Detection and resolution of closely spaced targets in a monopulse system

• US 7,667,638 B1
• Filed: 02/26/2009
• Issued: 02/23/2010
• Est. Priority Date: 02/26/2009
• Status: Expired due to Fees

• Current Assignee: Northrop Grumman Guidance And Electronics Company, Inc.
• Original Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Inventors: Karam, Mostafa
• Primary Examiner(s): Sotomayor; John B
• Application Number:
  US12/393,602
• Time in Patent Office: 362 Days
• Field of Search: 342/149, 342/152, 342/80, 342/91, 342194-195
• US Class Current: 342/149
• CPC Class Codes: G01S 13/44 Monopulse radar, i.e. simultaneous lobing
• CPC Class Codes: G01S 7/292 Extracting wanted echo-signals
• CPC Class Codes: G01S 7/2923 based on data belonging to a number of consecutive radar periods
• CPC Class Codes: G01S 7/295 Means for transforming co-ordinates or for evaluating data, e.g. using computers

Abstract

Systems and methods are provided for determining first and second azimuth angle values representing two closely spaced targets. Monopulse radar scan data is produced and processed to provide quadrature angle data, merged azimuth angle data, and a maximum magnitude of the quadrature angle. A quadrature angle methodology that derives the first and second azimuth angle values from an integration of the quadrature angle data over an angular region within the monopulse scan is applied if the maximum quadrature angle magnitude exceeds the threshold value. A merged azimuth angle methodology that fits the merged azimuth angle data to a polynomial as a function of a boresight angle of the monopulse radar to derive the first and second azimuth angle values is applied if the maximum quadrature angle magnitude does not exceed the threshold value. The first and second azimuth angle values are then displayed to a user.

Claims

1. A method for determining first and second azimuth angle values representing two closely spaced targets comprising:

performing a monopulse scan that includes the two closely spaced targets at a monopulse system to produce monopulse radar scan data;

processing the monopulse scan data to provide quadrature angle data, merged azimuth angle data, and a maximum magnitude of the quadrature angle;

comparing the maximum magnitude of the quadrature angle to a threshold value;

applying a quadrature angle methodology that derives the first and second azimuth angle values from an integration of the quadrature angle data over an angular region within the monopulse scan if the maximum magnitude of the quadrature angle exceeds the threshold value;

applying a merged azimuth angle methodology that fits the merged azimuth angle data to a polynomial as a function of a boresight angle of the monopulse radar to derive the first and second azimuth angle values if the maximum magnitude of the quadrature angle does not exceed the threshold value; and

displaying the first and second azimuth angle values to a user at an associated display.

2. The method of claim 1, the threshold value being determined as a function of a noise level of the monopulse radar scan data.

3. The method of claim 1, wherein applying a merged azimuth angle methodology comprises:

fitting the merged azimuth angle data to a cubic polynomial as a function of the boresight angle of the monopulse radar to provide a set of polynomial coefficients;

comparing a magnitude of a coefficient of a cubic term from the set of polynomial coefficients to a second threshold value;

calculating the first and second azimuth angle values from the set of polynomial coefficients if the magnitude of the coefficient of the cubic term does not fall below the second threshold value; and

determining the first and second azimuth angle values from a linear polynomial model if the magnitude of the coefficient of the cubic term falls below the second threshold value.

4. The method of claim 3, wherein calculating the first and second azimuth angle values from the set of polynomial coefficients comprises:

determining a mean of the first and second azimuth angle values as a cubic function of a boresight angle associated with the maximum quadrature angle;

determining a difference between the first and second azimuth angle values as a function of the boresight angle associated with the maximum quadrature angle and the set of polynomial coefficients;

determining the first azimuth angle value as the sum of the mean of the first and second azimuth angle values and one-half of the difference between the first and second azimuth angle values; and

determining the second azimuth angle value as difference between the mean of the first and second azimuth angle values and one-half of the difference between the first and second azimuth angle values.

