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Active Electronically Scanned Array (AESA) radar, or e-scan, is a well-established technology that has evolved in recent decades, becoming increasingly reliable and price competitive due to ongoing refinements in GaAs MMICs, higher integration of functions (IoF) per chip, increased process yields with reduced failure rates, pre-screening of key components such as power amplifiers (PAs), and other low cost manufacturing techniques.
Despite these advancements, the use of AESA radar technology has been largely restricted to highly customized solutions developed by prime aerospace contractors for use in their own proprietary systems. These custom designed radar systems are relatively costly, require extended lead-time to design and manufacture, and lack the flexibility to be easily configurable to new applications. The E-2D Advanced Hawkeye for example uses the new AN/APY-9 radar that has an active electronically scanned array (Figure 1).
The E-2D Advanced Hawkeye for example uses the new AN/APY-9 radar that has an active electronically scanned array.
With the threat of international terrorism on the rise, including a higher risk of asymmetrical attacks on military and civilian targets and key infrastructure, a clear need has emerged for a modular, stackable approach to AESA radar system design that addresses a growing range of applications, including radar-naval, airborne, vehicle-mounted, and ground-based-; coastal, harbor, and border security; air traffic control; foreign object detection (FOD) for airport runways; satellites; and data links.
How AESA Radar Works
AESA systems use numerous transmit/receive modules (TRMs) that transmit and receive high power radio waves of varying frequencies, scanning rates, and radiation patterns on demand to provide highly agile beam steering that is capable of tracking multiple targets simultaneously. A key advantage of AESA over conventional radar systems, including Passive Electronically Scanned Arrays (PESA), is its greater frequency agility, which results in more unpredictable scan patterns that are more difficult to detect by radar warning receivers (RWRs), particularly older systems.
Offering unmatched versatility, AESA radar systems can provide high jamming resistance by spreading signals across a range of frequencies. These systems can also be switched into a receiver-only mode to track the source of jamming signals or to act as a radar warning receiver. AESA radar can also serve as a high-speed data link, enabling such functions as peer-to-peer networking, where data from multiple platforms is combined to provide expanded radar coverage and resolution enhancement.
In general, the highest field of view (FOV) for a flat phased array antenna is 120 degrees (60 degrees left and 60 degrees right). Wider coverage can be obtained using multiple antenna face configurations or through the use of two rotating antenna faces. AESA radar systems mounted onto the nose of an aircraft can use a mechanical gimbal to provide additional mechanical steering, thus widening the aircraft's FOV. Thermal management requires careful consideration as the power amplifiers are distributed across the antenna face. Distributed cooling systems are required that can fit within the limited space envelope between elements and keep the thermal gradient across the antenna to a minimum.
Modular, Stackable Solution
In 2013, API Technologies introduced the first modular Quad Transmit Receive Module (QTRM) as part of its Active Antenna Array Unit (AAAU) solution, providing the industry's first modular and scalable building block solution available at X-band and C-Band, as well as a Dual Transmit Receive Module (DTRM) at S-Band. Figure 2 shows a QTRM.
This modular Quad Transmit Receive Module (QTRM) provides a scalable building block solution available at X-band and C-Band, as well as a Dual Transmit Receive Module (DTRM) at S-Band.
The AAAU consists of multiple QTRM sub-arrays. The QTRMs are also available as a standalone product, or as scalable Planks that typically consist of four QTRMs combined with an integrated linear 16 element antenna array, liquid cooling with quick release non-drip connections, and distribution networks to provide RF and DC control signals to each QTRM. Individual Planks plug into slots in the main array structure to create a 2D array solution Figure 3 shows how quad packs fit into the final 2D radar array.
Shown here QTRMs plug into slots in the main array structure to create a 2D array solution
A QTRM is self-contained unit consisting of four T/R channels, each consisting of a low noise amplifier (with receiver protection), a power amplifier (PA), as well as digitally controlled phase and gain control elements. Each QTRM module also contains local DC power supply conditioning, a built-in logic interface for serial control and BITE power supply monitoring, and a protective thermal shutdown facility. QTRMs come factory calibrated and individually addressed to provide ease of installation and integration, with external system loss, antenna offsets, and phase offsets being easily uploaded by the system integrator.
