AN/APG Active Electronically Scanned Array AESA
Active electronically scanned array [AESA] radar systems, with groundbreaking performance and tactical advantages, are the cornerstone of current and future aircraft. In a typical active electronically scanned array, also known as an active aperture, for a pulsed radar system, a large number of transmit/receive (T/R) circuits are arranged in a predetermined configuration and connected to an array of forwardly located radiator elements which collectively transmit and receive a beam of RF pulses to and from a target. The beam is typically energized, shaped and directed in azimuth and in elevation under the control of a beam steering controller assembly which forms part of the system.
While the ranges of the new lines of AESA radars are classified [and are of course dependent on target characteristics], there published estimates based on an estimated 55 mile range for the mechanically scanned F-15C radar. In comparison, the AESA radars ranges are estimated at about 90 miles for the smallest (aimed at the F-16 radar-upgrade market), a range of 100 miles for the F/A-18E/F and F-35, about 110-115-miles on the F-22, and about 125 miles for the largest AESA radars carried on the upgraded F-15Cs and Es.
The state of the art in airborne X-band AESAs moved impressively in the decade of the 1990s due to several prototyping efforts and the JSF Dem Val technology push. Factors of 3 to 5 or more in weight and cost reductions can be supported along with innovations in mechanical design to simplify manufacturability and maintenance. Transmit/receive modules are approaching commodity status, albeit with limited component suppliers. Raytheon is dominant in the surface-based X-band AESA market. Raytheon (radar supplier for the F-15 and F-18) and Northrop Grumman (radar supplier for the F-22 and F-16) are the dominant US contractors in the airborne market. Since Hughes and Texas Instruments (both now Raytheon) were involved in the F-22 radar, there is significant Raytheon participation in the F-22, although Northrop Grumman is the lead. Each contractor has been motivated to follow the family-of-radars construct to remain cost competitive. In all cases, components are being shared among various products, and in many cases, complete subsystems (in both hardware and software).
As of 2001 the DoD had plans to build AESAs for the F-22 (331 units), the F/A-18E/F (258 units), the F-15C (18 units), the F-16-UAE (80 units), and the JSF (about 3000 units). Raytheon (radar supplier for the F-15 and F-18) and Northrop Grumman (radar supplier for the F-22 and F-16) are the dominant U.S. contractors in this market. The very large, potential JSF procurement was in the competition phase, with Raytheon on the Boeing team and Northrop Grumman on the Lockheed Martin team. This procurement, which would extend through 2020, would require approximately 3.5 million transmit/receive modules, which was roughly 80% of the projected fighter market. Northrop Grumman won the contract to develop the AN/APG-81 active electronically scanned array (AESA) radar for the F-35 Lightning II. These AESA fighter radars, in addition to air-to-air modes, support ground-surveillance for stationary and moving targets, i.e., SAR and GMTI.
For a given size and weight, AESA technology provides a factor of 10-30 times more net radar capability than competing approaches due to power increases, lower losses, and increased flexibility. Also, AESA designs provide inherently superior countermeasure resistance, enhanced range resolution (for target identification), and more flexibility to support nontraditional radar modes such as jamming and ESM. In addition, AESA technology supports high reliability/low maintenance designs with the promise of attractive life cycle costs. These advantages are so compelling that it is unlikely that any new U.S. fighter radar will be procured in the future without AESA technology.
Although passive array technology provides electronic beam control, it has the disadvantage that phase control must be accomplished at high power to position the transmit beam. High-power phase control technology is dominated by power loss concerns. Typical total losses in early systems resulted in a factor of 10 reduction in radiated power; in modern systems these losses are still in the factor of 5 range.
The active array concept has the planar antenna face populated by discrete elements referred to as transmit/receive modules (T/R modules). In an active array the power source is now distributed and phase control to provide electronic beam steering can be accomplished at low power (and with only minor losses). Since large numbers of T/R modules can be required, viz., 3000 - 4000 per square meter, module cost has always been a primary concern with active arrays.
After many years of technology investment, by circa 1990, X-band active arrays arrived as serious contenders in the radar market. Pivotal developments along the way included improvement of gallium arsenide material and the development of monolithic microwave integrated circuit (MMIC) technology. MMIC technology uses lithographic-type processes to produce microwave circuits on chips at very high levels of integration. A modern X-band T/R module, in addition to a radiating element, will contain five to seven chips (MMICs) produced in a foundry and later integrated into a substrate with a few discrete components and cooling provisions, all filling a space on the order of 1/4 in3.
