Radar with multiband concentric rings of array elements

Abstract

A method and apparatus for a radar system having an array including a series of concentric rings of array elements, the concentric rings having a shape defined by respective ellipses increasing in size from a common center for the respective ellipses. The series of concentric rings may include multiple groups of concentric rings, with the concentric rings in each successive group being larger in size than the concentric rings in the first group.

Description

[Technical Field]

[0001] As is known in the art, radar systems transmit signals that can be reflected by a target and generate signal returns that can be processed by a receiver. Some radar systems are line-of-sight, meaning the signal travels directly from the transmitter to the target and back to the receiver. Some radar systems are over-the-horizon (OTH) radar systems, which can detect targets that are not directly in the line-of-sight of the transmitted signal. One type of OTH radar refracts signals through the ionosphere and is sometimes called skywave OTH radar. Signals transmitted obliquely into the sky are refracted by the ionosphere and directed toward the ground beyond the horizon. Targets may be illuminated by the refracted signal, which then refracts again through the ionosphere to generate signal returns that are detected by a receiver. [Background technology]

[0002] Traditional OTH radars use linear transmit arrays with no elevation scan, or large elevation arrays with limited elevation scan, resulting in wasted energy and many modes returned from the target, making it difficult to detect and track the target. Summary of the Invention [Means for solving the problem]

[0003] Exemplary embodiments of the present disclosure provide methods and apparatus for a radar with a concentric multiband array that enables elevation scanning capability and produces narrower elevation beams in addition to typical azimuth scanning. This arrangement allows the radar to include a scalable array layout that provides optimized RF performance for multiband systems with a minimal footprint. In some embodiments, the radar array has elliptical concentric rings to improve scanning performance for systems requiring different fields of view in azimuth and elevation. The array layout can be scalable because the element separation can be easily adjusted to balance the array's gain and sidelobe performance. Wider separation increases the array's gain but results in higher sidelobes. The distance between elements within each ring and the separation between rings can be adjusted to somewhat randomize sidelobes and avoid grating lobes. Ring separation typically increases gradually outward to create a "thinning" effect for the array, which is well known to have the benefit of increasing gain while minimizing sidelobe degradation. In some embodiments, the starting clock angle (φ) of each ring can also be adjusted to further randomize sidelobes. In a multi-band layout, high-band elements are placed in the inner ring of the array and low-band elements are placed in the outer ring. In embodiments, some dual-band or multi-band elements in some of the inner rings can be used to improve low-band performance. It will be understood that any practical number of bands can be used with any practical number and configuration of rings to meet the needs of a particular application.

[0004] In one aspect, a radar system includes a radar array including a series of concentric rings of array elements, the concentric rings having shapes defined by respective ellipses, the concentric rings increasing in size from a center common to the respective ellipses, the series of concentric rings including a first group of concentric rings, a second group of concentric rings, the concentric rings of the second group being larger in size than the concentric rings of the first group.

[0005] The system may further include one or more of the following features: a rotationally symmetric radiating element having an omnidirectional radiation pattern, at least one of the array elements of the concentric rings comprising a multiband array element, the array elements and spacing of a first group of the concentric rings configured for a first frequency band and the array elements and spacing of a second group of the concentric rings configured for a second frequency band, the first frequency band being higher than the second frequency band, the second group of the concentric rings comprising an outer ring and the first group of the concentric rings comprising an inner ring, and the positions of at least some of the array elements in at least one of the first and second groups of the concentric rings are configured to randomize and phase the side lobes. The radar system may be configured to minimize mutual coupling, the aspect ratio of the first and second groups of concentric rings may be configured based on a selected field of view (FOV), the third group of concentric rings may be larger in size than the second group of concentric rings, at least some elements in the radar array may be comprised of tri-band elements, the radar system may be comprised of over-the-horizon radar, the eccentricity of each of at least some of the concentric rings of the array elements may vary to lower grating lobes, and / or the first and second frequency bands may have a bandwidth ratio of 2:1 to 3:1.

[0006] In another aspect, a method includes a radar array including a series of concentric rings of array elements, the concentric rings having shapes defined by respective ellipses, the concentric rings increasing in size from a center common to the respective ellipses, the series of concentric rings including: a first group of concentric rings; a second group of concentric rings, the concentric rings of the second group being larger in size than the concentric rings of the first group.

