Method for range alignment and rotation correction of a high resolution image in an inverse synthetic aperture radar system

• US 6,255,981 B1
• Filed: 08/04/1999
• Issued: 07/03/2001
• Est. Priority Date: 08/04/1999
• Status: Expired due to Term

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Raytheon Company (Rtx Corporation)
• Inventors: Samaniego, Raymond
• Primary Examiner(s): Lobo, Ian J.
• Application Number:
  US09/368,867
• Time in Patent Office: 699 Days
• Field of Search: 342/25, 342/195, 342/191
• US Class Current: 342/25.00R
• CPC Class Codes: G01S 13/904 SAR modes

Abstract

A method for range alignment and rotation correction of a high resolution image in an inverse synthetic aperture radar (ISAR) system is provided that includes an ISAR image generator (14). The ISAR image generator (14) receives a full aperture (24) of data samples (20) that is then subdivided into a plurality of subapertures (26). A coarse image generator (40) generates a coarse image (70) for each subaperture (26). A composite image generator (42) generates a composite magnitude image (72) and a composite power image (74) from the coarse images (70). A point select module (44) uses the composite magnitude image (72) and the composite power image (74) to select a set of prominent points (76). A range alignment module (46) uses the coarse images (70) and the prominent points (76) to determine a range alignment correction for each coarse image (70). A rotation correction module (48) uses the composite power image (74) and the prominent points (76) to determine a point of rotation and rotational correction for each coarse image. A coarse image correction module (50) applies the range alignment correction and the rotational correction to each coarse image (70). An autofocus module (52) uses a phase gradient autofocus algorithm to correct phase errors occurring across coarse images (70). Coarse image combiner (54) combines the set of coarse images (70) into a single higher resolution image (75). Image formatter (56) generates ISAR image (58) for display on a display device (15) from high resolution image (75).

Claims

1. An Inverse Synthetic Aperture Radar imaging method, comprising:

generating a plurality of coarse images in response to receiving a full aperture of range compressed data samples, each coarse image generated from an overlapping subaperture of the range compressed data samples and containing a radar image of a target;

selecting prominent points within each of the plurality of coarse images;

aligning the radar images of the plurality of coarse images using the prominent points to remove translational movement and range migration between coarse images;

removing phase errors between the plurality of coarse images using a phase gradient autofocus;

combining the plurality of coarse images to form a high resolution image; and

formatting the high resolution image for display on a display device.

2. The method of claim 1 wherein the step of selecting prominent points includes:

generating a composite magnitude image and a composite power image form the plurality of coarse images;

locating prominent points in the composite power image by identifying bins with radar return intensity greater than a first threshold and variance below a second threshold.

3. The method of claim 1 wherein the step of formatting the high resolution image includes:

centering the high resolution image in a viewable area of the display device; and

mapping a value in each bin of the high resolution image to a gray scale corresponding to an intensity value of the bin.

4. An Inverse Synthetic Aperture Radar imaging method, comprising:

generating a plurality of coarse images in response to receiving a full aperture of range compressed data samples, each coarse image generated from an overlapping subaperture of the range compressed data samples and containing a radar image of a target;

selecting prominent points from the plurality of coarse images;

aligning the radar images on the plurality of coarse images using the prominent points to remove translational movement and range migration between coarse images;

removing phase errors between the plurality of coarse images using a phase gradient autofocus;

combining the plurality of coarse images to form a high resolution image;

formatting the high resolution image for display on a display device;

wherein the step of aligning the radar images includes;

locating the prominent points on each of the plurality of coarse images;

calculating a range alignment factor for each coarse image, the range alignment factor indicating direction and magnitude of a shift in the radar image to place the prominent points in the same relative location on each of the coarse images;

calculating a rotational correction factor for each column in each coarse image to compensate for range migration; and

applying the range alignment factors and the rotational correction factors to each coarse image.

5. The method of claim 4, wherein the step of calculating a range alignment factor for each coarse image, includes:

placing a range window in a range area where each prominent point is expected to be located;

calculating a prominent point centroid of bins within the range window, the centroid identifying the location of the prominent point;

calculating an overall centroid of the prominent point centroids, the overall centroid indicating a shift in range to range align the plurality of coarse images; and

reducing the range window size and repeating the calculating a range alignment factor step until the difference between the previous overall centroid and the current overall centroid does not exceed a third threshold value.

