Phase difference auto focusing for synthetic aperture radar imaging
First Claim
1. An apparatus for automatically focusing range compressed SAR data stored in an array of range bins and azimuth lines comprising:
- means for unpacking the range compressed data in each range bin;
means for generating a first and second subarray from the unpacked data;
means for conjugating the first subarray;
means for multiplying the conjugated first subarray and the second subarray to create a first product array;
means for performing a fast Fourier transform of the first product array to produce an FFT filter bank output;
means for calculating the magnitude of the FFT filter to form a correlation function;
means for integrating the correlation function over the range bins;
means for detecting a peak in the correlation function after integrating over range bins;
means for calculating a quadratic phase error responsive to the detected peak; and
means for removing the calculated phase error from the range compressed data.
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Abstract
The present invention discloses an autofocusing method and apparatus for Synthetic Aperture Radar which employs operation on two or three subarrays of the SAR compressed data to estimate quadratic and cubic phase errors in the data. Focusing of the SAR array may then be accomplished by removing the estimated phase error from the data to sharpen the image. Phase error is estimated by FFT filtering product arrays formed by multiplying the complex conjugate of one subarray by another of the subarrays to form a correlation function. The correlation functions are integrated. A peak detection of the integrated correlation functions yields a peak location in the filter which is proportional to the phase error which may then be calculated.
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Citations
10 Claims
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1. An apparatus for automatically focusing range compressed SAR data stored in an array of range bins and azimuth lines comprising:
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means for unpacking the range compressed data in each range bin; means for generating a first and second subarray from the unpacked data; means for conjugating the first subarray; means for multiplying the conjugated first subarray and the second subarray to create a first product array; means for performing a fast Fourier transform of the first product array to produce an FFT filter bank output; means for calculating the magnitude of the FFT filter to form a correlation function; means for integrating the correlation function over the range bins; means for detecting a peak in the correlation function after integrating over range bins; means for calculating a quadratic phase error responsive to the detected peak; and
means for removing the calculated phase error from the range compressed data. - View Dependent Claims (2, 3, 4, 9, 10)
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5. A method for autofocusing a quadratic phase error from range compressed SAR data contained in an array of range bins and azimuth lines comprising the steps of:
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creating a first subarray of the SAR data; creating a second subarray of the SAR data; calculating the complex conjugate of the first taking the product of the complex conjugate of the first subarray and the second subarray; performing a fast Fourier transform on the product to produce an FFT filter bank; detecting the magnitude of the FFT filters to form a correlation function; integrating the correlation function over the range bins; detecting a peak in the correlation function after integrating overall range bins; calculating a quadratic phase error responsive to the detected peak; and correcting the range compressed data for the quadratic phase error calculated. - View Dependent Claims (6, 7, 8)
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Specification