[0007] The method may further include one or more of the following features: a rotationally symmetric radiating element having an omnidirectional radiation pattern, at least one of the array elements of the concentric rings comprising a multiband array element, the array elements and spacing of a first group of the concentric rings configured for a first frequency band and the array elements and spacing of a second group of the concentric rings configured for a second frequency band, the first frequency band being higher than the second frequency band, the second group of the concentric rings comprising an outer ring and the first group of the concentric rings comprising an inner ring, and the positions of at least some of the array elements in at least one of the first and second groups of the concentric rings configured to randomize side lobes and minimize mutual coupling. , the aspect ratios of the first and second groups of concentric rings are configured based on a selected field of view (FOV), the third group of concentric rings is larger in size than the second group of concentric rings, at least some elements in the radar array are comprised of tri-band elements, the radar system is comprised of over-the-horizon radar, the eccentricity of each of at least some of the concentric rings of the array elements varies to lower grating lobes, and / or the first and second frequency bands have a ratio of approximately 2:1 and are positioned on a ground screen having a radius larger than the maximum ring radius to allow for a suitable takeoff angle.

[0008] The foregoing features of the present invention, as well as the invention itself, can be better understood from the following description of the drawings. [Brief explanation of the drawings]

[0009] [Figure 1]FIG. 1 is a conceptual diagram of an exemplary OTH radar system having multiple bands of concentric rings of array elements, in accordance with an exemplary embodiment of the present disclosure. [Figure 2-1] 1 is a layout of elements of a dual-band array having an outer oval ring of low-band elements and an inner oval ring of high-band and dual-band elements, according to an exemplary embodiment of the present disclosure. [Figure 2-2] 1A is a layout of elements of a tri-band array having an elliptical shape according to an exemplary embodiment of the present disclosure. [Figure 2-3] B is a graphical representation of an ellipse. [Figure 3] FIG. 10 is a flow diagram of example steps for generating a layout of a scalable array including an outer elliptical ring of low-band elements and an inner elliptical ring of high-band and dual-band elements, according to an example embodiment of the present disclosure. [Figure 4] FIG. 1 is a schematic diagram of an exemplary computer capable of performing at least a portion of the processes described herein. DETAILED DESCRIPTION OF THE INVENTION

[0010] FIG. 1 illustrates an exemplary embodiment of an over-the-horizon (OTH) radar system 100 having a multi-band concentric ring array, in accordance with an exemplary embodiment of the present disclosure. In the illustrated embodiment, a transmitter 102 and one or more receivers 104a, b are provided. While multiple receivers 104a, b are shown, it will be understood that the concepts and techniques described herein may be applied to any practical number of receivers. The exemplary OTH radar system 100 further includes an optional airborne repeater 106, an optional satellite-based communications (SATCOM) repeater 107, and an optional combat command platform (COCOM) 108. In certain embodiments, the various components 102, 104, 108, and / or the airborne repeater 106 include a GPS receiver for receiving GPS signals (including geographic location and time information) from multiple GPS satellites 110.

[0011] The transmitter 102 includes a radar transmitter coupled to a transmit antenna array, and the receiver 104 includes a radar receiver coupled to a receive antenna array. After reading this disclosure, it will be understood that both the transmit antenna array and the receive antenna array are capable of steering beams in a desired direction.

[0012] The transmitter 102, receiver 104, and COCOM 108 may communicate with the airborne repeater 106 using a suitable wireless communication link 122. The communication link 122 may include, but is not limited to, an RF link, a microwave link, and / or an optical link. Additionally, one or more of the system components 102, 104, 108 may communicate with the SATCOM relay 107 using any suitable one-way or two-way satellite communication link 124.

[0013] During operation, the transmitter 102 generates and transmits a radar signal 118, which is directed upward into the ionosphere 112 and refracted toward the ground as signal 118', using a process called ionospheric refraction. The refracted signal 118' is generally directed downward toward the ground to illuminate a selected target area 126. The signal 118' reflects off various objects within the target area to generate backscattered signals 120. Such objects include, for example, ships, aircraft, missiles, and land vehicles. The backscattered signals 120 reflect back into the ionosphere 112 and are then reflected to one or more of the receivers 104 (the received signals are generally shown in FIG. 1 as signals 120'). The receivers then process the received signals 120' to detect targets, such as the ship 114 and / or aircraft 116, within the illuminated target area. Because the targets are detected indirectly through the ionosphere, a direct line of sight between the transmitting and receiving vessels and the targets is not required.