High-power radar applications, such as fighters and JSTARS RTIP, use "filled" apertures, which at X-band is generally about 3000 modules per square meter of antenna. Fighter radars are usually in the 1000 to 2000 modules size range. Modules that are readily available have peak powers of about 10 W with average power of about 2 W; hence, for a square meter of filled X-band aperture the peak power is typically 30 kW and the average power about 6 kW.
Electronically scanned arrays (ESAs)
Electronically scanned arrays (ESAs) are made up of a plurality of antenna radiating elements or radiators, which together form a radiating surface. In one prior ESA implementation, each antenna subarray is configured with a plurality of radiators which are mounted on machined metal support structures. The radiators are located on precise and uniform spacings across the face of the antenna aperture. The radiators are connected to transmit and/or receive (T/R) components that are combined via an radio frequency (RF) distribution manifold. Phase shifters are provided to allow electronic steering of the antenna beam. Phase shifters may be a variety of devices, such as PIN diodes, MMIC's, ferrite phasors, or other phase shifting devices. Separate DC power and control signals are typically provided to the phase shifters or T/R components through distribution manifolds. A cooling manifold is also typically provided for dissipating heat generated by the phase shifter, T/R components, the DC and control manifold devices.
Passive ESA (PESA)
T/R components may be located immediately behind the ESA radiators to form an Active ESA (AESA). Alternatively, these T/R components may be located remote to the radiators to form a Passive ESA (PESA). Examples of RF generators in a PESA include traveling wave tube (TWT), magnetrons, or solid state transmitter (SST) components. In an AESA configuration, T/R components are usually located in hermetically sealed modules (T/R modules). RF losses are minimized in AESA configurations due to the close proximity of the T/R modules to the radiators. However, the requirement of having a discrete T/R module at each radiator site is costly. In a PESA configuration, the T/R components may be lumped together for more cost-efficient packaging because they are remote to the radiators. However, because these devices are remote from the radiators, increased RF losses tend to lower the overall system performance.
Although ESAs offer many advantages over mechanically scanned antennas, in many applications it is prohibitively expensive to substitute either AESA or PESA equipment for an equal performance mechanically scanned antenna. The most costly components of AESAs generally include the T/R modules and manifold structure required for the T/R modules. The most costly components of PESAs generally include the RF generator, phase shifters, distribution manifolding and structure required for the phase shifters. These problems reduce the cost competitiveness of ESAs compared to mechanically scanned antennas.
Active Electronically Scanned Array AESA
An antenna assembly for an active electronically scanned array includes, among other things: an array of antenna elements; an RF signal feed and circulator assembly coupled to said antenna elements and forming thereby an array of radiating structures; a generally planar RF manifold assembly having regularly spaced openings therein located behind and normal to the radiating structures; an array of T/R modules connected to the array of radiating structures and having respective RF connector assemblies forming a portion of an RF interface at one end portion of each of the modules which project upwardly through said spaced openings in the RF manifold and wherein the respective connector assemblies thereof connect to at least one immediately adjacent circulator as well as to transmit and receive manifold portions of the RF manifold.
In a phased array, the radar system sequentially generates RF transmit pulses which are distributed by means of a transmit manifold and microwave power generating circuitry to the array antenna elements. Between transmitted pulses, the radar system receives and processes successive return signals from the same antenna elements. The return signals are then processed and collected through a receive manifold and then processed in the radar system receiver circuitry for target identification and/or display.
In the transmit mode, a microwave transmit circuit in the T/R circuit operates on each RF pulse which is generated and fed to the transmit manifold, and thereafter controls the amplitude and phase of the RF pulse coupled to the antenna elements via a circulator device, a device well known in the art. In the receive mode, a microwave receive circuit in the T/R circuit, operates on each radar return signal coupled from the antenna elements via the circulator to control its amplitude and phase and which is then applied to the receive manifold where the signals are collected from all of the T/R circuits and fed back to RF demodulator circuitry in the system. Accordingly, one transmit circuit and one receive circuit form one transmit/receive channel.
Pulse radar transmitters generate pulse trains having a certain peak power and a certain average power. Since such a transmitter operates with a certain peak power, continuous wave (CW) signals cannot normally be generated because the average power of such a signal would be so high that the transmitter would be damaged. In other words, the pulse radar transmitters of today are designed for a maximum duty cycle that cannot be exceeded.
