FREE-HAND SCANNING AND IMAGING

• US 20140111374A1
• Filed: 10/21/2013
• Published: 04/24/2014
• Est. Priority Date: 10/19/2012
• Status: Active Grant

• Current Assignee: Curators of the University of Missouri (University of Missouri)
• Original Assignee: Curators of the University of Missouri (University of Missouri)
• Inventors: Ghasr, Mohammad Tayeb, Zoughi, Reza, Case, Joseph T.
• Granted Patent: US 9,316,734 B2
• Time in Patent Office: Days
• US Class Current: 342/25.F
• CPC Class Codes: G01S 13/88 Radar or analogous systems specially adapted for specific applications electromagnetic prospecting or detecting of objects, e.g. near-field detection, G01V3/00
• CPC Class Codes: G01S 13/885 for ground probing prospecting or detecting using electromagnetic waves G01V3/12
• CPC Class Codes: G01S 13/887 for detection of concealed objects, e.g. contraband or weapons
• CPC Class Codes: G01S 13/888 through wall detection
• CPC Class Codes: G01S 13/9011 with frequency domain processing of the SAR signals in azimuth G01S13/9005 takes precedence
• CPC Class Codes: G01S 15/8904 using synthetic aperture techniques
• CPC Class Codes: G01V 3/12 operating with electromagnetic waves operating with millimetre waves G01V8/005

Abstract

Wideband synthetic aperture radar (SAR) imaging. A probe transmits a signal through its aperture incident to an object located in a medium of interest remotely from the probe. The probe receives through the aperture a plurality of nonuniformly sampled reflected signals from the object as the probe moves in a measurement plane located a predetermined distance from the object. A processor executes a SAR-based reconstruction algorithm to generate an image.

Claims

1. A wideband synthetic aperture radar (SAR) imaging system, comprising:

a probe having an aperture through which a signal is transmitted incident to an object located in a medium of interest remotely from the probe and through which a plurality of nonuniformly sampled reflected signals from the object are received by the probe as the probe moves in a measurement plane located a predetermined distance from the object;

a memory for storing measurement data representative of the reflected signals collected by the probe;

a processor executing a plurality of computer-executable instructions for a SAR-based reconstruction algorithm, said instructions comprising;

instructions for performing a spectral estimation based on the measurement data;

instructions for transforming a frequency component of the spectral estimation as a function of the medium of interest;

instructions for obtaining a three-dimensional SAR image from the transformed spectral estimation data using Fourier transforms; and

a display responsive to the processor for presenting the three-dimensional SAR image to a user.

2. The imaging system of claim 1, wherein the measurement data is mapped to a densely sampled uniform grid.

3. The imaging system of claim 1, wherein the spectral estimation comprises application of a fast Fourier transform.

4. The imaging system of claim 2, wherein the spectral estimation comprises application of a nonuniform fast Fourier transform.

5. The imaging system of claim 1, wherein the spectral estimation includes a single spatial bandwidth encompassing all depths of a preliminary image, and further comprising instructions for performing an operation that reduces the error between the nonuniform measurement data and an inverse two dimensional nonuniform fast Fourier transform of the estimate of the spectrum.

6. The imaging system of claim 1, wherein the spectral estimation includes a single spatial bandwidth for each segment of a preliminary image.

7. The imaging system of claim 6, further comprising instructions for performing an optimization operation based on a comparison between original nonuniform measurement data and intermediate segmented image data.

8. The imaging system of claim 1, wherein the spectral estimation includes a spatial bandwidth unique for each depth in the preliminary image.

9. The imaging system of claim 1, further comprising instructions for performing an optimization operation based on components derived from a SAR-based transform and an R-SAR-based transform.