6. The method of claim 4, wherein the step of calculating a rotation correction factor includes:

locating a radar image pivot point by calculating a centroid of the prominent points in a composite power image; and

calculating the rotation correction factor using the radar image pivot point.

7. A method of range aligning and rotationally correcting radar image errors in an Inverse Synthetic Aperture Radar system, comprising:

dividing a full aperture of radar pulse return data into a plurality of overlapping subapertures, each overlapping subaperture including a coarse radar image formed from a plurality of radar pulse returns;

determining one or more prominent points from the overlapping subapertures;

locating the prominent points in each overlapping subaperture;

calculating a range alignment factor for each overlapping subaperture from the location of the prominent points in each overlapping subaperture, the range alignment factor indicating a shift in the radar image to align the prominent points in each overlapping subaperture;

calculating a rotation correction factor for each column in the coarse radar image in each overlapping subaperture, the rotation correction factor removing Doppler frequency shifts from the coarse radar image;

applying the range alignment factors and the rotation correction factors to the plurality of overlapping subapertures; and

combining the plurality of overlapping subapertures into a single high resolution image for display on a display device.

8. The method of claim 7, wherein the step of applying the range alignment factors and the rotation correction factors includes:

summing the range alignment factor for each overlapping subaperture and the rotational correction factor for each column in each overlapping subaperture giving a total correction; and

shifting the radar image in each overlapping subaperture by the direction and magnitude indicated by the total correction.

9. The method of claim 7, further comprising:

generating a composite magnitude image and a composite power image from the plurality of overlapping subapertures, the composite magnitude image and the composite power image used to determine the one or more prominent points.

10. An Inverse Synthetic Aperture Radar imaging system, comprising:

a coarse image generator operable to receive a full aperture of range compressed received reflected radar pulses and to create a plurality of coarse images from overlapping subapertures of the range compressed received reflected radar pulses, the coarse images including a radar image of a target;

a composite image generator operable to create a composite magnitude image by combining the data in the plurality of coarse images and a composite power image by combining the squares of the data in the plurality of coarse images;

a point select module operable to select one or more prominent points in the range compressed received reflected radar pulses by identifying bins with an intensity above a first threshold and a variance below a second threshold using the composite magnitude image and the composite power image;

an adjustment module operable to align the radar images in the plurality of coarse images in the same relative range location, the adjustment module further operable to remove range migration from the plurality of coarse images;

an autofocus module operable to apply a phase gradient autofocus to the plurality of coarse images to correct phase errors across the range aligned and rotationally corrected coarse images;

a coarse image combiner operable to combine the plurality of coarse images into a single high resolution image; and

an image formatter operable to prepare the single high resolution image for display on a display device.

11. The system of claim 10, wherein the adjustment module further comprises:

a range alignment module operable to determine a range alignment factor for each coarse image;

a rotational correction module operable to determine a rotation correction factor along a Doppler frequency axis to remove range migration from each coarse image; and

a coarse image correction module operable to shift the radar image in each of the plurality of coarse images using the range alignment factors and the rotation correction factor.

12. A method for aligning a plurality of coarse images in an inverse Synthetic Aperture Radar system, comprising:

selecting prominent points from a plurality of coarse images, each coarse image containing a radar image of a target;

locating the prominent points on each of the plurality of coarse images;

calculating a range alignment factor for each coarse image, the range alignment factor indicating direction and magnitude of a shift in the radar image to place the prominent points in the same relative location on each of the coarse images;

calculating a rotational correction factor for each column in each coarse image to compensate for range migration; and

applying the range alignment factors and the rotational correction factors to each coarse image.

13. The method of claim 12, wherein the step of calculating a range alignment factor for each coarse image, includes:

placing a range window in a range area where each prominent point is expected to be located;

calculating a prominent point centroid of bins within the range window, the centroid identifying the location of the prominent point;

calculating an overall centroid of the prominent point centroids, the overall centroid indicating a shift in range to range align the plurality of coarse images; and

reducing the range window size and repeating the calculating a range alignment factor step until the difference between the previous overall centroid and the current overall centroid does not exceed a third threshold value.

14. The method of claim 12, wherein the step of calculating a rotation correction factor includes:

locating a radar image pivot point by calculating a centroid of the prominent points in a composite power image; and

calculating the rotation correction factor using the radar image pivot point.