Method and system for forming very low noise imagery using pixel classification

• US 8,193,967 B2
• Filed: 09/14/2010
• Issued: 06/05/2012
• Est. Priority Date: 12/10/2008
• Status: Expired due to Fees

• Current Assignee: The United States of America As Represented By The Secretary of Agriculture
• Original Assignee: The United States Of America As Represented By The Secretary Of The Army
• Inventors: Nguyen, Lam Huy, Sichina, Jeffrey P.
• Primary Examiner(s): Brainard, Timothy A
• Application Number:
  US12/881,364
• Publication Number:
  US 20110012778A1
• Time in Patent Office: 630 Days
• Field of Search: 342/25.F, 342/190
• US Class Current: 342/25.F
• CPC Class Codes: G01S 13/9017 with time domain processing of the SAR signals in azimuth G01S13/9005 takes precedence
• CPC Class Codes: G01S 13/9089 SAR having an irregular aperture
• CPC Class Codes: G01S 7/414 Discriminating targets with respect to background clutter

Abstract

A method and system for generating images from projection data comprising inputting first values representing correlated positional and recorded data; forming an image by processing the projection data utilizing a pixel characterization imaging subsystem to form the SAR imagery utilizing one of a back-projection algorithm or range migration algorithm; integrating positional and recorded data from many aperture positions, comprising: forming the complete aperture A₀ comprising collecting the return radar data, the coordinates of the receiver, and the coordinates of the transmitter for each position k along the aperture of N positions; forming an imaging grid comprising M image pixels; selecting and removing a substantial number of aperture positions to form a sparse aperture A_ifor L iterations; classifying each pixel in the image into target class based on the statistical distribution of its amplitude across L iterations; otherwise, the pixel is given the value of zero.

Claims

1. A method for generating images from projection data comprising:

inputting from at least one data receiving element first values representing correlated positional and recorded data;

each of said first values forming a point in an array of N aperture positions;

forming an image by processing the projection data utilizing a pixel characterization imaging subsystem that combines the positional and recorded data to form the SAR imagery utilizing one of a back-projection algorithm or range migration algorithm;

integrating positional and recorded data from many aperture positions, comprising;

forming the complete aperture for SAR image formation comprising collecting the return radar data, the coordinates of the receiver, and the coordinates of the transmitter for each position along the complete aperture of N positions;

forming an imaging grid comprising M image pixels wherein each pixel P_i in the imaging grid is located at coordinate (xp(i),yp(i), z_p(i));

selecting and removing a substantial number of aperture positions from the aperture of N positions to form a sparse aperture array;

forming a preliminary image using the sparse aperature array;

repeating the selecting and removing step and forming a preliminary image step for L iterations;

classifying each pixel in the image into either target class based on the statistical distribution of its amplitude across L iterations;

whereby if an image pixel is classified so as to be associated with a physical object, its value is computed from its statistics;

otherwise, the pixel is assumed to come from a non-physical object and is given the value of zero.

2. The method of claim 1 wherein the number of aperture positions removed to form the sparse aperture is in the range of approximately twenty to fifty per cent.

3. The method of claim 1 wherein the step of selecting and removing aperture positions comprises arbitrarily removing approximately twenty to fifty percent of the aperture positions from the aperture of N positions each time from substantially random locations within the complete aperture, such that each time the step is performed, substantially the same number of aperture positions at substantially a different set of locations is removed.

4. The method of claim 1 wherein the step of selecting and removing aperture positions comprises arbitrarily removing a predetermined percentage of aperture positions each time from random locations within the complete aperture, such that each time the step is performed, substantially the same number of aperture positions at substantially a different set of locations is removed.

5. The method of claim 1, further comprising the step of processing return radar signals using a signal processing subsystem, the signal processing system comprising at least one signal processing algorithm performed to suppress unwanted noise from the returned radar data.

6. The method of claim 1 wherein the first aperture position in the array represents the positional information and the recorded data obtained during the first increment;

the k^th aperture position in the array represents the combined positional information and recorded data obtained by the at least one data receiving element during the k^th increment;

where k is a number greater than 1;

and wherein during the formation of the sparse aperture arrays approximately twenty to fifty percent of the aperture positions are removed from the complete aperture randomly.

7. The method of claim 1 wherein the steps of selecting and removing a substantial number of aperture positions to form a sparse aperture, and forming a preliminary image for L iterations comprises:

(a) forming the next array by randomly removing approximately twenty to fifty percent of the aperture positions from the from the complete aperture of N positions;

(b) forming the next preliminary image using the next array of data points; and

repeating (a) and (b) until L iterations are completed; and

wherein the step of classifying each pixel comprises analyzing the statistical distribution of each pixel across all iterations to classify the pixel into target class or noise class based upon the statistical analysis of the pixels;

whereby the values of each pixel are examined across all iterations and if the normalized fluctuation is greater than a threshold value, the pixel is assigned a value of zero and otherwise the pixel is assigned a value which is one of the maximum, average or mean value across the iterations.

