FIELD OF THE INVENTION 0001 This invention generally relates to radar systems and techniques for determining the angular location of a target and specifically to a monopulse radar processing system and technique for maintaining the accuracy of the monopulse ratio in the presence of multiple mainlobe jammers and multiple sidelobe jammers.

BACKGROUND 0002 One of the problems facing surveillance and fire control radar systems today is target detection and estimation of target angle in the presence of severe jamming interference. This problem is particularly important for next generation radar systems used in missile defense. Recently, interest has been generated toward a goal of implementing radar systems in airborne and spaceborne platforms for large area surveillance. A problem associated with achieving this goal is developing a radar system capable of detecting targets while rejecting unwanted information such as jammers and clutter. 0003 Radar systems implementing antenna arrays typically form beam patterns comprising a central beam, i.e., main lobe, and surrounding minor lobes, i.e., sidelobes. Typically, it is desired to have a narrow mainlobe having high gain, and low sidelobes. To detect a desired target and reject unwanted clutter and jamming, the mainlobe is steered toward the target of interest. The desired target within the mainlobe is enhanced and the response to clutter and jamming outside the mainlobe is attenuated. However, if a jammer is located within the mainlobe, it becomes difficult to detect the target of interest. This problem is exacerbated in the situation where multiple jammers exist. 0004 Radar systems have been developed to cancel a single jammer in the mainlobe. Such a system is described in U.S. Pat. No. 5,600,326 issued to Yu et al., which is incorporated herein by reference in its entirety. However, these systems require a priori knowledge of the jammer location. Thus, a need exists for a radar system having the ability to detect a target of interest in the presence of multiple mainlobe jammers. A need also exists for a radar system having the capability to cancel multiple mainlobe jammers without requiring a priori knowledge of jammer locations. Further, a need exists for a radar system having the capability to detect a target of interest in the presence of multiple mainlobe and multiple sidelobe jammers.

SUMMARY OF THE INVENTION 0005 A radar system for detecting and maintaining a detection of a target of interest in the presence of interference includes an antenna array, a summer, a sub-array beamformer, and a monopulse processor. Antenna array element data are provided to the summer to form a plurality of sub-arrays. The sub-array data are provided to the sub-array beamformer for forming respective sub-array beam patterns. The monopulse processor includes a monopulse sum-difference beamformer and a monopulse ratio former. The monopulse sum-difference beamformer forms sum and difference beams from the sub-array beam patterns, and the monopulse ratio former forms at least one monopulse ratio from the sum and difference beams.

BRIEF DESCRIPTION OF THE DRAWINGS 0006 The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: 0007 FIG. 1 is a block diagram of an exemplary monopulse sub-array sum-difference beamformer; 0008 FIG. 2 is a block diagram of an exemplary adaptive digital sub-array beamformer and monopulse sum and difference beamformer, with jamming cancellation and monopulse ratio preservation, in accordance with the present invention; 0009 FIG. 3 is a block diagram of an exemplary adaptive digital sub-array beamformer and monopulse sum and difference beamformer, with jamming cancellation and monopulse ratio preservation, for a one-dimensional array, in accordance with the present invention; 0010 FIG. 4 is a flow diagram of an exemplary process for detecting a target of interest in the presence of multiple mainlobe and sidelobe jammers, in accordance with the present invention; and 0011 FIG. 5 is a block diagram of an exemplary radar system in accordance with the present invention.

DETAILED DESCRIPTION 0012 Monopulse radar processing is a radar processing technique in which the angular location of a target (also referred to as direction of arrival) can be determined within fractions of a beamwidth by comparing measurements received from two or more simultaneous beams. This technique for estimating the direction of arrival (DOA) of a target is often implemented in surveillance and tracking radar systems comprising a phased array antenna and a digital beamforming (DBF) processor. Typically, one beam is formed for transmission and two beams are formed upon reception for angle measurement. The term monopulse refers to the fact that the echo from a single transmitted pulse returning from a target is used to measure the angle of the target. 0013 Monopulse processing may be implemented for a linear array of N antenna elements which provides respective signals x(0), . . . ,x(N1) to a beamforming network. The output signals of the beamforming network are the sum, , and difference, , signals which are processed to generate an output signal, , representing the estimated direction of arrival. The sum beam pattern has a symmetrical amplitude profile with respect to its maximum at the boresight, and the difference beam pattern has an antisymmetrical amplitude profile with respect to a zero response at the boresight. In the beamforming network, each of the N input signals is split into two paths, linearly weighted, and then added together. The DOA of a target signal is determined by evaluating (e.g., from a look up table or from a graph) the ratio of the difference signal over the sum signal, as indicated by the following equation. 1

