Patent Grant
8604976
This concepts and systems described herein relate generally to phased array antennas and more particularly to phased array antennas disposed on moving platforms including, but not limited to, ships and aircraft.
As is known in the art, a phased array antenna includes a plurality of antenna elements spaced apart from each other by known distances coupled through a plurality of phase shifter circuits to either or both of a transmitter or receiver. In some cases, the phase shifter circuits are considered part of the transmitter and/or receiver.
As is also known, phased array antenna systems are adapted to produce a beam of radio frequency energy (RF) and direct such beam along a selected direction by controlling the phase (via the phase shifter circuitry) of the RF energy passing between the transmitter or receiver and the array of antenna elements. In an electronically scanned phased array, the phase of the phase shifter circuits (and thus the beam direction) is set by sending a control signal or word to each of the phase shifter sections. The control word is typically a digital signal representative of a desired phase shift and may comprise a desired attenuation level and other control data.
Phased array antennas are often used in both defense and commercial electronic systems. For example, Active, Electronically Scanned Arrays (AESAs) are in demand for a wide range of defense and commercial electronic systems such as radar surveillance and track, terrestrial and satellite communications, mobile telephony, navigation, identification, and electronic counter measures. Military radar systems often require both long range operation for Ballistic Missile Defense (BMD) missions (requiring fully focused, high sensitivity beam patterns) and short range operation for volume surveillance missions (requiring spatially broadened beams to scan the surveillance volume faster). Such systems may also be used for electronic warfare (EW) and intelligence collection. Thus, the systems are often deployed on a single structure such as a ship, aircraft, missile system, missile platform, satellite, or a building.
When a phased array is deployed on a moveable platform (e.g., ship and airborne radar systems) it is often necessary to account for platform movement (e.g., ship pitch and roll) when pointing an antenna beam. Additionally, signal filtering or suppression to remove surface (horizon) clutter, electronic jamming, and other effects is typically necessary.
Many conventional phased array antennas accommodate movement by the use of agile beamforming and/or stabilized antenna mountings. However, such systems may be overly expensive or infeasible in the case of very large, high-powered phased arrays used for long-range surveillance. When the phased array antenna is mounted on the superstructure of a ship, for example, typical stabilization schemes are not possible. Often, existing algorithms employed to solve these problems employ an iterative “optimal search” scheme. In such applications, each antenna pattern variation or shaping for motion compensation requires independent real-time processing which becomes computationally intractable with narrow beam operation.
The combination of the long-range BMD and the short-range surveillance missions also poses a number of challenges to digital beamforming (DBF) in phased array systems. The typical high-power beam used for long-range searching is too narrow to effectively provide the large volume, rapid coverage needs of the short-range surveillance tasking. Although multiple simultaneous receive beams are typically formed to reduce occupancy time (and thereby improve volumetric coverage), such systems lack a sufficiently large transmit beam footprint to provide the necessary search coverage on time.
What is needed is an enhanced DBF approach capable of providing a flexible, broad beam transmit capability on a moving platform that can operate with multiple-beam simultaneous receive to provide clutter mitigation and pattern canting/shaping for motion compensation.
In contrast to the above-described conventional approaches, embodiments of the concepts and apparatus described herein are directed to techniques and systems for “spoiling” (or modifying from optimal configuration) the transmit beam of a phased array antenna in order to effect a shape change in the beam that pre-compensates for platform motion and clutter considerations in the local environment. The techniques and systems herein disclosed take advantage of the phase controlling capability of modern, high-power phased array elements to adjust the performance of each element in a way that de-focuses or spoils the transmit beam. This spoiling, in turn, enables the transmission of a broader, tailored beam that provides illumination over an area that would otherwise require multiple scans from the (normally) highly-focused, narrow transmit beams. The techniques described provide a closed-form solution that sacrifices some antenna pattern efficiency in exchange for greatly reduced computational complexity over prior art, optimal search techniques.