[0014] The transmitted radar signal 118 may be a high frequency (HF) radio wave, sometimes referred to as "short wave." It will be appreciated that the transmitted radar signal 118 must have high enough power to overcome both active and passive interference so that the received signal 120' has a sufficiently high signal-to-noise ratio (S / N) for target detection.

[0015] As known in the art, ionospheric refraction is the bending of electromagnetic waves / signals propagating through the ionosphere toward the ground through a complex process involving refraction. The amount of ionospheric bending depends on the degree to which the radar signal penetrates the ionosphere (a function of the signal frequency), the angle of incidence, the wave / signal polarization, and the ionospheric conditions, e.g., ionization density. OTH systems may use normal-incidence sounding ("overhead" sounding) or oblique sounding ("downrange" sounding) techniques to measure layer height and associated frequencies. These measurements, used to determine appropriate radar signal characteristics such as direction, frequency, and polarization, are based on current ionospheric conditions. Additional signal characteristics, such as bandwidth, modulation type, pulse width, and pulse repetition frequency, can be selected based on the type of target to be detected (e.g., ships, aircraft, missiles, etc.).

[0016] To illuminate a target area, the transmitters and receivers can be dynamically reconfigured / recalibrated in a coordinated manner to establish appropriate signal paths from the transmitter 102 to the target area (via the ionosphere) and from the target area to the receivers 104a,b (also via the ionosphere). Because environmental conditions may be continually changing, an adaptive process is employed to select appropriate transmit signal characteristics in real time or near real time based on the current transmitter and receiver locations. Other factors may also be considered, including the maximum available frequency obtained from sounding data and the presence of a clear channel (i.e., a channel free of other external sources of interference), which may be derived from a continuously running passive spectrum monitor receiver.

[0017] Although the exemplary system 100 is shown using a fixed transmitter 102 and receiver 104 configuration, in some embodiments, mobile transmitter(s) and mobile receiver(s) may be used. U.S. Patent No. 9,423,495, incorporated herein by reference, shows a vessel-based transmitter and receiver configuration.

[0018] FIG. 2 illustrates an exemplary dual-band array 200 with elliptical rings that increase in size with increasing distance from the center. In the illustrated embodiment, the outer ring group 202 is composed of low-band array elements 204, and the inner ring group 206 is composed of high-band array elements 208 with several dual-band array elements 210 scattered throughout the inner ring. In the illustrated embodiment, the outer ring group 202 and the inner ring group 206 are concentric ellipses. In other words, the rings share the same center, which is in the middle of the array, as indicated by the location of the center element C. Because 2D OTH arrays typically have different scan fields in the azimuth and elevation directions, the eccentricity of the ellipses can be selected to optimize the array gain and minimize sidelobe RMS and mutual coupling. In the illustrated example, the eccentricity of all ellipses is 0.47, i.e., the same for all rings. However, in some applications requiring additional randomization for lower grating lobes, the eccentricity of some of the rings, particularly the outer rings, may be different.

[0019] In the dual-band example shown in Figure 2, the bandwidth ratio between the low and high bands is approximately 1:2, and in an embodiment, one of every four high-band elements would be selected as the dual-band element. This ensures that both bands occupy the entire aperture area without leaving any "holes" in the array aperture for optimal RF performance. Note that the positions of the dual-band elements are randomized for lower sidelobes.

[0020] In the illustrated embodiment, the elliptical outer ring 202 and inner ring 206 are sized for OTH radar, as shown in feet on the X and Y axes. This exemplary array has 100 high-band elements and 136 low-band elements. The inner 100 elements include 75 high-band only elements and 25 dual-band elements. The outer 111 elements are low-band only.

[0021] FIG. 2A shows a tri-band array 250 having an outer ring of low-band elements 252, a second ring of mid-band elements and some dual-band (mid-low) elements 254, and an inner ring of high-band elements and some tri-band (low, mid, high) elements 256.

[0022] The major axis of the ellipse (the y-axis in the example) can correspond to the direction of the narrower scan, while the minor axis can correspond to the direction of the wider scan. This is consistent with array theory, which states that elements need to be closely spaced to achieve a wider scan range. Separate transmit and receive arrays can be configured identically, but this usually requires tradeoffs with other considerations.

