Phase correction system for automatic focusing of synthetic aperture radar
First Claim
Patent Images
1. A synthetic aperture radar comprising:
- a directional radar antenna directed at a target area to be imaged and displaying relative movement with respect thereto;
means for supplying radar interrogation pulses to said antenna;
image formation processing means for receiving position indicating reflections from said target area and comparing said position indicating reflections with said interrogation pulses produced by said means for supplying, said image formation processing means using multiple samples of said position indicating reflections to develop target range and azimuth data representative of a target image;
at least one of said target range and azimuth data exhibiting arbitrary phase error between said samples, resulting in smearing of the target image; and
phase correction autofocus means for estimating and subtracting said phase error from said target image, said phase correction autofocus means including,autofocus processor means for,(a) dividing said target image into a plurality of target image portions,(b) locating the strongest highlight in each of said target image portions,(c) estimating the width of the smearing of each highlight in each dimension of said phase error for each of said target image portions,(d) estimating of the phase error in each said dimension thereof from said estimated smear width of said highlights, and(e) compensating for said phase error in said target image to produce compensated data for said target image.
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Abstract
A phase gradient autofocus system for use in synthetic aperture imaging accurately compensates for arbitrary phase errors in each imaged frame by locating highlighted areas and determining the phase disturbance or image spread associated with each of these highlight areas. An estimate of the image spread for each highlighted area in a line in the case of one dimensional processing or in a sector, in the case of two-dimensional processing, is determined. The phase error is determined using phase gradient processing. The phase error is then removed from the uncorrected image and the process is iteratively performed to substantially eliminate phase errors which can degrade the image.
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Citations
33 Claims
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1. A synthetic aperture radar comprising:
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a directional radar antenna directed at a target area to be imaged and displaying relative movement with respect thereto; means for supplying radar interrogation pulses to said antenna; image formation processing means for receiving position indicating reflections from said target area and comparing said position indicating reflections with said interrogation pulses produced by said means for supplying, said image formation processing means using multiple samples of said position indicating reflections to develop target range and azimuth data representative of a target image; at least one of said target range and azimuth data exhibiting arbitrary phase error between said samples, resulting in smearing of the target image; and phase correction autofocus means for estimating and subtracting said phase error from said target image, said phase correction autofocus means including, autofocus processor means for, (a) dividing said target image into a plurality of target image portions, (b) locating the strongest highlight in each of said target image portions, (c) estimating the width of the smearing of each highlight in each dimension of said phase error for each of said target image portions, (d) estimating of the phase error in each said dimension thereof from said estimated smear width of said highlights, and (e) compensating for said phase error in said target image to produce compensated data for said target image. - View Dependent Claims (2, 3, 4, 5, 6, 8, 9, 10, 11)
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7. The synthetic aperture radar of claim further comprising display means for displaying said target image from said compensated data.
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12. A method of producing an image of a target object from radar signals transmitted and received from a radar antenna exhibiting movement relative to said target comprising:
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(a) transmitting radar interrogation signals from said antenna moving relative to said target object; (b) receiving position indicating reflections from said target object produced from said radar interrogation signals; (c) comparing said position indicating reflections with said interrogation signals and using multiple samples of said position indicating reflections to develop target range and azimuth data representative of a target image, at least one of said target range and azimuth data exhibiting phase errors between said samples, resulting in smearing of the target image; and (d) estimating and subtracting said phase error from said at least one of said target range and azimuth data to produce a phase corrected target image by, (1) dividing said target image into a plurality of target image portions, (2) locating the strongest highlight in each of said target image portions, (3) estimating the width of the smearing of each highlight in each dimension of said phase error for each of said target image portions, (4) estimating of the phase error in each said dimension thereof from said estimated smear width of said highlights, and (5) compensating for said phase error in said target image to produce compensated data for said target image. - View Dependent Claims (13, 14, 15, 16, 17, 18, 19)
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20. A system of correcting phase error in a synthetic aperture imaging system which develops range and azimuth data of individual image frames, at least one of said range and azimuth data exhibiting phase error resulting in smear of the image formed thereby, said system comprising:
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input means for receiving said range and azimuth data from said synthetic aperture imaging system; means for dividing said target image into a plurality of target sub-image portions; highlight locating means receiving each sub-image portion developed by said means for dividing and locating the strongest highlight therein; first means for estimating the amount of phase error in each sub-image portion by estimating the degree of smear of the brightest highlight contained therein; second means for estimating the phase error in said image frame from the aggregate processing of the image error for each sub-image portion; and correction means for removing the estimated phase error from the target image frame. - View Dependent Claims (21, 22, 23, 24, 25, 26)
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27. A method of correcting phase errors in a synthetic aperture imaging system which develops range and azimuth data of individual image frames, at least one of said range and azimuth data exhibiting phase error resulting in smear of the image formed thereby, said method comprising:
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(a) dividing said range and azimuth data representing a single image frame into a number of data portions defining sub-image portions; (b) determining the brightest highlight in each sub-image portion; (c) using the brightest highlight in each sub-image portion to estimate phase error induced image smear for each sub-image portion of said image frame; and (d) estimating the phase error in said single image frame 15 from the aggregate processing of the estimated image smear of each said sub-image portion; and (e) removing the estimated phase error from said range and azimuth data defining said single image frame to form a phase error corrected image. - View Dependent Claims (28, 29, 30, 31, 32, 33)
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Specification