The foregoing and other objects, features and advantages of the concepts and apparatus described herein will be apparent from the following description of particular embodiments, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the concepts and apparatus described herein.
Embodiments of the concepts and apparatus described herein are directed to techniques and systems for “spoiling” (or modifying from optimal configuration) the transmit beam of a phased array antenna in order to effect a shape change in the beam that pre-compensates for platform motion and clutter considerations in the local environment. The techniques and systems herein disclosed take advantage of the phase controlling capability of modern, high-power phased array elements to adjust the performance of each element in a way that de-focuses or spoils the transmit beam. This spoiling, in turn, enables the transmission of a broader, tailored beam that provides illumination over an area that would otherwise require multiple scans from the (normally) highly-focused, narrow transmit beams. The techniques described provide a closed-form solution that sacrifices some antenna pattern efficiency in exchange for greatly reduced computational complexity over prior art, optimal search techniques.
Digital beamforming (DBF) techniques may be used at the element (or sub-array/sub-aperture of elements) level to provide the necessary multiple simultaneous receive beams for radar applications employing transmit beam spoiling. In such applications, the receive beam (or beams) must be synchronized in both time and space in order to receive a return signal from the target(s). However, such an application requires flexibility in the modes and directions of transmit beam spoiling in order to take advantage of the receive-side DBF. For example, transmit spoiling in one or two dimensions and/or pattern canting (angular rotation around beam center or boresight) may be used to accommodate platform motion and/or local clutter. Digital beamforming in the receive beams may then be used to match the number and composite coverage of the receive beams to the spoiled pattern.
The unspoiled transmit beam 110 in the upper left quadrant represents a nominal spot beam shape and relative size for a high-power phased array antenna. The corresponding receive beam 115 superimposed thereon illustrates the difficulty of covering a large volume of space with the narrow beam. Using the systems and techniques herein disclosed, a transmit beam may be spoiled in two dimensions (u and v) resulting in a much larger illuminated region 120 as shown. Here, the beam spoiling factor (BSF) is roughly equal in both axes, resulting in a circular spoiled pattern. A plurality of receive beams 125 (here shown as seven beams) operating in simultaneous receive mode, covers the wider, spoiled area. Of course, the BSF applied in each axis need not be equal, which would give transmit patterns of elliptical shape.
A single dimension of beam spoiling may be illustrated by transmit beam 130, which is (in this example) spread out horizontally along the aperture's visible horizon 150. Again, a plurality of simultaneous receive beams 135 may be employed to identify and track targets illuminated by the wider transmit beam 130.
Finally, a canted 1-D beam broadening may be illustrated by transmit beam 140. As in the case of transmit beam 130, the beam is spread out in one dimension. However, the pattern may be canted to follow the visible horizon 150 by using one-dimensional beam spoiling along the axis of canting. A corresponding plurality of simultaneous receive beams 145 may be employed to identify and track targets illuminated by the wider transmit beam 140.
In general, the spoiled transmit beam will be effective whenever the transmit loss incurred by spoiling (due to the “inefficient” power distribution relative to the unspoiled spot beam) may be less than or equal to the geometric loss due to the off-boresight location of the target.
While the techniques and systems disclosed herein are generally described with reference to the transmit direction, one of ordinary skill in the art will readily see that these techniques are equally applicable to spoiling in the receive direction. Such embodiments may provide broad area receive coverage with arbitrary alignment with the horizon or another reference orientation and similar clutter mitigation properties. Accordingly, the concepts and apparatus described herein are not limited in their applicability to either the transmit or receive directions.
One of ordinary skill in phased array antenna applications will also appreciate that the systems and techniques herein described are applicable to a wide range of such application beyond radar. Various communications (including search and rescue [SAR]), electronic warfare, and signals intelligence applications may also employ the concepts and apparatus described herein. Accordingly, the concepts, systems, and techniques described herein are not limited to any particular phased array antenna application, nor to any particular operating frequency band or bands.
