Method and apparatus for 3-D sub-voxel position imaging with synthetic aperture radar

• US 7,511,655 B2
• Filed: 09/25/2006
• Issued: 03/31/2009
• Est. Priority Date: 09/25/2006
• Status: Active Grant

• Current Assignee: Navy The USA As Represented By The Secretary of The
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Barnwell, William A., Buss, James R., Willey, Jefferson M., Szu, Harold H.
• Primary Examiner(s): Tarcza; Thomas H
• Assistant Examiner(s): Barker; Matthew M
• Application Number:
  US11/534,732
• Publication Number:
  US 20080074313A1
• Time in Patent Office: 918 Days
• Field of Search: 342/25.A
• US Class Current: 342/25.00A
• CPC Class Codes: G01S 13/87 Combinations of radar systems, e.g. primary radar and secondary radar
• CPC Class Codes: G01S 13/9023 combined with interferometric techniques
• CPC Class Codes: G01S 13/904 SAR modes

Abstract

A method and apparatus for determining three dimensional sub-voxel positions using synthetic aperture radar. The apparatus includes at least four non-coplanar, phase-coherent synthetic aperture radar (SAR) platforms comprising a plurality of phase-synchronized local oscillators cohered to a common reference clock, a first SAR platform of the SAR platforms operable to transmit a radar waveform, and the SAR platforms operable to receive scattered energy waveforms resulting from the radar waveform and operable to generate two-dimensional (2-D) SAR images based on the received scattered energy waveforms. The apparatus also includes a synchronizing processor operable to communicate with the SAR platforms and operable to synchronize a plurality of SAR transmission and receiving intervals for the SAR platforms. Optionally, the apparatus also includes an image-coregistration processor operable to receive at least four 2-D SAR images from the SAR platforms, and operable to generate a 3-D SAR image having 3-D voxel positions by coregistering the 2-D SAR images. Optionally, the apparatus also includes a sub-voxel position processor operable to calculate sub-voxel positions of single point scatterers from differential phase measurements on a given voxel across all of the SAR platforms.

Claims

1. A method comprising:

a) providing at least four phase-coherent synthetic aperture radar (SAR) platforms;

b) transmitting a radar waveform from one of the at least four SAR platforms;

c) receiving scattered energy waveform by the at least four SAR platforms, the scattered energy comprising a superposition of a plurality of point scatters in range, cross-range, and height convolved with the radar waveform;

d) generating a plurality of complex two dimensional (2-D) SAR images by forming a complex 2-D SAR image at each SAR platform based on the scattered energy received at each SAR platform, each complex 2-D SAR image comprising a plurality of pixels, each pixel comprising a magnitude and a phase;

e) co-registering the plurality of complex 2-D SAR images to generate a discretized 3-D space comprising a plurality of 3-D voxels, each 3-D voxel comprising a range bound, a cross-range bound, and a height bound;

f) calculating three differential paired phase functions for each 3-D voxel of the plurality of 3-D voxels;

g) calculating a plurality of sub-voxel positions;

h) for each sub-voxel position calculation, determining whether a point scatter of the plurality of point scatters falls within a single 3-D voxel of the plurality of 3-D voxels;

i) reporting a range, a cross-range, and a height of the point scatter, based on the range bound, cross-range bound, and height bound of the single 3-D voxel of the plurality of 3-D voxels, if the calculated position of the point scatter falls within the single 3-D voxel of the plurality of 3-D voxels;

j) reporting a non-compliant scatter, if the calculated position of the point scatter of the plurality of point scatters does not fall within the single 3-D voxel; and

k) repeating steps i) through j) until a 3-D image with sub-voxel position is generated.

2. The method according to claim 1, wherein the at least four SAR platforms comprise a tetrahedral arrangement.

3. The method according to claim 1, wherein the at least four SAR platforms are time-coherent.

4. The method according to claim 1, wherein calculating a plurality of sub-voxel positions comprising one of inverting the three differential paired phase functions and using numerical optimization to minimize quadratic error functions.

5. The method according to claim 1, further comprising:

reducing sub-pixel position error in a presence of at least one of phase bias and platform flight perturbations by dilation of a constellation geometry of the four SAR platforms.

6. A method comprising:

a) providing at least four phase-coherent synthetic aperture radar (SAR) platforms;

b) generating a two-dimensional (2-D) SAR image for each SAR platform of the at least four phase-coherent synthetic aperture radar (SAR) platforms; and

c) generating a three-dimensional (3-D) SAR image by co-registering the generated 2-D SAR images, the 3-D SAR image comprising sub-voxel position,

wherein the 3-D image comprises a plurality of voxels, wherein said generating a three-dimensional (3-D) SAR image comprises calculating a plurality of sub-voxel positions by one of using numerical optimization to minimize quadratic error functions and inverting three differential paired phase functions calculated for each voxel of the plurality of voxels.

7. An apparatus comprising:

at least four non-coplanar, phase-coherent synthetic aperture radar (SAR) platforms, each SAR platform of said SAR platforms comprising a corresponding phase-synchronized local oscillator cohered to a common reference clock, a first SAR platform of said SAR platforms operable to transmit a radar waveform, and said SAR platforms operable to receive scattered energy waveforms resulting from the radar waveform and operable to generate two-dimensional (2-D) SAR images based on the received scattered energy waveforms; and

a synchronizing processor operable to communicate with said SAR platforms and operable to synchronize a plurality of SAR transmission and receiving intervals for said SAR platforms,

an image-coregistration processor operable to receive at least four 2-D SAR images from said SAR platforms, and operable to generate a 3-D SAR image having 3-D voxel positions by coregistering the 2-D SAR images; and

a sub-voxel position processor operable to calculate sub-voxel positions of single point scatters from differential phase measurements on a given voxel across said SAR platforms.