5. The method of claim 3, wherein determining the first and second azimuth angle values from a linear polynomial model comprises:

integrating the merged azimuth angle data over an angular region of the monopulse scan to produce an integration value;

fitting the merged azimuth data to a linear polynomial function of the boresight angle to produce a first order coefficient and a constant value;

determining a mean of the first and second azimuth angle values as a function of the first order coefficient, the constant value, and the integration value;

determining a difference between the first and second azimuth angle values as a function of the first order coefficient;

determining the first azimuth angle value as the sum of the mean of the first and second azimuth angle values and one-half of the difference between the first and second azimuth angle values; and

determining the second azimuth angle value as difference between the mean of the first and second azimuth angle values and one-half of the difference between the first and second azimuth angle values.

6. The method of claim 1, wherein applying a quadrature angle methodology comprises:

integrating the quadrature angle data over an angular region of the monopulse scan having first and second limits of integration to produce an integration value;

determining a mean of the first and second azimuth angle values as a function of the integration value, respective merged azimuth angle values at each of the first and second limits of integration, and respective quadrature angle values at each of the first and second limits of integration;

determining a difference between the first and second azimuth angle values as a function of the integration value, respective azimuth angle values at each of the first and second limits of integration, respective quadrature angle values at each of the first and second limits of integration, and the mean of the first and second azimuth angles

determining the first azimuth angle value as the sum of the mean of the first and second azimuth angle values and one-half of the difference between the first and second azimuth angle values; and

determining the second azimuth angle value as difference between the mean of the first and second azimuth angle values and one-half of the difference between the first and second azimuth angle values.

7. A system for processing monopulse scan data, the monopulse scan data comprising a quadrature angle value and a merged azimuth angle value for each of a plurality of boresight angles, comprising:

a multilinear regression component configured to fit the merged azimuth angle values to a cubic polynomial of the boresight angle of the monopulse radar to provide a set of polynomial coefficients; and

a model selection component configured to select a cubic polynomial model if a magnitude of a coefficient of a cubic term of the set of polynomial coefficients exceeds a threshold value and select a linear polynomial model if the magnitude of the coefficient of the cubic term does not exceed the threshold.

8. The system of claim 7, wherein the linear polynomial model comprises:

an azimuth integration component configured to integrate the merged azimuth angle values over an angular region of the monopulse scan to produce an integration value;

a linear regression element configured to fit the merged azimuth angle values to a linear polynomial function of the boresight angles to produce a first order coefficient and a constant value;

an azimuth angle calculator configured to determine a mean of a first azimuth angle value and a second azimuth angle value representing two closely spaced targets and a difference between the first and second azimuth angle values as a function of the first order coefficient, the constant value, and the integration value, determine a difference between the first and second azimuth angle values as a function of the first order coefficient, and determine the first and second azimuth angle values from the determined mean of the first and second azimuth angle values and difference between the first and second azimuth angle values.

9. The system of claim 7, further comprising a path selection element that compares a maximum value of a magnitude of quadrature angle values associated with the plurality of boresight angles to a threshold value, each of the multilinear regression component, the model selection component, the cubic polynomial model, and the linear polynomial model being utilized to process the monopulse scan data only if the maximum value of the quadrature angle values fails to exceed the threshold value.

10. The system of claim 9, further comprising a quadrature angle element configured to determine a first azimuth angle value and a second azimuth angle value representing two closely spaced targets, comprising:

a quadrature integration element configured to integrate the quadrature angle values over an angular region of the monopulse scan having first and second limits of integration to produce an integration value; and

an azimuth angle calculator configured to determine a mean of the first and second azimuth angle values and a difference between the first and second azimuth values as a function of the integration value, respective merged azimuth angle values at each of the first and second limits of integration, and respective quadrature angle values at each of the first and second limits of integration and determine the first and second azimuth angle values from the determined mean of the first and second azimuth angle values and difference between the first and second azimuth angle values;

the quadrature angle element being utilized to determine only if the maximum value of the quadrature angle values exceeds the threshold value.

11. The system of claim 7, further comprising a multiple target detector that determines a number of targets represented by monopulse scan data, the multiple target detector comprising:

a descriptive statistics generator configured to generate a mean and a variance of the merged azimuth angle values;

a maxima detector configured to determine the number and location of local maxima within the magnitude of quadrature angle values; and

a multiple target logic component configured to determine a number of targets represented by the monopulse scan data from the variance of the merged azimuth angle values and the number of local maxima within the quadrature angle values.