\begin{matrix} m = \frac{}{} & (1) \end{matrix}

0014 where m is referred to as the monopulse ratio. 0015 Monopulse processing may also be implemented for planar arrays in which the target azimuth and elevation angles are of interest. In this case the sum and difference signals represent sum and difference signals for angles in elevation and azimuth. These angles are represented by the following symbols: E represents the difference signal in elevation, A represents the difference signal in azimuth, represents the sum signal in elevation as well as in azimuth. 0016 In an exemplary embodiment of the invention, the accuracy of the monopulse ratio, m, is maintained in the presence of multiple mainlobe and multiple sidelobe interference (e.g., caused by multiple jammers). The accuracy of the monopulse ratio, m, is maintained by adaptively suppressing the interference before forming the monopulse sum and difference output beams. Sub-arrays are formed using digital beamforming, DBF. Beam patterns are formed for each sub-array and jamming interference is cancelled for each sub-array by steering beam pattern nulls at the interference. The sub-array beam patterns are then used to form the sum, , and difference, , beams to determine the target DOA. Thus, unbiased angle estimation is achieved by exploiting the available degrees of freedom across the aperture while jamming is canceled in the sub-arrays. 0017 FIG. 1 is a block diagram of an exemplary monopulse sub-array sum-difference beamformer 100. Antenna array data are digitally summed by summer 2, to form a number, N, of overlapped sub-arrays. The amount of overlap may vary and is determined by factors such as the beamwidth of the mainlobe and available degrees of freedom in the aperture. Monopulse processing performance, as measured by the monopulse ratio, m, is determined, in part, by sub-array separation. In an exemplary embodiment of the invention, the array is partitioned into quadrants for forming respective quadrant beams in sub-array beamformer 4. The sub-array quadrant beams are denoted by Q1(Tx,Ty), Q2(Tx,Ty), Q3(Tx,Ty), and Q4(Tx,Ty), located at (Dx,Dy), (Dx,Dy), (Dx,Dy), and (Dx,Dy), respectively, with respect to the center of the array. Tx and Ty denote the conventional directional cosines, representing azimuth angle (AZ) and elevation angle (EL) information, respectively. Thus:

Txcos(EL)sin(AZ), (2)

0018 and

Tysin(EL). (3)

0019 The quadrant beams differ from each other in phase. The quadrant beams are represented by the following equations. 2

\begin{matrix} Q_{1} (T_{x}, T_{y}) = Q (T_{x}, T_{y})^{\frac{j 2}{} (T_{x} D_{x} + T_{y} D_{y})} & (4) \\ Q_{2} (T_{x}, T_{y}) = Q (T_{x}, T_{y})^{\frac{j 2}{} (- T_{x} D_{x} + T_{y} D_{y})} & (5) \\ Q_{3} (T_{x}, T_{y}) = Q (T_{x}, T_{y})^{\frac{j 2}{} (- T_{x} D_{x} - T_{y} D_{y})} & (6) \\ Q_{4} (T_{x}, T_{y}) = Q (T_{x}, T_{y})^{\frac{j 2}{} (T_{x} D_{x} - T_{y} D_{y})} & (7) \end{matrix}

