Estimation and correction of phase for focusing search mode SAR images formed by range migration algorithm

• US 6,781,541 B1
• Filed: 07/30/2003
• Issued: 08/24/2004
• Est. Priority Date: 07/30/2003
• Status: Expired due to Term

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Raytheon Company (Rtx Corporation)
• Inventors: Cho, Kwang M.
• Primary Examiner(s): Lobo, Ian J.
• Application Number:
  US10/630,889
• Time in Patent Office: 391 Days
• Field of Search: 342/25, 342/195, 342/196
• US Class Current: 342/25.00D
• CPC Class Codes: G01S 13/9011 with frequency domain processing of the SAR signals in azimuth G01S13/9005 takes precedence
• CPC Class Codes: G01S 13/9019 Auto-focussing of the SAR signals
• CPC Class Codes: G01S 13/904 SAR modes

Abstract

A method for acquiring synthetic aperture images of stationary targets converts a plurality of radar signals stored as digital values and motion compensated to a first order into a well focused image. The digital values are Fourier transformed, match filtered and interpolated using a Stolt interpolator, then skewed to reorient distortions arising from imperfect motion compensation, generating an image data, descriptive of the stationary targets in the range and azimuth direction. The image data is divided into a plurality of overlapping sub-patches in, preferably, the cross track (azimuth) direction. Each sub-patch containing a portion of the image data and overlapping data. The overlapping data is part of the image data and common between two or more of the overlapping sub-patches.

Each of the overlapping sub-patches is individually focused using autofocus means to obtain focused sub-patches having a phase. The phase is adjusted for each of the focused sub patches to obtain a continuous phase with respect to one or more of the focused sub-patches, thereby obtaining smooth phase transition sub-patches. The synthetic aperture images of stationary targets are computed after concatenating the smooth phase transition sub-patches.

Claims

1. A radar system for acquiring synthetic aperture images of stationary targets, said radar system located on a moving platform, said moving platform moving in an along track direction, said along track direction perpendicular to a cross track direction, said radar system comprising:

an analog to digital converter for converting a plurality of radar signals reflected from stationary targets located on a ground patch into digital values;

memory for storing said digital values;

motion compensation means for aligning said digital values stored within said memory along a motion compensation line to a first approximation of motion of said moving platform to generate motion compensated digital values;

Fourier transform means for the conversion of said motion compensated digital values in said along track direction to a frequency domain;

match filter function means to generate range compressed data for said stationary targets on said motion compensation line within said frequency domain;

a Stolt interpolator for interpolating said range compressed data to generate interpolated values;

means for skewing said interpolated values to reorient distortions arising from said first approximation of motion thereby generating an image data, said image data descriptive of said stationary targets in a range direction and an azimuth direction, said azimuth direction generally in said cross track direction;

means for dividing said image data into overlapping sub-patches, each sub-patch containing a portion of said image data and overlapping data, said overlapping data part of said image data and common between two or more of said overlapping sub-patches;

means for focusing each of said overlapping sub-patches individually using autofocus means to obtain focused sub-patches having a phase;

means for adjusting said phase of each of said focused sub-patches to obtain a continuous phase with respect to one or more of said focused sub-patches, thereby obtaining smooth phase transition sub-patches;

means for extracting said synthetic aperture images of stationary targets after concatenating said smooth phase transition sub-patches.

2. A radar system as described in claim 1 wherein said means for skewing said interpolated values is first computed in one direction and subsequently in a second direction.

3. A radar system as described in claim 2 wherein said means for skewing said second direction is perpendicular to said first direction.

4. A radar system as described in claim 1 wherein said autofocus means comprise a phase gradient autofocus algorithm.

5. A radar system as described in claim 1 wherein said autofocus means comprise a multiple discrete autofocus algorithm.

6. A radar system as described in claim 1 wherein said overlapping sub-patches divide said image data in said azimuth direction.

7. A radar system as described in claim 1 wherein said phase is calculated by integrating twice the second derivatives of target phase descriptive of all of said stationary targets contained within each of said focused sub-patches and computing the weighted average of said second derivatives.

8. A radar system as described in claim 1 wherein said phase is calculated by removing constant and linear terms in said phase using said overlapping data.

9. A method for acquiring synthetic aperture images of stationary targets using a radar system, said radar system located on a moving platform, said moving platform moving in an along track direction, said along track direction perpendicular to a cross track direction, comprising the steps of:

converting a plurality of radar signals reflected from stationary targets located on a ground patch into digital values;

storing said digital values;

motion compensating said digital values stored within said memory to a first approximation of motion of said moving platform to generate motion compensated data;

transforming said motion compensated data in said along track direction into a frequency domain using a Fourier transform to generate frequency domain data;

performing a matched filter operation on said frequency domain data to generate matched data;

interpolating said matched data to generate interpolated values;

skewing said interpolated values to reorient distortions arising from said first approximation of motion in a preferred direction thereby generating an image data, said image data descriptive of said stationary targets in a range direction and an azimuth direction;

dividing said image data into a plurality of overlapping sub-patches in said cross track direction, each sub-patch containing a portion of said image data and overlapping data, said overlapping data part of said image data and common between two or more of said overlapping sub-patches;

focusing each of said overlapping sub-patches individually using autofocus methods to obtain focused sub-patches having a phase;

adjusting said phase of each of said focused sub patches to obtain a continuous phase with respect to one or more of said focused sub-patches, thereby obtaining smooth phase transition sub-patches;

extracting said synthetic aperture images of stationary targets after concatenating said smooth phase transition sub-patches.

10. A method as described in claim 9 wherein skewing said interpolated values is first computed in one direction and subsequently in a second direction.

11. A method as described in claim 10 wherein said second direction is perpendicular to said first direction.

12. A method as described in claim 9 wherein said autofocus means comprise a phase gradient autofocus algorithm.

13. A method as described in claim 9 wherein said autofocus means comprise a multiple discrete autofocus algorithm.

14. A method as described in claim 9 wherein said overlapping sub-patches divide said image data in said azimuth direction.

15. A method as described in claim 9 wherein said phase is calculated by integrating twice the second derivatives of target phase descriptive of all of said stationary targets contained within each of said focused sub-patches and computing the weighted average of said second derivatives.

16. A method as described in claim 9 wherein said phase is calculated by removing constant terms and linear terms in said phase using said overlapping data.