[0023] As used herein, the term "ellipse" or "elliptical" refers to a shape defined by a planar curve enclosing two foci, in which for every point on the curve, the sum of the two distances to the foci is a constant. Figure 2B shows an example ellipse with parameters including a semimajor axis a, a semiminor axis b, and a linear eccentricity c.

[0024] Furthermore, it is understood that a circle is a special case of an ellipse with overlapping foci. The elongation of an ellipse is defined by its eccentricity. A standard ellipse, centered at the origin, with width 2a and height 2b, can be defined as follows:

number

[0025] The eccentricity e of an ellipse is defined as follows:

number

[0026] The linear eccentricity c is the distance from the center C to the focus and is defined as:

number

[0027] In applications where the desired scan angles are similar between azimuth and elevation, the concentric rings become circular, which is a special case of an ellipse with an eccentricity of zero.

[0028] The multiband concentric ring array configuration maximizes array gain and minimizes sidelobes through proper control of element spacing. The array layout is defined by several variables, starting with the semimajor and semiminor radii (a and b) of the initial ellipse, followed by the ratio of the radii, the number of elements, and the starting element clock position of each additional ring. In embodiments, the eccentricity of the elliptical rings can be adjusted based on the desired field of view (FOV) and associated scan angle to optimize scanning performance. The array layout is fully scalable by simply changing the radii (a and b) of the initial ellipse and leaving all other variables the same, or by modifying some as needed. Furthermore, concentric rings offer significant footprint savings compared to multiple discrete arrays.

[0029] It will be appreciated that any practical number of additional frequency band ellipses may be added to meet the needs of a particular application.

[0030] FIG. 3 illustrates an exemplary sequence for generating an exemplary array layout having a first group of rings, e.g., an inner ring, with high-band array elements and a second group of rings, e.g., an outer ring, with low-band array elements, according to an exemplary embodiment of the present disclosure. The first step 300 is to determine the aspect ratio of an ellipse based on the required FOV. In step 302, a first group of rings is configured. In an exemplary embodiment, the first group of rings comprises an elliptical inner ring with high-band array elements. In step 304, a second group of rings is configured. In an exemplary embodiment, the second group of rings comprises an elliptical outer ring with low-band elements. In optional step 306, the first group of rings is configured to have at least some multi-band elements. In step 308, a portion of the rings is configured to randomize sidelobes generated by the array by adjusting the ring's starting clock angle. It is understood that the ring's starting clock angle refers to rotating one ring relative to at least one other ring. For example, as shown in FIG. 2, a subset of elements 220 are offset relative to the x-axis. In this arrangement, the array elements may have offsets that may be random to reduce side lobes and mutual coupling.

[0031] 4 illustrates an exemplary computer 400 capable of performing at least a portion of the processes described herein. For example, the computer 400 may perform processes to generate signals for configuring first-tier, second-tier, and / or third-tier circuit elements. The computer 400 includes a processor 402, volatile memory 404, non-volatile memory 406 (e.g., a hard disk), output devices 407, and a graphical user interface (GUI) 408 (e.g., a mouse, keyboard, display, etc.). The non-volatile memory 406 stores computer instructions 412, an operating system 416, and data 418. In one example, the computer instructions 412 are executed by the processor 402 from the volatile memory 404. In one embodiment, an article 420 includes non-transitory computer-readable instructions.

[0032] The processing may be implemented in hardware, software, or a combination of the two. The processing may be implemented in a computer program running on a programmable computer / machine, each of which includes a processor, a processor-readable storage medium or other article of manufacture (including volatile and non-volatile memory and / or storage elements), at least one input device, and one or more output devices. The program code may be applied to data entered using the input device to perform processing and generate output information.

[0033] The system may execute processes, at least in part, via a computer program product (e.g., in a machine-readable storage device) for execution by or to control the operation of a data processing device (e.g., a programmable processor, computer, or multiple computers). Each such program may be implemented in a high-level procedural or object-oriented programming language for communicating with a computer system. However, the program may also be implemented in assembly or machine language. The language may be a compiled or interpreted language, and may be deployed in any form, including as a stand-alone program or module, as a component, subroutine, or other unit suitable for use in a computing environment. A computer program may be deployed to run on one computer, or on multiple computers at one location, or on multiple computers distributed across multiple locations and interconnected by a communications network. The computer program may be stored on a general-purpose or special-purpose programmable computer-readable storage medium or device (e.g., a CD-ROM, hard disk, or magnetic diskette) such that the computer is configured to operate when the storage medium or device is read by the computer. The process may also be implemented as a machine-readable storage medium configured with a computer program that, when executed, causes a computer to operate according to instructions in the computer program.