0020 where Q(Tx,Ty) is a common sub-array beam located at the center of the array, is the wavelength of the transmitted radar pulse, and Dx is the distance between the center of the sub-array and the center of the array in the x direction (azimuth), in meters, and Dy is the distance between the center of the sub-array and the center of the array in the y direction (elevation), in meters. The quadrant beams are summed, by the monopulse sum-difference beamformer 6, to form the sum, (Tx,Ty), azimuth difference, A(Tx,Ty), and elevation difference, E(Tx,Ty), outputs in accordance with the following equations. 3

\begin{matrix} \begin{matrix} (T_{x}, T_{y}) = Q_{1} (T_{x}, T_{y}) + Q_{2} (T_{x}, T_{y}) + Q_{3} (T_{x}, T_{y}) + Q_{4} (T_{x}, T_{y}) \\ = Q (T_{x}, T_{y}) 4 \cos (\frac{2}{} T_{x}, T_{y}) 4 \cos (\frac{2}{} T_{x} D_{x}) \cos (\frac{2}{} T_{y} D_{y}) \end{matrix} & (8) \\ \begin{matrix} _{A} (T_{x}, T_{y}) = Q_{1} (T_{x}, T_{y}) - Q_{2} (T_{x}, T_{y}) - Q_{3} (T_{x}, T_{y}) + Q_{4} (T_{x}, T_{y}) \\ = Q (T_{x}, T_{y}) 4 j \sin (\frac{2}{} T_{x}, D_{x}) \cos (\frac{2}{} T_{y} D_{y}) \end{matrix} & (9) \\ \begin{matrix} _{E} (T_{x}, T_{y}) = Q_{1} (T_{x}, T_{y}) + Q_{2} (T_{x}, T_{y}) - Q_{3} (T_{x}, T_{y}) - Q_{4} (T_{x}, T_{y}) \\ = Q (T_{x}, T_{y}) 4 j \cos (\frac{2}{} T_{x} D_{x}) \sin (\frac{2}{} T_{y} D_{y}) . \end{matrix} & (10) \end{matrix}

0021 The azimuth monopulse ratio, mA, and the elevation monopulse ratio, mE, are calculated in accordance with the following equations by monopulse ratio former 34. 4

\begin{matrix} m_{A} = \frac{_{A} (T_{x}, T_{y})}{(T_{x}, T_{y})} = \frac{Q (T_{x}, T_{y}) j4 \sin (\frac{2}{} T_{x} D_{x}) \cos (\frac{2}{} T_{y} D_{y})}{Q (T_{x}, T_{y}) 4 \cos (\frac{2}{} T_{x} D_{x}) \cos (\frac{2}{} T_{y} D_{y})} = j \tan (\frac{2}{} T_{x} D_{x}) & (11) \\ m_{E} = \frac{_{E} (T_{x}, T_{y})}{(T_{x}, T_{y})} = \frac{Q (T_{x}, T_{y}) j4 \cos (\frac{2}{} T_{x} D_{x}) \sin (\frac{2}{} T_{y} D_{y})}{Q (T_{x}, T_{y}) 4 \cos (\frac{2}{} T_{x} D_{x}) \cos (\frac{2}{} T_{y} D_{y})} = j \tan (\frac{2}{} T_{y} D_{y}) & (12) \end{matrix}

0022 In an exemplary embodiment of the invention, adaptive processing techniques are implemented to reduce interference due to jamming. Various adaptive processing techniques may be implemented depending upon the number of available antenna element data. Examples of adaptive processing techniques include a main auxiliary adaptation technique, an adaptive-adaptive processing technique, and a fully adaptive array technique. 0023 In an exemplary embodiment of the invention, gain at the center of the main beam is maintained during the application of each of these techniques. In the main auxiliary adaptation technique, main beam and auxiliary beams (beams having approximately omnidirectional beam patterns and relatively low gain) are formed from available array elements, which are capable of being shared among multiple beams. The auxiliary beams are used to cancel the sidelobe jamming in the sum beam. 0024 In the adaptive-adaptive processing technique, the auxiliary beams are steered in the direction of the jammers. As is the case in the main auxiliary adaptation technique, the auxiliary beams are used to cancel jamming in the sum beam. In the fully adaptive array technique, all elements of the array are used to cancel jamming while the sum beam is formed from all elements of the array. 0025 For example, in the context of fully adaptive array processing, in which all antenna element data are available, the adaptive processing can be formulated using the sub-array degrees of freedom (i.e., the number of elements available in the sub-array) to cancel jamming subject to the constraint of sub-array bore-sight gain. To reduce jammer interference, the jammer power is minimized. To reduce jammer interference and maintain target detection, jammer power is minimized subject to a constraint of maintaining sub-array boresight gain. Jammer power is given by the equation:

