Communications: directive radio wave systems and devices (e.g. – Synthetic aperture radar
Reexamination Certificate
1999-08-04
2001-07-03
Lobo, Ian J. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Synthetic aperture radar
C342S195000
Reexamination Certificate
active
06255981
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates in general to the field of radar imaging systems, and more particularly to a method for range alignment and rotation correction of a high resolution image in an Inverse Synthetic Aperture Radar system.
BACKGROUND OF THE INVENTION
Inverse Synthetic Aperture Radar (ISAR) is a radar signal processing technique used to form images of moving targets. The ISAR may be mounted on a movable platform such as an airplane or ship. For example, an airplane may use an ISAR to produce radar images of a speedboat. A two-dimensional image can be formed by separating radar returns from an object in range and frequency. Different points along a rotating object will have different line of sight velocities in the direction of the radar sensor thus giving different Doppler shifts. If no corrections are made, nonlinear Doppler shifts can often result in defocused images. Likewise, uncorrected range variations can cause blurred images.
Radar imaging systems generally use a tracker to maintain a target within the radar range gate. The tracker locates a high intensity point on the target to track, determines the range to the target, and controls the radar pulse timing. One of the challenges in ISAR is keeping a track on the moving target in real time in order to keep radar returns from each point on the object in the same radar sensor range bin over the data collection interval. For very high resolution radar systems, the challenge becomes even more difficult.
With ISAR, imaging of dynamic targets causes range tracker jitter (uncompensated translational motion) and range migration (rotational motion) that results in unfocused images with poor image quality. If digital pulse compression is used, range tracker jitter can induce a large amount of phase noise in the target radar returns resulting in poor image quality.
Due to the aforementioned problems, conventional ISAR systems are inadequate to generate high quality ultra-high resolution radar images.
SUMMARY OF THE INVENTION
From the foregoing, a need has arisen for a method for range alignment and rotation correction of a high resolution image in an Inverse Synthetic Aperture Radar system that provides a high resolution radar image. In accordance with the present invention, a method for range alignment and rotation correction of a high resolution image in an Inverse Synthetic Aperture Radar system is provided that substantially eliminates or reduces disadvantages or problems associated with conventional Inverse Synthetic Aperture Radar systems.
According to one embodiment of the present invention, there is provided a method for range alignment and rotation correction of a high resolution image in an Inverse Synthetic Aperture Radar system that comprises generating a plurality of coarse images from a full aperture of range compressed radar data samples. Each coarse image is generated from an overlapping subaperture of the data samples contained in the full aperture of data samples. Prominent points are selected from the plurality of coarse images. The radar images existing on the plurality of coarse images are aligned using the prominent points to remove translational movement and range migration that may exist between the coarse images. Phase errors are then removed from the plurality of coarse images using a phase gradient autofocus. The plurality of coarse images are next combined to form a single image of higher resolution. The higher resolution image is then formatted for display on a display device.
Technical advantages of the present invention include providing an efficient method for range alignment and rotation correction of a high resolution image in an Inverse Synthetic Aperture Radar system. In particular, the system range aligns a series of coarse radar images to compensate for range tracker jitter errors. In addition, the system compensates for rotational movement of a target in the coarse radar images. The system combines these range-aligned and rotation-corrected coarse images to create a higher resolution radar image. Other examples may be readily ascertainable by those skilled in the art from the following figures, descriptions, and claims.
REFERENCES:
patent: H1720 (1998-04-01), Chen
patent: 4924229 (1990-05-01), Eichel et al.
patent: 5343204 (1994-08-01), Farmer et al.
Baker & Botts L.L.P.
Lobo Ian J.
Raytheon Company
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