APPARATUS AND METHOD FOR RADAR IMAGING BY MEASURING SPATIAL FREQUENCY COMPONENTS

• US 20080291077A1
• Filed: 05/16/2008
• Published: 11/27/2008
• Est. Priority Date: 05/21/2007
• Status: Active Grant

• Current Assignee: Spatial Digital Systems Inc
• Original Assignee: Spatial Digital Systems Inc
• Inventors: CHANG, DONALD CHIN-DONG
• Granted Patent: US 7,609,198 B2
• Time in Patent Office: Days
• US Class Current: 342/59
• CPC Class Codes: G01S 13/89 for mapping or imaging
• CPC Class Codes: G01S 13/90 using synthetic aperture techniques , e.g. synthetic aperture radar [SAR] techniques
• CPC Class Codes: G01S 13/9023 combined with interferometric techniques
• CPC Class Codes: G01S 13/9043 Forward-looking SAR
• CPC Class Codes: G01S 13/9047 Doppler beam sharpening mode
• CPC Class Codes: H01Q 3/26 varying the relative phase or relative amplitude of energisation between two or more active radiating elements; varying the distribution of energy across a radiating aperture H01Q3/12, H01Q3/22, H01Q3/24 take precedence

Abstract

A radar imaging system is provided that directly measures the spatial frequency components of a scene via digital-beam-forming techniques applied along the cross-track dimension. Separate transmit and receive antennas provide increased integration time for the receive function, thus improving the signal-to-noise ratio. A segmented receive antenna is employed and processed as a series of interferometers sensitive to spatial frequency components of the scene corresponding to the separation between pairs of antenna elements. Range gating is used in the along-track dimension to divide the return from an illuminated swath into multiple range bins that may be processed independently. The system provides an improved signal-to-noise ratio and lends significant flexibility to the image formation process, improving the quality of the radar imaging. An embodiment having multiple transmit antennas is also provided that enables the generation of three-dimensional stereoscopic radar images.

Claims

1. A radar imaging system comprising:

a transmit antenna adapted to transmit radar waveforms to illuminate a target region extending in a range direction and in an azimuth direction;

a receive antenna divided into a plurality of array segments, wherein the receive antenna is adapted to receive a radar return from the target region and to generate a plurality of return signals associated with corresponding ones of the plurality of array segments;

a range processor adapted to receive the plurality of return signals from the receive antenna and to divide them into a plurality of range slices, wherein each one of the plurality of range slices comprises a plurality of azimuth signals associated with corresponding ones of the plurality of array segments;

at least one modulation-transfer-function (MTF) processor adapted to receive the plurality of azimuth signals from one of the plurality of range slices at a time and to auto-correlate the plurality of azimuth signals to generate a plurality of spatial-frequency signals;

at least one Fourier transform processor adapted to receive the plurality of spatial-frequency signals and to generate an inverse Fourier transform of the spatial-frequency signals to generate an azimuth image slice associated with the one of the plurality of range slices received by the at least one MTF processor; and

an image processor adapted to combine azimuth images slices associated with ones of the plurality of range slices to create a two-dimensional radar image.

2. The radar imaging system of claim 1, wherein the plurality of array segments of the receive antenna comprise a sparse linear array whereby ones of the plurality of array segments are positioned along a straight line such that spacings between pairs of the plurality of range segments are irregular.

3. The radar imaging system of claim 2, wherein the sparse linear array is further adapted to comprise a minimally redundant array whereby a number of pairs of array segments having equal spacing is minimized.

4. The radar imaging system of claim 1, wherein the at least one MTF processor further comprises a weighting unit adapted to apply a plurality of complex weighting factors to corresponding ones of the plurality of spatial-frequency signals.

5. The radar imaging system of claim 4, wherein the plurality of complex weighting factors applied by the weighting unit is adapted to low-pass filter the plurality of spatial-frequency signals.

