first and second elevation are extracted from a terrain elevation database (DTED). Horizontal displacement of returns (range deviation) is computed for each strip using the elevation information from the terrain elevation database. Taylor series coefficients are computed for the horizontal displacement due to terrain elevation using the ideal path, the actual path and central reference point. Actual flight path deviation is available at each pulse position while azimuth frequency is given in azimuth angle off mid angle point. Remapping between indices in two arrays is also computed. Phase error compensation and compensation in azimuth (spacial frequency) is computed using the Taylor series coefficients, a Fast Fourier Transform and an inverse Fast Fourier Transform for each strip. Phase error compensation is applied to the digital returns from each strip to obtain the SAR image. The SAR image is further improved by having the first strip corrected data and the second strip corrected data merged over the overlap portion to generate a relatively seamless SAR image.

Claims

1. A radar for acquiring a SAR image of radar scatterers in an area around a central reference point, said area having a first elevation and a second elevation, said radar on a moving platform, said moving platform moving along an actual path, said actual path displaced from an ideal path, said radar comprising:

an analog to digital converter for converting radar returns from said area to digital returns;

a navigation system for motion compensating said digital returns with respect to said central reference point and said actual path;

a computer for;

dividing said digital returns descriptive of said SAR image into a first strip and an adjacent strip, said first strip covering a first part of said area, said first part having said first elevation, said adjacent strip covering second part of said area said second area having said second elevation, said first strip overlapping said adjacent strip over an overlap portion;

extracting said first elevation from a terrain elevation database;

extracting said second elevation from said terrain elevation database;

computing a first horizontal displacement for said first strip using said first elevation;

computing a second horizontal displacement for said adjacent strip using said second elevation;

computing Taylor series coefficients using said ideal path, said actual path and said central reference point for said first strip and said adjacent strip;

computing phase error compensation using said Taylor series coefficients, a Fast Fourier Transform and an inverse Fast Fourier Transform for said first strip to obtain first strip corrected data and for said adjacent strip to obtain second strip corrected data;

applying said phase error compensation to said digital returns to obtain said SAR image.

2. A radar as described in claim 1, wherein said first strip corrected data and said second strip corrected data are merged over said overlap portion to generate said SAR image.

3. A radar as described in claim 2 wherein said Taylor series is truncated after a second order term.

4. A radar as described in claim 3 wherein said terrain elevation database is stored in increments and said first strip and said second strip contain an integral number of said increments.

5. A radar as described in claim 3 wherein said terrain elevation database is interpolated for target elevation using cubic spline approximation.

6. A radar as described in claim 3 wherein said terrain elevation database is interpolated for target elevation using a least squares polynomial approximation.

7. A method for acquiring a SAR image of radar scatterers using a radar in an area around a central reference point, said area having a first elevation and a second elevation, said radar on a moving platform, said moving platform moving along an actual path, said actual path displaced from an ideal path, said method comprising the steps of:

converting radar returns from said area to digital returns;

motion compensating said digital returns with respect to said central reference point and said actual path;

dividing said digital returns descriptive of said SAR image into a first strip and an adjacent strip, said first strip covering a first part of said area, said first part having said first elevation, said adjacent strip covering second part of said area said second area having said second elevation, said first strip overlapping said adjacent strip over an overlap portion;

extracting said first elevation from a terrain elevation database;

extracting said second elevation from said terrain elevation database;

computing a first horizontal displacement for said first strip using said first elevation;

computing a second horizontal displacement for said adjacent strip using said second elevation;

computing Taylor series coefficients using said ideal path, said actual path and said central reference point for said first strip and said adjacent strip;

computing phase error compensation using said Taylor series coefficients, a Fast Fourier Transform and an inverse Fast Fourier Transform for said first strip to obtain first strip corrected data and for said adjacent strip to obtain second strip corrected data;

applying said phase error compensation to said digital returns to obtain said SAR image.

8. A method as described in claim 7, wherein said first strip corrected data and said second strip corrected data are merged over said overlap portion to generate said SAR image.

9. A method as described in claim 8 wherein said Taylor series is truncated after a second order term.

10. A method as described in claim 9 wherein said terrain elevation database is stored in increments and said first strip and said second strip contain an integral number of said increments.

11. A method as described in claim 10 wherein said terrain elevation database is interpolated for target elevation using cubic spline approximation.

12. A method as described in claim 10 wherein said terrain elevation database is interpolated for target elevation using a least squares polynomial approximation.