Auto-focusing correction for rotational acceleration effects on inverse synthetic aperture radar images

Claim 1

A method for improving an inverse synthetic aperture radar image by enhancing the radar return image signals that are utilized to form an image of a target that is undergoing rotational acceleration and that may also be undergoing range change and translational acceleration so that range change, translational acceleration and rotational acceleration distortion components that result from the relative target motion may be present in said radar return image signals comprising the steps of: (a) sampling said radar return image signals at a plurality of initially equal time intervals to provide a plurality of digital sub-image data representations of said radar return image signals wherein said sub-images collectively span said image of said target, (b) forming a plurality of sub-aperture digital data storage areas for each of said sub-images into which said digital sub-image data representations are respectively collected over said image of said target, (c) computing a frequency spectrum for each sub-aperture to produce a magnitude versus frequency data plot for each sub-aperture, (d) subdividing said frequency spectrum of each sub-image into a plurality of magnitude versus frequency plots of sub-swaths which collectively span said associated sub-image, (e) estimating and storing change-of-frequency data in each sub-swath between selected pairs of sub-apertures, (f) generating a change-of-frequency versus frequency data plot for all of said selected sub-swaths for each selected pair of sub-apertures, (g) selecting a plurality of times each of which is a mid-point time for a selected sub-aperture pair, (h) curve-fitting said change-of-frequency versus frequency data to form a plurality of data plots which are each associated with one of said mid-point times, (i) measuring the change-of-frequency intercept value for each of said data plots on the change-of frequency axis of said change-of-frequency versus frequency data plot, (j) measuring the slope value of each of said data plots of the change-of-frequency versus frequency data plot, (k) providing a polynomial fitting of the polynomial represented by said intercept values versus scan-time, (l) providing a polynominal fitting of the polynominal representation by said slope values, (m) twice integrating said intercept polynomials and slope polynomials with respect to time, and (n) replacing said radar return image signals with image signals that have: (1) range change and translational acceleration distortion components eliminated by utilizing the values obtained from the twice-integrated change-of-frequency intercept values versus time data plots to develop phase correction signals for said radar return image signals, and (2) rotational acceleration distortion components eliminated by utilizing the values obtained from the twice-integrated slope values versus time data plot to modify said initial equal sampling time intervals to compensate for frequency-stretch errors of said radar return image signals.

Claim 2

A method for improving an inverse synthetic aperture radar image by enhancing the radar return image signals that are utilized to form an image of a target that is undergoing rotational acceleration and that may also be undergoing range change and translational acceleration so that range change, translational acceleration and rotational acceleration distortion components that result from the relative target motion may be present in said radar return image signals comprising the steps of: (a) sampling said radar return image signals at a plurality of initially equal time intervals to provide a plurality of digital sub-image data representations of said radar return image signals wherein said sub-images collectively span said image of said target, (b) forming a plurality of sub-aperture digital data storage areas for each of said sub-images into which said digital sub-image data representations are respectively collected over said image of said target, (c) computing a frequency spectrum for each sub-aperture to produce a magnitude versus frequency data plot for each sub-aperture, (d) subdividing said frequency spectrum of each sub-image into a plurality of magnitude versus frequency plots of sub-swaths which collectively span said associated sub-image, (e) estimating and storing change-of-frequency data in each sub-swath between selected pairs of sub-apertures, (f) generating a change-of-frequency versus frequency data plot for all of said selected sub-swaths for each selected pair of sub-apertures, (g) selecting a plurality of times each of which is a mid-point time for a selected sub-aperture pair, (h) curve-fitting said change-of-frequency versus frequency data to form a plurality of data plots which are each associated with one of said mid-point times, (i) measuring the change-of-frequency intercept value for each of said data plots on the change-of frequency axis of said change-of-frequency versus frequency data plot, (j) measuring the slope value of each of said data plots of the change-of-frequency versus frequency data plot, (k) providing a polynomial fitting of the polynomial represented by said intercept values versus scan-time, (l) providing a polynominal fitting of the polynominal representation by said slope values, (m) twice integrating said intercept polynomials and slope polynomials with respect to time, and (n) replacing said radar return image signals with image signals that have: (1) range change and translational acceleration distortion components eliminated by utilizing the values obtained from the twice-integrated change-of-frequency intercept values versus time data plots to develop phase correction signals for said radar return image signals, and (2) rotational acceleration distortion components eliminated by utilizing the values obtained from the twice-integrated slope values versus time data plot to modify said initial equal sampling time intervals to compensate for frequency-stretch errors of said radar return image signals.

Claim 13

A method as claimed in any of the claims 1-6 or 10-11 wherein said computing step is accomplished by using a Fast Fourier Transformation.