Conventional, electronically scanned arrays and phased arrays are realized in two geometries, including a passive electronically scanned array using ferrite phase shifters, and an active electronically scanned array using transceiver modules. At millimeter-wave frequencies, the center-to-center antenna element spacing ranges from 0.200 inches at Ka-band to 0.060 inches at W-band. Within a square cross-section of this dimension, an active transceiver module or a reciprocal phase shifter assembly must be mounted and control lines must be made accessible.
In order to illustrate the magnitude of this antenna design problem, consider as an example a 25.times.25, fully populated Ka-band active electronically scanned array. Also assume five power and signal control lines are needed per antenna element. This means that 625 modules must be packaged with 3,125 power and control lines, a 625 way RF power divider network and sufficient heat sinking to dissipate the heat from the modules. The present invention will reduce considerably the amount of hardware necessary for a millimeter-wave phased array.
Conventional, electronically scanned, phased arrays are not yet practical for millimeter-wave applications. The center-to-center element spacing, 0.060 inches at W-band (94 GHz) and 0.100 inches at V-band (60 GHz) and 0.200 inches at Ka-band (35 GHz), is not conducive to the packaging of such arrays. Passive ferrite phase shifters above Ka-band (35 GHz) have only recently become available and are generally lossy, current controlled devices and active transceiver modules are in their infancy of development. W-band transmit/receive module electronically scanned array antennas are not feasible with conventional technology.
An electronically scanned semiconductor antenna is manufactured using conventional semiconductor device fabrication technology. The antenna is fashioned in the form of a continuous transverse stub array geometry but uses a semiconductor substrate, such as silicon, gallium arsenide, or indium phosphide, for example. The antenna has a semiconductor substrate having a plurality of stubs projecting from one surface. The semiconductor substrate may be silicon, gallium arsenide, or indium phosphide, for example. A first conductive layer formed on the surfaces of the semiconductor substrate and along sides of the stubs so that the stubs are open at their terminus. The conductive layers form a parallel plate waveguide region. A diode array having a plurality of diode elements is formed in the semiconductor substrate that are disposed transversely across the semiconductor substrate and longitudinally down the semiconductor substrate between selected ones of the plurality of stubs. The diode array provides a voltage variable capacitive reactance in selective regions of the waveguide region. A beam steering computer is coupled to the plurality of diode elements of the diode array which controls the voltage applied thereto to control steering of a beam radiated by the antenna.
Electronically scanned arrays (ESAs) may be set up with phase shifters servicing array elements and subarrays steered by adjustable time delay. Subarray combinations may be in either an analog or digital sense. Digital combination allows limited scan, multiple full aperture beams. Beams may be steered electronically through corresponding settings in both the phase shifters and adjustable time delay elements. An exemplary array may be arranged horizontally and be horizontally subdivided into a number of horizontally adjacent subarrays. The array elements may be arranged in horizontal rows and vertical columns. All of the subarrays typically extend the full vertical height of the array. Horizontally contiguous subarrays do not share elements with adjacent, contiguous subarrays. Horizontally overlapping subarrays may share elements with adjacent, overlapping subarrays.
For example, in the case of uniformly-sized subarrays with 50% horizontal overlap, an array which is horizontally adjacent to two other arrays will share the left half of its elements with the horizontally adjacent array on its left and the right half of its elements with the horizontally adjacent subarray on its right. In the area of overlap, the arrays overlap throughout the full height of the array. Overlapped subarrays may decrease the width of respective subarray beam patterns and may provide some degree of grating lobe suppression. Shared-element, overlapping, full-height subarrays may be more costly to manufacture and introduce an added level of complication to achieve desired calibration of the array, in comparison with non-overlapping full-height subarrays. A complex, calibration correction term associated with a single array element location may be applied to multiple signal paths if the element is shared between two subarrays. For 50% overlap, for example, two signal paths may be required. Elemental phase shifters may perform electronic beam steering in the vertical orientation along with associated array calibration for signals in one of two subarrays by which the column of elements is shared. For the other subarray, a manifold phase shifter may apply an additional calibration setting for the signal path to the other subarray.
The additional manifold phase shifters required for more optimal calibration may increase costs and add complexity to the array architecture. Subarrays with a higher percentage of overlap result in a greater number of parallel signal paths with a corresponding requirement for additional phase shifters to achieve desired levels of calibration. As a result, array architecture may be more complex because a manifold phase shifter may be required to account for differences in signal path for shared-element signal paths in adjacent sub-arrays. The use of such overlapped subarrays may therefore result in increased complexity where optimal calibration is desired.
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