Some examples of the prior art in iterative gradient descent searches are discussed in Brown, G. C.; Kerce, J. C.; Mitchell, M. A.; “Extreme Beam Broadening using Phase Only Pattern Synthesis,” Sensor Array and Multichannel Processing, Fourth IEEE Workshop on, pp. 36-39, 12-14 Jul. 2006 (DOI: 10.1109/SAM.2006.1706079), incorporated herein by reference in its entirety. These approaches are computationally expensive and require the use of stored, pre-calculated or otherwise quantized patterns. Accordingly, these prior art methods are not well-suited to the real-time needs of long-range BMD or theatre defense missions employing a narrow beam antenna array.
By contrast, in some exemplary embodiments of the concepts and apparatus described herein, the beam broadening approach may be based on a combination of a homotropy from a known optimal beam with BSF-1 for a perturbation stable solution of broadening phase values and a stochastic gradient descent technique used to optimize the cost functional for a more realistic aperture having irregular shape. This novel approach is further described below.
An exemplary embodiment of a system 200 constructed according to the teachings herein is depicted as a high-level block diagram in
Communications processor 220 conveys the beam direction vector, spoiling direction vector, magnitude (i.e., the beam spoiling factor), and (in some embodiments) a dimensionality indicator (i.e., 1-D or 2-D shape) to the beam steering generator (BSG) 230. In one exemplary embodiment, the beam spoiler 240 may be a component of the BSG. Alternatively, the beam spoiler may be implemented in a separate unit or module configured as a pre-processor to the BSG 230.
Beam spoiler 240 utilizes the input signals fro INS 210 and communications processor 220 to determine adjusted phase weights necessary to spoil the beam and conveys these weights to BSG 230 for the completion of the conventional phased array beamforming computations. BSG 230 in turn drives elements 160A-160N of phased array aperture 170. Note that elements 160D through 160H here represent of the elements in sub-aperture 180.
In one exemplary embodiment, aperture 170 may be partitioned into a number of concentric sub-apertures 180, arranged as shown in
As is further described below, beam spoiler 240 determines the spoiling direction, magnitude, and shape by which the system will spoil or broaden the beam based on the platform's tracking algorithm, surveillance map and platform motion status. The magnitude of the effect may be referred to herein as the beam spoiling factor (BSF) and is equal to the spoiled beamwidth divided by the non-spoiled beamwidth. The shape of the broadening depends on the application, and may be defined as either one or two-dimensional, denoting broadening in either one or both axes normal to the boresight. (These orthogonal co-planar dimensions are conventionally referred to as the u- and v-axes [or azimuth and elevation] in the horesight coordinate frame of reference and are mutually perpendicular to the direction of propagation of a transmit wavefront.) Table 1 defines some exemplary one and two-dimensional (1-D and 2-D, respectively) spoiling in terms of (u, v) coordinates.
The combination of the spoiling direction, magnitude, and shape forms a beam spoiling vector according to the equation:
Beam Spoiling Vector=BSF*S (Eqn. 1)
The spoiling direction factor S may be a function of a dimensionality indicator D, which is equal to 1 for either uspoil=0 or vspoil=0 or equal to 2 for uspoil, vspoil≠0. The use of this factor in defining S is optional.
In each sub-aperture, the specified beam steering vector (BSV) perturbs the beam in the direction of the BSV. Note that
In
The beamforming element of the system then computes the individual phase weights for each element in each sub-aperture as follows:
where φi represents the phase coefficient for each element i; (xi, yi) represent the location of element i; and (uspoil, vspoil) represent the beam spoiling vector for the sub-aperture decomposed into its (u, v) coordinate components. Usteer and Vsteer represent the standard beam steering vector for each element decomposed into its (u, v) coordinate components. The second term in Eqn, 2 represents the conventional beam steering component; the first term represents the beam spoiling contribution. These calculations may be performed in real-time, using the ship's (or the platform's, generally speaking) inertial navigation system (INS) for pattern placement and synthesis relative to array foresight.
