Extraction of relative signals from closely spaced targets in a monopulse system

• US 7,911,371 B2
• Filed: 02/26/2009
• Issued: 03/22/2011
• Est. Priority Date: 02/26/2009
• Status: Active Grant

• Current Assignee: Northrop Grumman Guidance And Electronics Company, Inc.
• Original Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Inventors: Karam, Mostafa
• Primary Examiner(s): Tarcza; Thomas H
• Assistant Examiner(s): Brainard; Timothy A
• Application Number:
  US12/393,656
• Publication Number:
  US 20100214156A1
• Time in Patent Office: 754 Days
• Field of Search: 342/25
• US Class Current: 342/25.00C
• CPC Class Codes: G01S 13/4454 phase comparisons monopulse, i.e. comparing the echo signals received by an interferometric antenna arrangement

Abstract

Systems and methods are provided for extracting relative signal parameters representing two closely spaced targets from monopulse scan data. A maximum quadrature angle value from the scan data is compared with a threshold quadrature value representing a noise level. A linear polynomial model is utilized if the maximum quadrature angle exceeds the threshold value. The linear polynomial model fits a function of the azimuth angle values and quadrature angle values to a linear function of an exponential parameter derived from the boresight angles to produce polynomial coefficients and determines the relative signal parameters from the polynomial coefficients. A cubic polynomial model is utilized if the maximum quadrature angle fails to exceed the threshold value. The cubic polynomial model fits azimuth angle values to a cubic function of corresponding boresight angles to produce a set of polynomial coefficients and determines the relative signal parameters from the set of polynomial coefficients.

Claims

1. A method for extracting at least one relative signal parameter representing two closely spaced targets from monopulse scan data, comprising a quadrature angle value and a merged azimuth angle value for each plurality of boresight angles, from a monopulse scanner comprising:

comparing, at a monopulse processor, a maximum quadrature angle value from the quadrature angle values associated with the plurality of boresight angles with a threshold quadrature value representing a noise level of the monopulse scan data stored in a memory;

extracting, at the monopulse processor the at least one relative signal parameter according to a cubic polynomial model that is configured to fit the merged azimuth angle values to a cubic function of their corresponding boresight angles to produce a set of polynomial coefficients and determine the at least one relative signal parameter from the set of polynomial coefficients if the maximum quadrature angle fails to exceed the threshold quadrature value; and

extracting, at the monopulse processor, the at least one relative signal parameter according to a linear polynomial model that is configured to fit a function of the merged azimuth angle data stored in the memory associated with each boresight angle and the quadrature angle value associated with the boresight angle to a linear function of an exponential parameter that is an exponential function of the boresight angle to produce a first order coefficient and a constant value and determine the at least one relative signal parameter from the first order coefficient and the constant value if the maximum quadrature angle exceeds the threshold quadrature value

wherein the monopulse processor comprises a processing unit for accessing the memory and executing computer executable instructions stored in the memory.

2. The method of claim 1, wherein extracting the at least one relative signal parameter according to a cubic polynomial model comprises:

fitting the merged azimuth angle values to a cubic function of their corresponding boresight angles to produce the set of polynomial coefficients;

determining an azimuth difference as a difference between a first azimuth angle value for a first target and a second azimuth angle value for a second target and an azimuth mean as a mean of the first azimuth angle value and a second azimuth angle value from the set of polynomial coefficients; and

calculating at least one relative signal parameter representing the two closely spaced targets as a function of the azimuth difference, the azimuth mean, and the set of polynomial coefficients.

3. The method of claim 1, wherein extracting the at least one relative signal parameter according to a linear polynomial model comprises:

calculating, for each antenna boresight angle, the exponential parameter as an exponential function of the boresight angle;

computing, for each boresight angle, a modified ratio value as a function of the merged azimuth angle data associated with the boresight angle and the quadrature angle value associated with the boresight angle;

fitting the modified ratio values for the plurality of boresight angles to a linear function of the exponential parameters for the plurality of boresight angles via a linear regression analysis to provide the first order coefficient value and the constant value; and

calculating at least one relative signal parameter representing the two closely spaced targets as a function of the first order coefficient value and the constant value.

4. The method of claim 1, the at least one relative signal parameter comprising a phase difference between respective signals representing the two closely spaced targets and a radar cross section of the targets.

5. The method of claim 1, further comprising displaying a representation of the at least one relative signal parameter at an associated display.

6. A system for extracting at least one relative signal parameter representing two closely spaced targets from monopulse scan data, comprising a quadrature angle value and a merged azimuth angle value for each plurality of boresight angles, from a monopulse scanner comprising:

an exponential parameter generator configured to calculate, for each antenna boresight angle, an exponential parameter that is an exponential function of the boresight angle;

a modified ratio generator configured to compute, for each boresight angle, a modified ratio value as a function of the merged azimuth angle data associated with the boresight angle and the quadrature angle value associated with the boresight angle;

a linear regression component configured to fit the modified ratio values for the plurality of boresight angles to a linear function of the exponential parameters for the plurality of boresight angles via a linear regression analysis to provide a first order coefficient value and a constant value; and

a relative signal determination component configured to calculate at least one relative signal parameter representing the two closely spaced targets as a function of the first order coefficient value and the constant value.