6. The radar imaging system of claim 4, wherein the plurality of complex weighting factors applied by the weighting unit is adapted to high-pass filter the plurality of spatial-frequency signals.

7. The radar imaging system of claim 4, wherein the plurality of complex weighting factors applied by the weighting unit is adapted to band-pass filter the plurality of spatial-frequency signals.

8. The radar imaging system of claim 1, wherein the range processor further comprises a range-Fourier-transform processor adapted to perform short-time-window fast Fourier transforms of the plurality of return signals to divide the plurality of return signals into the plurality of range slices.

9. The radar imaging system of claim 1, wherein:

the at least one MTF processor further comprises N MTF processors; and

the at least one Fourier-transform processor further comprises N Fourier-transform processors;

wherein N is a positive integer corresponding to a number of range slices generated by the range processor; and

wherein the N MTF processors and the N Fourier transform processors are adapted to parallel process the plurality of azimuth signals from N range slices generated by the range processor.

10. A radar imaging system comprising:

a transmit antenna adapted to transmit radar waveforms to illuminate a target region extending in a range direction and in an azimuth direction;

a receive antenna divided into a plurality of array segments, wherein;

the plurality of array segments are adapted to comprise a minimally redundant array whereby a number of pairs of array segments having equal spacing is minimized; and

the minimally redundant array is adapted to receive a radar return from the target region and to generate a plurality of return signals associated with corresponding ones of the plurality of array segments;

a range processor adapted to receive the plurality of return signals from the minimally redundant array and to divide them into a plurality of range slices, wherein each one of the plurality of range slices comprises a plurality of azimuth signals associated with corresponding ones of the plurality of array segments;

a modulation-transfer-function (MTF) processor adapted to receive the plurality of azimuth signals from one of the plurality of range slices at a time and to auto-correlate the plurality of azimuth signals to generate a plurality of spatial-frequency signals;

a Fourier transform processor adapted to receive the plurality of spatial-frequency signals and to generate an inverse Fourier transform of the spatial-frequency signals to generate an azimuth image slice associated with the one of the plurality of range slices received by the MTF processor; and

an image processor adapted to combine azimuth images slices associated with ones of the plurality of range slices to create a two-dimensional radar image.

11. The radar imaging system of claim 10, wherein the MTF processor further comprises a weighting unit adapted to apply a plurality of complex weighting factors to corresponding ones of the plurality of spatial-frequency signals.

12. The radar imaging system of claim 11, wherein the plurality of complex weighting factors applied by the weighting unit is adapted to low-pass filter the plurality of spatial-frequency signals.

13. The radar imaging system of claim 11, wherein the plurality of complex weighting factors applied by the weighting unit is adapted to high-pass filter the plurality of spatial-frequency signals.

14. The radar imaging system of claim 11, wherein the plurality of complex weighting factors applied by the weighting unit is adapted to band-pass filter the plurality of spatial-frequency signals.

15. The radar imaging system of claim 10, wherein the range processor further comprises a range-Fourier-transform processor adapted to perform short-time-window fast Fourier transforms of the plurality of return signals to divide the plurality of return signals into the plurality of range slices.

16. The radar imaging system of claim 10, further comprising

N modulation-transfer-function (MTF) processors;

and

N Fourier-transform processors;

wherein N is a positive integer corresponding to a number of range slices generated by the range processor; and

wherein the N MTF processors and the N Fourier transform processors are adapted to parallel process the plurality of azimuth signals from N range slices generated by the range processor.

