Subaperture processing for clutter reduction in synthetic aperture radar images of ground moving targets

• US 6,426,718 B1
• Filed: 03/14/2000
• Issued: 07/30/2002
• Est. Priority Date: 03/14/2000
• Status: Expired due to Term

• Current Assignee: The Boeing Co.
• Original Assignee: The Boeing Co.
• Inventors: Ridgway, Robert I.
• Primary Examiner(s): Lobo, Ian J.
• Application Number:
  US09/525,642
• Time in Patent Office: 868 Days
• Field of Search: 342/25, 342/159, 342/160
• US Class Current: 342/160
• CPC Class Codes: G01S 13/534 based upon amplitude or phase shift resulting from movement of objects, with reference to the surrounding clutter echo signal, e.g. non coherent MTi, clutter referenced MTi, externally coherent MTi
• CPC Class Codes: G01S 13/9029 specially adapted for moving target detection within a single SAR image or within multiple SAR images taken at the same time

Abstract

A radar tracking system extracts moving target content from a single radar pulse stream. The radar tracking system has a single phase center antenna for receiving the radar pulse stream. The tracking system further includes a signal processing system for converting the radar pulse stream into a plurality of SAR images. Each image has a corresponding moving target content and a corresponding clutter content. The tracking system also includes a targeting system for canceling identical clutter content between the images. The signal processing system includes a synthetic subaperture system for generating a plurality of synthetic subapertures and defining a common reference point. The common reference point has known slant ranges with respect to the plurality of synthetic subapertures. A deramping module uses a unique deramping function to compute a deramped signal for each synthetic subaperture based on the known slant ranges. The signal processing system further includes an imaging system for generating SAR images for the deramped signals. The processing of a radar pulse stream from a single antenna allows antenna size to be reduced by a factor of two or more, and allows tracking of slowly moving targets.

Claims

1. A method for extracting moving target content from a radar pulse stream, the method comprising the steps of:

receiving the radar pulse stream from a single phase center antenna;

converting the radar pulse stream into a plurality of synthetic aperture radar images, each image having a corresponding moving target content and a corresponding clutter content; and

canceling identical clutter content between the images.

2. The method of claim 1 further including the steps of:

generating a plurality of synthetic subapertures based on the radar pulse stream;

defining a common reference point, the common reference point having known slant ranges with respect to the plurality of synthetic subapertures; and

computing a deramped signal for each synthetic subaperture based on the known slant range to the common reference point.

3. The method of claim 2 further including the step of generating synthetic aperture radar images for the deramped signals.

4. The method of claim 3 further including the step of performing a fast fourier transform on the deramped signals.

5. The method of claim 2 further including the step of generating a complex phase history for the radar pulse stream, wherein the complex phase history has a first dimension and a second dimension, the first dimension containing range data and the second dimension containing pulse-to-pulse data.

6. The method of claim 2 further including the step of applying a unique deramping function to each synthetic subaperture.

7. The method of claim 6 further including the step of multiplying each synthetic aperture by a complex conjugate of a complex phase modulated signal from the common reference point.

8. The method of claim 2 further including the step of storing the deramped signals to a computer readable storage medium.

9. The method of claim 1 further including the steps of:

sampling fine time pulses from the radar pulse stream at a fine time sample rate; and

sampling the fine time pulses at a coarse time sample rate.

10. The method of claim 9 wherein the fine time sample rate is selected such that range details can be resolved.

11. The method of claim 9 wherein coarse time sample rate is selected such that the fine time samples can be aligned.

12. The method of claim 1 wherein the plurality of synthetic aperture radar images includes first image and a second image, the method further including the step of subtracting the second image from the first image such that the moving target content is extracted from the images.

13. A method for converting a radar pulse stream into a plurality of synthetic aperture radar images, the method comprising the steps of:

generating a plurality of synthetic subapertures based on the radar pulse stream;

defining a common reference point, the common reference point having known slant ranges with respect to the plurality of synthetic subapertures;

computing deramped signals for the synthetic subapertures based on the known slant ranges and the radar pulse stream; and

generating synthetic aperture radar images for the deramped signals.

14. The method of claim 13 further including the step of applying a unique deramping function to each synthetic aperture.

15. The method of claim 14 further including the step of multiplying each synthetic aperture by a complex conjugate of a complex phase modulated signal from the common reference point.

16. The method of claim 13 further including the step of generating a complex phase history for the radar pulse stream, wherein the complex phase history has a first dimension and a second dimension, the first dimension containing range data and the second dimension containing pulse-to-pulse or azimuth data.

17. A radar tracking system comprising:

a single phase center antenna for receiving a radar pulse stream;

a signal processing system for converting the radar pulse stream into a plurality of synthetic aperture radar images, each image having a corresponding moving target content and a corresponding clutter content; and

a targeting system for cancelling identical clutter content between the images.

18. The tracking system of claim 17 wherein the signal processing system includes:

a synthetic subaperture system for generating a plurality of synthetic subapertures based on the radar pulse stream, the synthetic subaperture system further defining a common reference point having known ranges with respect to the plurality of synthetic subapertures;

a deramping module for computing deramped signals for the synthetic subapertures based on the known ranges and the radar pulse stream; and

an imaging system for generating synthetic aperture radar images for the deramped signals.

19. The tracking system of claim 18 wherein the deramping module includes a plurality of deramping submodules, where each deramping subaperture applies a unique deramping function to a corresponding synthetic subaperture.

20. The tracking system of claim 18 wherein the signal processing system further includes a phase history module for generating a complex phase history for the radar pulse stream.