The phase calculation of Eqn. 2 is repeated for each element in each sub-aperture via computation means well known in the digital beam steering art in, for example and not by way of limitation, a radar mission processor, beam steering computer, or other such system. Unlike in the prior art, where the computational time required is generally proportional to the square of the number of array elements, the present technique allows parallel computation of the phase weights for the beam spoiling component of Eqn. 2. In one exemplary embodiment, the radar mission processor calculates required beam “steer” direction from the target/surveillance direction and platform state (i.e., heading and inertial position; also referred to in terms of pitch/roll/yaw). The beam “spoil” parameters (φ and ki, the BSF) are determined from the radar mode and the platform state. Based on the conventional tracking algorithm, surveillance map, and ship motion status, the radar processor calculates the beam spoiling vector according to Eqn. 2. The beam spoiling vector may be sent to the beam steering generator, which (on turn) computes the phase weights and sets the phase shift for each array element to produce the broadened transmit beam.
Experiments have shown that the spoiling transmit loss will be less than the array's geometric losses when the BSF is greater than 3. BSF values of less than 3 result in excessive spoiling losses and reduced efficacy.
The iterative process of computing the appropriate array element phase weight (or coefficient) begins in step 440, where the phase coefficients for each element in the SA (step 454) are computed. As each element is adjusted by the phase coefficient (step 458), the process iterates for every element in the SA, step 450. As each SA is completed, the outer loop of the process (step 440) iterates for every sub-aperture until all computations are complete.
Referring to
The processes described herein are not limited to use with the hardware and software of
The system may be implemented, 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, data processing apparatus (e.g., a programmable processor, a computer, or multiple computers). Each such program may be implemented in a high level procedural or object-oriented programming language to communicate with a computer system. However, the programs may be implemented in assembly or machine language. The language may be a compiled or an interpreted language and it may be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program may be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. A computer program may be stored on a storage medium or device (e.g., DVD, CD-ROM, hard disk, or magnetic diskette) that is readable by a general or special purpose programmable computer for configuring and operating the computer when the storage medium or device is read by the computer to perform the processes described herein. The processes described herein may also be implemented as a machine-readable storage medium, configured with a non-transitory computer program, where upon execution, instructions in the computer program cause the computer to operate in accordance with processes 300 and 550.
The processing blocks associated with implementing the system 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 field programmable gate array [FPGA] and/or an application-specific integrated circuit [ASIC]).
Elements of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Other embodiments not specifically described herein are also within the scope of the following claims.
The order in which the steps of the present method are performed is purely illustrative in nature. In fact, the steps can be performed in any order or in parallel, unless otherwise indicated by the present disclosure.
The method of the present invention may be performed in either hardware, software, or any combination thereof, as those terms are known in the art. In particular, the present method may be carried out by software, firmware, and/or microcode operating on a computer or processor (or plural elements thereof) of any type. Additionally, software embodying the concepts described herein may comprise computer instructions in any non-transitory form (e.g., source code, object code, and/or interpreted code, etc.) stored in any computer-readable medium. Accordingly, the implementation of the concepts and apparatus described herein is not limited to any particular platform, unless specifically stated otherwise in the present disclosure.
While particular embodiments of the present concepts and apparatus have been shown and described, it will be apparent to those skilled in the art that various changes and modifications in form and details may be made therein without departing from the spirit and scope of the invention as defined by the following claims. Accordingly, the appended claims encompass within their scope all such changes and modifications.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5592178 | Chang et al. | Jan 1997 | A |
| 7728769 | Chang et al. | Jun 2010 | B2 |
| Entry |
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| Brown, G.C.; Kerce, J.C.; Mitchell, M.A.; “Extreme Beam Broadening using Phase Only Pattern Synthesis,” Sensor Array and Multichannel Processing, Fourth IEEE Workshop on, pp. 36-39, Jul. 12-14, 2006 (DOI: 10.1109/SAM.2006.1706079). |
