Auto-focus for circular synthetic aperture sonar

Patent metadata

Field	Value
Publication number	US-8937849-B1
Application number	US-201213568677-A
Country	US
Kind code	B1
Filing date	Aug 7, 2012
Priority date	Aug 7, 2012
Publication date	Jan 20, 2015
Grant date	Jan 20, 2015

Abstract

A method of focusing fully-coherent circular synthetic aperture sonar (CSAS) imagery is provided. A k-space representation of the CSAS image is generated using the two dimensional Fast-Fourier Transform (FFT). Sub-aperture images are generated by windowing the k-space representation and applying the two dimensional inverse FFT to the windowed spectrum. All adjacent complex sub-aperture images are correlated, the correlation peaks are detected and the relative shifts in X and Y as a function of look-angle are recorded. The relative shifts between adjacent sub-apertures are integrated and the means are subtracted to find the absolute shift as a function of look-angle. A motion-solution is calculated by exploiting the relationship between apparent scene shift and actual vehicle sway. The motion estimation is used to generate a phase-correction matrix that is multiplied by the k-space representation of the fully-coherent image.

First claim

What is claimed is:

1. A method of focusing a circular synthetic aperture sonar image of a scene taken from a platform, said method comprising the steps of:
generating a k-space representation of said sonar image;
generating sub-aperture images based on windowing said k-space representation over chosen look-angles;
correlating adjacent ones of said sub-aperture images;
detecting correlation peaks based on said correlating;
obtaining absolute shifts of said scene as a function of said look-angles, said absolute shifts based on said correlation peaks;
calculating a sway motion estimation of said platform based on said absolute shifts;
generating a phase-correction matrix based on said sway motion estimation; and
applying said phase-correction matrix to said sonar image to recover a focused image of said scene.

2. The method of claim 1 , further comprising iteratively returning to generating sub-aperture images utilizing a k-space representation of said focused image.

3. The method of claim 2 , wherein said step of generating sub-aperture images further comprises applying an inverse Fast Fourier Transform to each windowed k-space representation.

4. The method of claim 2 , comprising first generating said sonar image using a phase-preserving method applied to raw echo data of said scene, said raw echo data having uncompensated motion errors.

5. The method of claim 4 , wherein said phase-preserving method comprises at least one of projection-slice imaging and time-delay and sum imaging.

6. The method of claim 2 , wherein said step of detecting correlation peaks further comprises interpolating and filtering correlated sub-aperture images.

7. The method of claim 6 , wherein said step of obtaining absolute shifts comprises:
expressing said correlation peaks as a function of said look angles to obtain relative shifts;
subtracting a mean from said relative shifts; and
accumulating said relative shifts with said mean subtracted therefrom.

8. The method of claim 2 , wherein said step of calculating a sway motion estimation comprises:
calculating axial scene shifts based on said absolute shifts; and
integrating and scaling said axial scene shifts to obtain a scalar function A(θ), wherein platform sway is proportional to said scalar function and wherein θ corresponds to an angle in spatial coordinates X and Y of said scene.

9. The method of claim 8 , wherein said scalar function comprises an expression:

A
⁡

(

θ
n

)

=

C
⁡

[

∑

n
-
1

N

⁢

⁢

X
n
2

+

Y
n
2

*

cos
⁡

(

θ
n

+

arctan
⁡

(

Y
n

X
n

)

)

N

+

2
⁢

∑
l
n

⁢

⁢

X
n
2

+

Y
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sin
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(

θ
n

+

arctan
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X
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)

Assignees

• Marston Timothy M
• Us Navy

Inventors

• Marston Timothy M

Classifications

• G01S15/89

  for mapping or imaging · CPC title

• G01S7/52003 Primary

  Techniques for enhancing spatial resolution of targets (G01S7/52046 takes precedence) · CPC title