[0034] Processing may be performed by one or more programmable processors executing one or more computer programs to perform the functions of the system. All or part of the system may be implemented as special purpose logic circuitry (e.g., an FPGA (field programmable gate array) and / or an ASIC (application specific integrated circuit)).

[0035] While exemplary embodiments of the present invention have been described, it will now become apparent to those skilled in the art that other embodiments incorporating these concepts may also be used. The embodiments contained herein should not be limited to the disclosed embodiments, but rather should be limited only by the spirit and scope of the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.

[0036] Elements of different embodiments described herein may be combined to form other embodiments not specifically described above. Various elements described in the context of a single embodiment may also be provided separately or in any suitable subcombination. Other embodiments not specifically described herein are also within the scope of the following claims.

Claims

1. 1. A radar system comprising:

1. A radar array comprising: A series of concentric rings of array elements, said concentric rings having shapes defined by respective ellipses, said concentric rings increasing in size from a center common to said respective ellipses, said series of concentric rings comprising: the first group of concentric rings; a second group of concentric rings, the concentric rings in the second group being larger in size than the concentric rings in the first group.

2. The system of claim 1 , wherein some of the array elements in the first group of concentric rings include multi-band array elements.

3. 10. The system of claim 1, wherein a majority of the array elements in a first group of the concentric rings are configured for a first frequency band and a portion of the array elements in the first group of the concentric rings are configured for dual bands.

4. 4. The system of claim 3, wherein the second group of array elements of the concentric rings are configured for a second frequency band, and the first frequency band is higher than the second frequency band.

5. The system of claim 1 , wherein the second group of concentric rings comprises an outer ring and the first group of concentric rings comprises an inner ring.

6. The system of claim 1 , wherein the positions of at least some of the array elements in at least one of the first and second groups of concentric rings are configured to randomize side lobes.

7. The system of claim 1 , wherein the aspect ratio of the first and second groups of concentric rings is configured based on a selected field of view (FOV).

8. The system of claim 1 , further comprising a third group of concentric rings, wherein the size of the concentric rings in the third group is greater than the size of the concentric rings in the second group.

9. The system of claim 8 , wherein at least some of the elements of the radar array include tri-band elements.

10. The system of claim 1 , wherein the radar system comprises an over-the-horizon radar.

11. 10. The system of claim 1, wherein the eccentricity of each of some of the concentric rings of array elements can be varied to reduce grating lobes.

12. 4. The system of claim 3, wherein a ratio of the bandwidths of the first frequency band and the second frequency band is between 2:1 and 3:

1.

13. 1. A method comprising:

1. A radar array using a series of concentric rings of array elements, the concentric rings having shapes defined by respective ellipses, the concentric rings increasing in size from a center common to the respective ellipses, the series of concentric rings comprising: the first group of concentric rings; a second group of concentric rings, wherein the concentric rings in the second group are larger in size than the concentric rings in the first group.

14. The method of claim 13 , wherein at least one of the array elements of the concentric rings comprises a multi-band array element.

15. The method of claim 13 , wherein a first group of array elements of the concentric rings are configured for a first frequency band.

16. 16. The method of claim 15, wherein the second group of array elements of the concentric rings are configured for a second frequency band, and the first frequency band is higher than the second frequency band.

17. The method of claim 13 , wherein the second group of concentric rings comprises an outer ring and the first group of concentric rings comprises an inner ring.

18. The method of claim 13 , wherein the positions of at least some of the array elements in at least one of the first and second groups of concentric rings are configured to randomize side lobes.

19. The method of claim 13 , wherein the aspect ratio of the first and second groups of concentric rings is configured based on a selected field of view (FOV).

20. 14. The method of claim 13, further comprising using a third group of the concentric rings, the concentric rings of the third group being larger in size than the concentric rings of the second group, and at least some of the elements of the radar array including tri-band elements.

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