7. The system of claim 6, the exponential parameter generator being receiving a difference between the azimuth angles of the closely spaced targets and a mean of the azimuth angles of the closely spaced targets and calculating, for each boresight angle, an exponential parameter that is an exponential function of the boresight angle.

8. The system of claim 6, the modified ratio generator being configured to compute, for each boresight angle, a modified ratio value as a ratio of a deviation of the merged azimuth angle data associated with the boresight angle from an azimuth angle of one of the targets and the quadrature angle value associated with the boresight angle.

9. The system of claim 6, the relative signal determination component being configured to determine a phase difference between respective signals representing the two targets as the arctangent of the inverse of the constant value produced by the linear regression component.

10. The system of claim 9, the relative signal determination component being configured to select one of a plurality of angular ranges for the determined phase difference according to the signs of the first order coefficient value and the constant value produced by the linear regression component.

11. The system of claim 9, the relative signal determination component being configured to determine a radar cross section ratio between the two closely spaced targets as the square of the product of the first order coefficient and a sine of the phase difference.

12. The system of claim 6, further comprising a model selection element that is configured to compare a maximum quadrature angle value from the quadrature angle values associated with the plurality of boresight angles to a threshold value representing a noise level of the monopulse scan data, each of the exponential parameter generator, the modified ratio generator, the linear regression component, and the relative signal determination component operating to calculate at least one relative signal parameter only if the maximum quadrature angle value exceeds the threshold value.

13. The system of claim 12, further comprising a cubic polynomial model that is configured to fit the merged azimuth angle values to a cubic function of their corresponding boresight angles to provide a set of polynomial coefficients and determine the at least one relative signal parameter from the set of polynomial coefficients, the model selection component selecting the cubic polynomial model if the maximum quadrature angle value fails to exceed the threshold value.

14. A system for extracting at least one relative signal parameter representing two closely spaced targets from monopulse scan data, comprising a quadrature angle value and a merged azimuth angle value for each plurality of boresight angles, from a monopulse scanner comprising:

a multilinear regression element configured to fit the merged azimuth angle values to a cubic function of their corresponding boresight angles to produce a set of polynomial coefficients;

an azimuth angle calculator configured to determine an azimuth difference as a difference between a first azimuth angle value for a first target and a second azimuth angle value for a second target and an azimuth mean as a mean of the first azimuth angle value and a second azimuth angle value from the set of polynomial coefficients; and

a relative signal determination element configured to calculate at least one relative signal parameter representing the two closely spaced targets as a function of the azimuth difference, the azimuth mean, and the set of polynomial coefficients.

15. The system of claim 14, the azimuth angle calculator being configured to calculate the azimuth mean as a cubic function of a central boresight angle in which each coefficient in the cubic function of the central boresight angle is a function of at least one of the set of polynomial coefficients.

16. The system of claim 14, the azimuth angle calculator being configured to calculate a magnitude of a signal ratio of the closely spaced targets at the central boresight angle as a function of the set of polynomial coefficients and calculate the azimuth difference as a function of the magnitude of the signal ratio of the closely spaced targets at the central boresight angle.

17. The system of claim 16, the relative signal determination element being configured to determine a radar cross section ratio between the two closely spaced targets as a function of the magnitude of the signal ratio of the closely spaced targets at the central boresight angle, the central boresight angle, the azimuth mean, and the azimuth difference.

18. The system of claim 16, the relative signal determination element being configured to determine a phase difference between respective signals representing the two closely spaced targets as a function of the azimuth difference, the magnitude of the signal ratio of the closely spaced targets at the central boresight angle, and a quadrature angle value associated with the central boresight angle.

19. The system of claim 14, further comprising a model selection element that is configured to compare a maximum quadrature angle value from the quadrature angle values associated with the plurality of boresight angles to a threshold value representing a noise level of the monopulse scan data, each of the multilinear regression element, the azimuth angle calculator, and the relative signal determination element operating to calculate at least one relative signal parameter only if the maximum quadrature angle value fails to exceed the threshold value.

20. The system of claim 19, further comprising a linear polynomial model that is configured to fit a function of the merged azimuth angle data associated with each boresight angle and the quadrature angle value associated with the boresight angle to a linear function of an exponential parameter that is an exponential function of the boresight angle to produce a first order coefficient and a constant value and determine the at least one relative signal parameter from the first order coefficient and the constant value, the model selection component selecting the linear polynomial model if the maximum quadrature angle value fails to exceed the threshold value.