8. A method for generating images of a scanned area from projection data comprising:

inputting scanned data having first values representing correlated positional and recorded data;

each of said first values forming an aperture position;

forming an initial aperture array consisting of N aperture positions, each aperture position comprising correlated positional and recorded data;

forming an imaging grid comprising image pixel locations;

forming an initial image by generating backprojection values for each pixel location by coherently summing the scanned data collected at each aperture position into the initial aperture array of positions;

for a series of iterations, generating a series of secondary aperture arrays using at least one processor by using different subsets of aperture positions from the initial aperture array for each iteration;

forming pixels for the imaging grid from the secondary aperture arrays;

comparing the values of the pixels generated from the secondary aperture arrays and classifying each pixel as either target data or that originating from noise or a sidelobe based upon the magnitude and frequency of occurrence during the iterations;

forming the image

whereby the value of each pixel is examined across L iterations to determine whether or not the pixel represents a physical object.

9. The method of claim 8 wherein each pixel used to form the image has its maximum value across all iterations and wherein for each pixel location in the image grid, the decision as to whether or not the pixel represents a target is based upon a decision statistic derived from the standard deviation of the amplitudes of the pixel P_ij across L iterations, and this standard deviation is normalized by the mean value of the amplitudes.

10. The method of claim 8 wherein the step of generating a series of secondary aperture arrays comprises removing approximately twenty to fifty percent of the aperture positions from the initial array to form a secondary aperture array and wherein the same predetermined percentage of aperture positions are removed for each iteration with the selection of the aperture positions subject to removal being computer generated.

11. The method of claim 10 wherein each time the step of removing approximately twenty to fifty percent of the aperture positions from the array comprises removing a predetermined percentage of aperture positions each time from random locations within the array, such that each time the step is performed, substantially the same number of aperture positions at substantially a different set of locations is removed.

12. The method of claim 11 wherein the array of aperture positions represents radar data obtained by k radar receivers, each row of data being derived from a separate radar receiver, and wherein columns may be formed in the array comprising data from a different time interval in which data is received by each of the k radar receivers.

13. The method of claim 8 wherein the step of inputting data comprises scanning an area using at least one data receiving element to obtain first values representing combined positional and recorded data;

each of said first values forming a point in an array of k aperture positions.

14. The method of claim 8 wherein the steps of generating series of secondary aperture arrays and forming pixels for the imaging grid from the secondary aperature arrays are repeated over a series of iterations until the desired image resolution is obtained.

15. A system for generating images from projection data comprising:

at least one processor for processing image information;

the at least one processor having an input for inputting scanned data having first values representing correlated positional and recorded data;

each of said first values forming a point in an array of aperture positions;

the at least one processor operating to perform the following steps;

(a) forming an initial aperture array consisting of N aperture positions, each aperture position comprising radar receiving position information, radar transmitting information, and the data record that was measured at the target location;

(b) forming an imaging grid comprising image pixel locations;

(c) forming an initial image using backprojection values for each pixel location by coherently summing the scanned data collected at each data point into the initial aperture array of aperture positions;

(d) for a series of iterations, generating a series of secondary aperture arrays using at least one processor by using different subsets of aperture positions from the initial aperture array for each iteration;

(e) comparing the values of the pixels generated from the secondary aperture arrays and classifying each pixel as either target data or that originating from noise or a sidelobe based upon the magnitude and frequency of occurrence during the iterations;

(f) forming the image

whereby the value of each pixel is examined across L iterations to determine whether or not the pixel represents a physical object.

16. The system of claim 15 wherein the step of generating a series of secondary aperture arrays comprises removing approximately twenty to fifty percent of the aperture positions from the initial array to form a secondary aperture array and wherein the same predetermined percentage of aperture positions are removed for each iteration with the selection of the aperture positions subject to removal being computer generated.

17. The system of claim 15 wherein the scanned data is inputted from a plurality of data receiving elements that scan substantially the same area and three dimensional images are obtained using signal data from each receiving element that includes the magnitude of the signal, the angle from which the signal was derived, the time of receipt, and the positional information of the receiving element.

18. The system of claim 17 wherein a target can be located by inputting data from two receiving elements, determining the respective angles from signals reflected from the target obtained by both receiving elements, and determining the target location based upon the angles at which the signals are reflected, the time at which the signal is received, and the positional information of the receiving elements.

19. The system of claim 15 wherein the step of generating a series of secondary aperture arrays comprises forming the series of secondary aperture arrays by randomly deleting aperture positions from the secondary aperature arrays for each iteration.

20. The system of claim 15 wherein the step of generating a series of secondary aperture arrays comprises, for each iteration, deleting approximately twenty to fifty percent of the aperture positions each time from random locations within the array, such that each time the step is performed, substantially the same number of aperture positions at substantially a different set of locations is removed.