10. The imaging system of claim 1, wherein the probe is manually moved in the measurement plane as a scan is performed.

11. The imaging system of claim 1, wherein the signal comprises microwave or millimeter wave electromagnetic energy.

12. The imaging system of claim 1, wherein the signal comprises ultrasonic waves or acoustic energy.

13. The imaging system of claim 1, wherein the probe comprises an open-ended waveguide antenna.

14. The imaging system of claim 1, wherein spectral estimation includes the step of performing a reference shift to align the measurement plane with the top of the medium of interest.

15. A method of generating a three dimensional image of a specimen under test (SUT) comprising:

transmitting, via the probe, a signal within a predetermined operating bandwidth;

tracking nonuniform two-dimensional movement of a probe within a measurement plane remote from the SUT;

receiving, via the probe, signals reflected from the SUT during the movement of the probe;

storing reflection coefficient data based on the reflected signals as distributed measurement positions within the measurement plane by recording the signals at discrete frequencies throughout the operating bandwidth;

processing the stored data into a wide band, synthetic aperture image by implementing a synthetic aperture radar (SAR) algorithm;

displaying the image to a user in real-time;

further processing the image to perform an optimization objective; and

further displaying the optimized image to the user.

16. The method of claim 15, wherein the further processing comprises performing spectral estimation on the reflection coefficient data.

17. The method of claim 16, wherein the optimization includes reducing the error between nonuniform input data and an inverse two-dimensional nonuniform fast Fourier transform of the spectral estimation.

18. The method of claim 15, wherein the movement of the probe is tracked.

19. The method of claim 18, wherein nonuniform points are sampled from the tracked movement, and the points are mapped to a densely sampled uniform grid.

20. The method of claim 15, wherein said processing comprises of a two-dimensional fast Fourier transform.

21. The method of claim 20, wherein said two-dimensional fast Fourier transform comprises a nonuniform two-dimensional fast Fourier transform.

22. The method of claim 15, wherein said processing comprises segmenting the images for which partial data corresponds to a single spatial bandwidth.

23. The method of claim 15 wherein the optimization includes reducing the error between components derived from a SAR-based transform and an R-SAR-based transform.

24. A wideband synthetic aperture radar (SAR) imaging system, comprising:

a signal source for generating a signal with a predetermined operating bandwidth;

a transceiver antenna coupled to the signal source, the antenna having an aperture through which the signal is transmitted incident to an object located in a medium and through which a plurality of nonuniformly sampled reflected signals from the object are received by the antenna as the antenna moves nonuniformly in a plane located a predetermined distance from the object;

a memory for storing signal data, the signal data comprising nonuniformly sampled reflected signals collected at the aperture;

a processor executing a plurality of computer-executable instructions for a real-time, post-processing, reconstruction algorithm comprising;

instructions for estimating a two-dimensional spatial spectrum based on the signal data to provide a uniformly sampled spectrum;

instructions for estimating the uniformly sampled spectrum to remove or minimize image artifacts;

instructions for reconstructing uniformly sampled data from nonuniformly sampled data to remove or minimize image artifacts;

instructions for forming a SAR image of the object from the estimated uniform spectrum;

instructions for dividing the reconstructed SAR image into a plurality of segments;

instructions for applying a R-SAR transform to each of the segments;

instructions for filtering and reconstructing the data for each segment and summing each filtered segment;

a display for presenting the three-dimensional SAR image in real-time to a user.

25. The imaging system of claim 24, wherein the system is utilized for nondestructive evaluation applications.

26. The imaging system of claim 24, wherein the transceiver antenna is manually moved in the plane as a scan is performed.

27. The imaging system of claim 24, wherein the signal source transmits the signal at a plurality of discrete frequencies in the operating bandwidth.

28. The imaging system of claim 24, wherein nonuniform sampling occurs below a Nyquist sampling rate.

29. The imaging system of claim 24, further comprising instructions for applying a SAR transform to the summed segments to create an optimized three-dimensional SAR image with a reduced amount of image artifacts.

30. The imaging system of claim 24, further comprising instructions for applying SAR-based and R-SAR-based transforms for estimating the 2-D spatial spectrum of the SAR image.