17. In a radar imaging system comprising a transmit antenna and a receive antenna array comprising a plurality of segments, a method of creating a radar image comprises:

transmitting radar waveforms from the transmit antenna to illuminate a target area;

receiving a radar echo at the receive antenna array that produces a plurality of return signals from corresponding ones of the plurality of segments of the receive antenna array;

range gating the plurality of return signals to divide them into a plurality of range slices, wherein each one of the plurality of range slices comprises a plurality of azimuth signals associated with corresponding ones of the plurality of segments of the receive antenna array;

performing an autocorrelation of the plurality of azimuth signals associated with a first one of the plurality of range slices to generate a first modulation transfer function comprising a first plurality of spatial-frequency components;

performing an inverse Fourier transform of the first plurality of spatial-frequency components to generate a first azimuth image line associated with the first one of the plurality of range slices;

repeating the steps of performing an autocorrelation and performing an inverse Fourier transform of a plurality of azimuth signals associated with one of the plurality if range slices until all of the range slices have been processed;

arranging azimuth image lines associated with ones of the plurality of range slices to form a two-dimensional image of the target area.

18. The method of claim 17, wherein the step of performing an autocorrelation of the plurality of azimuth signals further comprises applying complex weighting factors to the modulation transfer function generated from the plurality of azimuth signals to effect a spatial filtering transformation of the modulation transfer function.

19. The method of claim 18, wherein the step of applying complex weighting factors further comprises low-pass filtering the modulation transfer function.

20. The method of claim 18, wherein the step of applying complex weighting factors further comprises high-pass filtering the modulation transfer function.

21. The method of claim 18, wherein the step of applying complex weighting factors further comprises band-pass filtering the modulation transfer function.

22. The method of claim 17, wherein the step of range gating the plurality of return signals further comprises performing short-time-window fast Fourier transform on the plurality of return signals to divide the plurality of return signals into the plurality of range slices.

23. The method of claim 17, wherein the step of repeating the steps of performing an autocorrelation and performing an inverse Fourier transform is performed in parallel such that each one of the plurality of range slices is processed in parallel.

24. A three-dimensional radar imaging system comprising:

at least a first transmit antenna and a second transmit antenna adapted to alternately transmit radar waveforms to illuminate a target region extending in a range direction and in an azimuth direction wherein;

the first transmit antenna illuminates the target region from a first angle; and

the second transmit antenna illuminates the target region from a second angle;

a receive antenna divided into a plurality of array segments, wherein the receive antenna is adapted to receive a radar return from the target region and to generate a plurality of return signals associated with corresponding ones of the plurality of array segments;

a range processor adapted to receive the plurality of return signals from the receive antenna and to divide them into a plurality of range slices, wherein each one of the plurality of range slices comprises a plurality of azimuth signals associated with corresponding ones of the plurality of array segments;

a modulation-transfer-function (MTF) processor adapted to receive the plurality of azimuth signals from one of the plurality of range slices at a time and to auto-correlate the plurality of azimuth signals to generate a plurality of spatial-frequency signals;

a Fourier transform processor adapted to receive the plurality of spatial-frequency signals and to generate an inverse Fourier transform of the spatial-frequency signals to generate an azimuth image slice associated with the one of the plurality of range slices received by the MTF processor; and

an image processor adapted to combine azimuth images slices associated with ones of the plurality of range slices from the first transmit antenna and azimuth image slices associated with ones of the plurality of range slices from the second transmit antenna to create a three-dimensional radar image.

25. The three-dimensional radar imaging system of claim 24, wherein the first transmit antenna and the second transmit antenna are separated in a substantially horizontal direction.

26. The three-dimensional radar imaging system of claim 24, wherein the first transmit antenna and the second transmit antenna are separated in a substantially vertical direction.

27. The three-dimensional radar imaging system of claim 24, wherein the plurality of array segments of the receive antenna comprise a sparse linear array whereby ones of the plurality of array segments are positioned along a straight line such that spacings between pairs of the plurality of range segments are irregular.

28. The three-dimensional radar imaging system of claim 27, wherein the sparse linear array is further adapted to comprise a minimally redundant array whereby a number of pairs of array segments having equal spacing is minimized.

29. The three-dimensional radar imaging system of claim 24, wherein the range processor further comprises a range-Fourier-transform processor adapted to perform short-time-window fast Fourier transforms of the plurality of return signals to divide the plurality of return signals into the plurality of range slices.