J1W1HR11W1, (13)

0026 where J1 is the received power of the jammer, W1 is the adaptive weight for sub-array number 1, and R11 is the covariance matrix measurement of the first sub-array, and the superscript H indicates the complex conjugate transpose. The constraining equation is:

S1HW1g1, (14)

0027 where S1 is the sub-array steering vector, and g1 is the bore-sight sub-array gain. Thus, combining equations (13) and (14) results in the following equation represent jammer power subject to the constraint of maintaining sub-array boresight gain.

J1W1HR11W1(S1HW1g1), (15)

0028 where is the LaGrange multiplier for constrained optimization. 0029 Minimizing equation (15) with respect to W1 (e.g., equating the first derivative of J1 with respect to W1 to zero) and solving for W1 results in the following equation for the adaptive weights for the sub-array, when the constraint of equation (15) is observed. 5

\begin{matrix} W_{1} = \frac{{R_{11}}^{- 1} S_{1}}{S_{1}^{H} R_{11}^{- 1} S_{1}} g_{1} & (16) \end{matrix}

0030 The adaptive weights for the other sub-arrays are derived in a similar manner. Thus, 6

\begin{matrix} W_{2} = \frac{R_{11}^{- 1} S_{2}}{S_{2}^{H} R_{33}^{- 1} S_{3}} g_{2} & (17) \\ W_{3} = \frac{R_{33}^{- 1} S_{3}}{S_{3}^{H} R_{33}^{- 1} S_{3}} g_{3} and & (18) \\ W_{4} = \frac{R_{44}^{- 1} S_{4}}{S_{4}^{H} R_{44}^{- 1} S_{4}} g_{4} . & (19) \end{matrix}

0031 FIG. 2 is a block diagram of an exemplary system 200 including adaptive digital sub-array beamformer 8 and monopulse processor 34, with jamming cancellation and monopulse ratio preservation, in accordance with the present invention. Antenna element data are formed into sub-arrays by summer 2, and the sub-array data are provided to adaptive sub-array beamformer 8. Adaptive sub-array beamformer 8 forms beam patterns for each sub-array, with nulls adaptively located (steered) to suppress jamming interference. The nulls may be steered in the direction of jamming interference as a result of the adaptation. Adapted of estimated values are represented by placing a circumflex ({circumflex over ()}) over the estimated variable or quantity. The adaptive beam pattern formed from elements of each sub-array result in identical nulls responsive to mainlobe or sidelobe jammers. The adaptive sub-array beams can also be related to a common adaptive sub-array beam located at the center of the array ({circumflex over (Q)}(Tx,Ty)). In the exemplary situation where quadrant beams are formed, the estimated quadrant beams are represented by the following equations. 7

\begin{matrix} {\hat{Q}}_{1} (T_{x}, T_{y}) = \hat{Q} (T_{x}, T_{y})^{\frac{j2}{} (T_{x} D_{x} + T_{y} D_{y})} & (20) \\ {\hat{Q}}_{2} (T_{x}, T_{y}) = \hat{Q} (T_{x}, T_{y})^{\frac{j2}{} (- T_{x} D_{x} + T_{y} D_{y})} & (21) \\ {\hat{Q}}_{3} (T_{x}, T_{y}) = \hat{Q} (T_{x}, T_{y})^{\frac{j2}{} (- T_{x} D_{x} - T_{y} D_{y})} & (22) \\ {\hat{Q}}_{4} (T_{x}, T_{y}) = \hat{Q} (T_{x}, T_{y})^{\frac{j2}{} (T_{x} D_{x} - T_{y} D_{y})} & (23) \end{matrix}