12. The system of claim 11, wherein the multiple target logic component is configured to determine that the monopulse scan data represents one target if the variance of the merged azimuth angle values is less than a threshold value, to determine that the monopulse scan data represents more than two targets if the variance of the merged azimuth angle values exceeds the threshold value and the number of maxima within the magnitude of quadrature angle values is greater than one, and to determine that the monopulse scan data represents two targets if the variance of the merged azimuth angle values exceeds the threshold value and the number of maxima within the magnitude of quadrature angle values is not greater than one.

13. The system of claim 7, the cubic polynomial model being configured to calculate a first azimuth angle value and a second azimuth angle value representing two closely spaced targets from the set of polynomial coefficients.

14. A system for processing monopulse scan data, the monopulse scan data comprising a quadrature angle value and a merged azimuth angle value for each of a plurality of boresight angles, comprising:

a quadrature integration element configured to integrate the quadrature angle values over an angular region of the monopulse scan having first and second limits of integration to produce an integration value; and

an azimuth angle calculator configured to determine respective first and second azimuth angle values for respective first and second targets represented by the monopulse scan data as a function of the integration value, respective azimuth angle values at each of the first and second limits of integration, and respective quadrature angle values at each of the first and second limits of integration and determine the first and second azimuth angle values from the determined mean of the first and second azimuth angle values and difference between the first and second azimuth angle values.

15. The system of claim 14, further comprising a path selection element that compares a maximum value of quadrature angle values associated with the plurality of boresight angles to a threshold value, each of the quadrature integration element and the azimuth angle calculator being utilized to process the monopulse scan data only if the maximum value of the quadrature angle values exceeds the threshold value.

16. The system of claim 15, further comprising a merged azimuth angle element configured to determine the first and second azimuth angle values, the merged azimuth angle element comprising:

a multilinear regression component configured to fit the merged azimuth angle values to a cubic polynomial of the boresight angle of the monopulse radar to provide a set of polynomial coefficients; and

a model selection component that selects one of a cubic polynomial model and a linear polynomial model according to a magnitude of a coefficient of a cubic term from the set of polynomial coefficients.

17. The system of claim 16, the linear polynomial model comprising:

an azimuth integration component configured to integrate the merged azimuth angle values over an angular region of the monopulse scan to produce an integration value;

a linear regression element configured to fit the merged azimuth angle values to a linear polynomial function of the boresight angles to produce a first order coefficient and a constant value;

an azimuth angle calculator configured to determine a mean of a first azimuth angle value and a second azimuth angle value representing two closely spaced targets and a difference between the first and second azimuth angle values as a function of the first order coefficient, the constant value, and the integration value, determine a difference between the first and second azimuth angle values as a function of the first order coefficient, and determine the first and second azimuth angle values from the determined mean of the first and second azimuth angle values and difference between the first and second azimuth angle values.

18. The system of claim 16, the cubic polynomial model being configured to calculate a first azimuth angle value and a second azimuth angle value representing two closely spaced targets from the set of polynomial coefficients.

19. The system of claim 14, further comprising a multiple target extractor that determines a number of targets represented by monopulse scan data, the multiple target extractor comprising:

a descriptive statistics generator configured to generate a mean and a variance of the merged azimuth angle values;

a maxima detector configured to determine the number and location of local maxima within the magnitude of quadrature angle values; and

a multiple target logic component configured to determine a number of targets represented by the monopulse scan data from the variance of the merged azimuth angle values and the number of local maxima within the magnitude of quadrature angle values.

20. The system of claim 19, wherein the multiple target logic component is configured to determine that the monopulse scan data represents one target if the variance of the merged azimuth angle values is less than a threshold value, to determine that the monopulse scan data represents more than two targets if the variance of the merged azimuth angle values exceeds the threshold value and the number of maxima within the magnitude of quadrature angle values is greater than one, and to determine that the monopulse scan data represents two targets if the variance of the merged azimuth angle values exceeds the threshold value and the number of maxima within the quadrature angle values is not greater than one.