Methods and Systems for Near-Field Microwave Imaging

• US 20190324135A1
• Filed: 06/06/2019
• Published: 10/24/2019
• Est. Priority Date: 12/17/2015
• Status: Active Grant

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Moulder, William F., KRIEGER, James D., MAURAIS-GALEJS, Denise T., NGUYEN, Huy T., HERD, Jeffrey S.
• Granted Patent: US 11,194,038 B2
• Time in Patent Office: Days
• CPC Class Codes: G01S 13/003 Bistatic radar systems; Multistatic radar systems
• CPC Class Codes: G01S 13/865 Combination of radar systems with lidar systems
• CPC Class Codes: G01S 13/867 Combination of radar systems with cameras
• CPC Class Codes: G01S 13/887 for detection of concealed objects, e.g. contraband or weapons
• CPC Class Codes: G01S 13/89 for mapping or imaging
• CPC Class Codes: G01V 3/12 operating with electromagnetic waves operating with millimetre waves G01V8/005

Abstract

A multistatic array topology and image reconstruction process for fast 3D near field microwave imaging are presented. Together, the techniques allow for hardware efficient realization of an electrically large aperture and video-rate image reconstruction. The array topology samples the scene on a regular grid of phase centers, using a tiling of multistatic arrays. Following a multistatic-to-monostatic correction, the sampled data can then be processed with the well-known and highly efficient monostatic Fast Fourier Transform (FFT) imaging algorithm. In this work, the approach is described and validated experimentally with the formation of high quality microwave images. The scheme is more than two orders of magnitude more computationally efficient than the backprojection method. In fact, it is so efficient that a cluster of four commercial off-the-shelf (COTS) graphical processing units (GPUs) can render a 3D image of a human-sized scene in 0.048-0.101 seconds.

Claims

1. A method of near-field microwave imaging of a scene, the method comprising:

acquiring multistatic array data of the scene with a multistatic array;

acquiring a depth map of the scene with an image sensor;

applying a multistatic-to-monostatic correction to the multistatic array data to form corrected multistatic array data; and

forming a representation of the scene, based on

the corrected multistatic array data and the depth map of the scene.

2. The method of claim 1, wherein forming the representation of the scene comprises fusing the corrected multistatic array data and the depth map.

3. The method of claim 1, wherein forming the representation of the scene comprises Fourier transforming the corrected multistatic array data to produce a three-dimensional image of the scene.

4. The method of claim 3, wherein forming the representation of the scene comprises constraining an image domain for Fourier transforming the corrected multistatic array data based on the depth map.

5. The method of claim 3, wherein forming the representation of the scene comprises restricting a size, shape, and/or position of an image domain for Fourier transforming the corrected multistatic array data based on the depth map.

6. A method of near-field microwave imaging of a scene, the method comprising:

at each of a plurality of frequency steps, acquiring and digitizing multistatic array data of the scene with a multistatic array within a period of 200 ns or less;

applying a multistatic-to-monostatic correction to the multistatic array data to form corrected multistatic array data; and

generating a three-dimensional representation of the scene based on the corrected multistatic array data at a frame rate of at least 1 Hz and a processing load of less than 2000 giga-floating point operations per second (GFLOPS).

7. The method of claim 6, wherein the plurality of frequency steps includes at least 56 frequency steps, the multistatic array comprises at least 25 elements, and the three-dimensional representation has at least 21 depth planes.

8. The method of claim 6, wherein the plurality of frequency steps includes at least 160 frequency steps.

9. The method of claim 6, wherein the processing load is less than 200 GFLOPS.

10. The method of claim 6, wherein generating the three-dimensional representation of the scene based on the corrected multistatic array data comprises Fourier transforming the corrected multistatic array data to yield Fourier-domain data, applying a phase shift to the Fourier-domain data to yield phase-shifted Fourier-domain data, and inverse Fourier transforming the phase-shifted Fourier-domain data to form the three-dimensional representation of the scene.

11. A multistatic imaging system comprising:

an array of supertiles, each supertile covering a corresponding partition of a full aperture of the array of supertiles configured to acquire and digitize radio-frequency returns representing a scene; and

a processor, operably coupled to the array of supertiles, to generate a three-dimensional representation of the scene by (i) applying a multistatic-to-monostatic correction to the radio-frequency returns and (ii) applying Fourier-transform beam propagation to the radio-frequency returns.

12. The multistatic imaging system of claim 11, wherein the supertiles in the array of supertiles transmit and receive orthogonal waveforms.

13. The multistatic imaging system of claim 11, wherein the supertiles in the array of supertiles are configured for purely digital input and purely digital output.

14. The multistatic imaging system of claim 11, wherein each supertile in the array of supertiles comprises:

a transceiver to generate radio-frequency signals;

a switched multistatic antenna array to transmit the radio-frequency signals toward the scene and to receive the radio-frequency returns from the scene;

a data acquisition block to digitize the radio-frequency returns.

15. The multistatic imaging system of claim 11, further comprising:

an adjunct sensor to generate a depth map of the scene.

16. A method of applying a multistatic-to-monostatic correction for a multistatic microwave imaging array, the method comprising:

measuring multistatic reflections of a scene with the multistatic microwave imaging array;

calculating multistatic reflections of a point scatterer, sampled on a grid of phase centers of the multistatic microwave imaging array, that would be received by the multistatic microwave imaging array;

calculating monostatic reflections of the point scatterer, sampled on the grid of phase centers of the multistatic microwave imaging array, that would be received by a monostatic element; and

multiplying the multistatic reflections of the scene by a ratio of the monostatic reflections of the point scatterer to the multistatic reflections of the point scatterer.