0032 where equations (20), (21), (22), and (23) are analogous to equations (4), (5), (6), and (7), respectively, wherein the beam patterns represented by equations (20), (21), (22), and (23) are adaptively estimated. 0033 The estimated sub-array beams are provided to the monopulse sum and difference beamformer 10. Monopulse sum and difference beamformer 10 provides estimated sum, and difference beams in elevation and azimuth, {circumflex over ()}(Tx,Ty), {circumflex over ()}A(Tx,Ty), and {circumflex over ()}A(Tx,Ty), respectively, to monopulse ratio former 34. Monopulse ratio former 34 calculates the monopulse ratio in azimuth and elevation in accordance with the following equations. Thus leading to the following undistorted estimated monopulse ratios given by: 8

\begin{matrix} {\hat{m}}_{A} = \frac{{\hat{}}_{A} (T_{x}, T_{y})}{\hat{} (T_{x}, T_{y})} = \frac{\hat{Q} (T_{x}, T_{y}) j4 \sin (\frac{2}{} T_{x} D_{x}) \cos (\frac{2}{} T_{y} D_{y})}{\hat{Q} (T_{x}, T_{y}) 4 \cos (\frac{2}{} T_{x} D_{x}) \cos (\frac{2}{} T_{y} D_{y})} = j \tan (\frac{2}{} T_{x} D_{x}), & (24) \\ {\hat{m}}_{E} = \frac{{\hat{}}_{E} (T_{x}, T_{y})}{(T_{x}, T_{y})} = \frac{\hat{Q} (T_{x}, T_{y}) j4 \cos (\frac{2}{} T_{x} D_{x}) \sin (\frac{2}{} T_{y} D_{y})}{\hat{Q} (T_{x}, T_{y}) 4 \cos (\frac{2}{} T_{x} D_{x}) \cos (\frac{2}{} T_{y} D_{y})} = j \tan (\frac{2}{} T_{y} D_{y}), & (25) \end{matrix}

0034 where {circumflex over (m)}A is the estimated monopulse ratio in azimuth and {circumflex over (m)}E is the estimated monopulse ratio in elevation. 0035 Thus, the jamming is cancelled by the adaptive sub-array beamformer 8, using the spatial degrees of freedom of each sub-array (i.e., the number of available elements in each sub-array) to steer at least one null of each sub-array beam toward the jamming interference. Further, the spatial degrees of freedom across the sub-arrays (i.e., the number of available elements in the total array) are used to form conventional monopulse beams for angle estimation in monopulse sum and difference beamformer 10. 0036 FIG. 3 is a block diagram of an exemplary adaptive digital sub-array beamformer and monopulse processor, with jamming cancellation and monopulse ratio preservation, for a one-dimensional array, in accordance with the present invention. Processing for the one-dimensional array is similar to that of the two-dimensional array, except that the beam patterns are in one less dimension in the one-dimensional array implementation than in the two-dimensional array implementation. Also, the estimated adaptive processing is applied to sub-arrays which differ by a linear phase factor. Sub-array data are provided to one-dimensional adaptive sub-array beamformer 12. Estimated sub-array beams (i.e., A1(Tx), A2(Tx), A3(Tx), and A4 (Tx)) are formed by one-dimensional adaptive sub-array beamformer 12, in which the nulls are adaptively located to suppress jammer interference. The nulls may be steered in the direction of jamming interference as a result of adaptation. The estimated sub-array beams are provided to monopulse sum and difference beamformer 14. Monopulse sum and difference beamformer 14 forms estimated sum and difference beams (i.e., {circumflex over ()}(Tx) and {circumflex over ()}(Tx)), which are use to calculate the monopulse ratio, {circumflex over (m)}, in monopulse ratio former 36, in accordance with the following equation. Thus the unbiased monopulse angle estimate is given by the following equation. 9

\begin{matrix} \hat{m} = \frac{\hat{} (T_{x})}{\hat{} (T_{x})} = \frac{{\hat{S}}_{A} (T_{x}) j 4 \sin (\frac{2}{} T_{x} D_{x}) \cos (\frac{2}{} T_{x} D_{x})}{{\hat{S}}_{A} (T_{x}) 4 \cos (\frac{2}{} T_{x} D_{x}) \cos (\frac{2}{} T_{x} D_{x})} = j \tan (\frac{2}{} T_{x} D_{x}), & (26) \end{matrix}

0037 where Dx is the distance between the center of the sub array and the center of the array in the x direction, in meters. 0038 A radar processing system and technique utilizing the versatility of digital beamforming to form sub-arrays for canceling jamming interference, and adaptively suppressing jamming interference prior to using conventional deterministic methods (i.e., equations (24) and (25)) to form the sum, , and difference, , beams for monopulse processing provide the ability to detect a target of interest, provide an undistorted monopulse ratio, m, and maintain target angle estimation, in the presence of multiple mainlobe and multiple sidelobe jammers. Further, this system and technique are not constrained by requiring a priori knowledge of the jamming interference. 0039 FIG. 4 is a flow diagram of an exemplary process for detecting a target of interest and maintaining a monopulse ratio in the presence of multiple mainlobe jammers and multiple sidelobe jammers in accordance with the present invention. Sub-arrays are formed using digital beamforming, from the full antenna array in step 16. Antenna array element data are digitally summed to form the sub-arrays. Step 16 may be performed by summer 2 of FIG. 1. The amount that the sub-arrays overlap (share the same antenna element data) is discretionary. Factors to consider when determining which antenna elements are assigned to the various sub-arrays include the desired beamwidth of each sub-array, the degrees of freedom available for adaptive processing, and the impact on monopulse processing performance parameters. 0040 In step 18, sub-array beams are adaptively formed for each sub-array. Each sub-array beam pattern comprises a mainlobe and at least one null. In an exemplary embodiment of the invention, four quadrant sub-arrays are formed. Nulls are adaptively formed to suppress jammer interference by multiplying sub-array element data by the adaptively formed weights formed in accordance with equations (16), (17), (18), and (19). The adaptively formed sub-array beam patterns are in accordance with equations (20), (21), (22), and (23). At least one null of each sub-array beam pattern is adaptively steered toward the jamming interference, while the boresight gain of each sub-array beam pattern is maintained. Monopulse sum, and difference beams in elevation and azimuth, {circumflex over ()}(Tx,Ty), {circumflex over ()}E(Tx,Ty), and {circumflex over ()}A(Tx,Ty), respectively, are determined using the full antenna array element data in step 24. Monopulse ratios are determined in step 26, in accordance with equations (24) and (25), to provide undistorted monopulse ratios {circumflex over (m)}A and {circumflex over (m)}E. The monopulse ratios are used to determine the estimated angle of arrival of the target of interest. This may be accomplished through the use of a look up table or from extracting data from a plot of monopulse ratio versus arrival angle. The monopulse ratio is maintained in step 28 by updating the sub-array beam patterns to steer the null(s) toward the jamming interference. The sum and difference beams are recalculated using the updated beam patterns, and updated monopulse ratios are calculated to maintain the accuracy of the estimated arrival angle. 0041 The present invention may be embodied in the form of computer-implemented processes and apparatus for practicing those processes. FIG. 5 is a block diagram of a radar system comprising an antenna array 30 and computer processor 32 in accordance with an exemplary embodiment of the invention. The exemplary array 30 has four sub-arrays 34A, 34B, 34C, and 34D, each sub-array including a plurality of antenna elements 36. Data received by antenna array 30 is transmitted to computer processor 32. Computer processor 32 performs processes for detecting a target of interest and maintaining a monopulse ratio in the presence of multiple mainlobe jammers and multiple sidelobe jammers in accordance with the present invention, as herein described with reference to FIG. 4. Received signal processing may also be performed by special purpose hardware. 0042 The present invention may also be embodied in the form of computer program code embodied in tangible media, such as floppy diskettes, read only memories (ROMs), CD-ROMs, hard drives, high density disk, or any other computer-readable storage medium, wherein, when the computer program code is loaded into and executed by computer processor 32, the computer processor 32 becomes an apparatus for practicing the invention. The present invention may also be embodied in the form of computer program code, for example, whether stored in a storage medium, loaded into and/or executed by computer processor 32, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the computer program code is loaded into and executed by computer processor 32, the computer processor 32 becomes an apparatus for practicing the invention. When implemented on a general-purpose processor, the computer program code segments configure the processor to create specific logic circuits. 0043 Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.