Each T/R channel is individually controlled, so any individual T/R channel failure will not affect the other channels within the module, resulting in graceful degradation. This solution eliminates a major shortcoming of legacy radar systems that could potentially face a total loss of operation due to a single Point of Failure (PoF), such as the loss of the travelling wave tube (TWT) power amplifier. These modular, plug and play QTRMs are Line Replaceable Units (LRU) that use COTS components to reduce first line repair costs. Individual QTRMs can also be rapidly swapped out without incurring any system downtime. By contrast, non-modular legacy AESA systems require the entire platform to go off-line for repairs or upgrades.
Designing the Ideal AESA Solution
Application-specific requirements can limit design options, so a flow-down approach is required. Key design parameters include a number of elements. First there's optimizing the physical size of the T/R channels to minimize the antenna spacing to a half wavelength or below, which reduces undesirable grating lobes. Next there's the matter of combining a low noise receive channel with high output power to extend signal transmission range. Improving Power Added Efficiency (PAE) and reducing DC power requirements is also key.
System designers should reduce the minimum detectable range (MDR) by minimizing T/R switching speed, limiter recovery time, and DC supply gating circuit requirements. It's also important to cutting RMS phase and amplitude errors to reduce side-lobes. System integrators should have access to upload phase and attenuation offsets introduced by external RF circuitry such as cabling. Unique address codes should be applied so that modules can be placed anywhere within the overall array. Using GaN technology and space-saving cooling solutions helps to improve efficiency and to reduce footprint and mass, particularly for airborne applications.
The available space envelope is generally dictated by the need to maintain a half wavelength (or less) antenna spacing and by the array configuration, with total power output limited by the module's frequency, size, and allowable heat dissipation. Generally, a number of modules are packaged into a single housing, usually four for the higher frequencies (C-Band and above), and two for S-Band frequencies. These configurations provide enough space to include full digital functionality, local power supply conditioning, and a single, all-encompassing environmental seal, which is generally more cost effective than multiple channel-by-channel sealing techniques.
Circuit Technology Issues
The circuit technology is largely influenced by the required frequency band and hence the available space envelope. Lower frequency designs lend themselves to a single layer RF pcb design mounted onto a backplane, with surface mount packaged MMICs, and drop-in devices such as circulators or packaged discrete transistors.
With higher frequency designs, the space envelope gets reduced, making it increasingly difficult to fit all the RF functionality and associated interconnects onto a single layer. Therefore, the use of packaged devices is usually prohibited, thus requiring a chip and wire approach using a highly integrated MMIC chip set. A multilayer approach may also be considered, such as LTCC packaging or a mixed-media multilayer board.
The functional building blocks of a typical T/R channel are similar regardless of overall system requirements. For frequencies at C-Band and above, a MMIC 'core chip' will most likely be utilized along with a low noise amplifier MMIC in the receive path and a power amplifier MMIC in the transmit path. The MMICs will likely be designed as a chip set, allowing the power amplifier to be driven directly from the core chip.
Core Chip Functions
The core chip itself typically includes a digital phase shifter and an attenuator, along with low noise and medium power amplifiers that interface directly with the receive and transmit path MMICs. Switches within the core chip allow the attenuator and phase shifter functions to be used in both transmit and receive paths and thus form a common leg circuit.
The LNA requires a limiter circuit to protect the device from high power RF signals generated either from the transmit side or from external sources. The antenna port feed to the T/R channel usually passes through a ferrite circulator, often with a ferrite isolator to protect the power amplifier, or occasionally with a high power T/R switch that can terminate the receive path with a load during the transmit pulse cycle.
Lower frequency designs can utilize a combination of discrete surface mount MMIC devices to realize the core chip functionality, along with discrete high power transistors with external matching circuits for the power amplifier. In all cases, adjacent T/R channels will need to be isolated using either channelized grounded cavities or metal covers.
Application Dictates Frequency Range
Lower frequencies such as S-Band are ideal for long-range applications, including seaborne surveillance and tracking, achieving higher output power per element and lower atmospheric attenuation. S-Band systems typically utilize Silicon LDMOS or GaN discrete transistors for the output stage of the PA.
C-Band is commonly used for short range or medium range mobile battlefield surveillance and missile control applications that require rapid relocation and deployment. This higher frequency allows the size of the antenna to be reduced, while also improving accuracy and resolution, thus creating the potential for radar systems that are mounted on mobile platforms.
X-Band offers even further size reduction and enhanced resolution, as over 1,000 elements can be concentrated within a square meter, which is ideal for airborne applications where minimizing size and mass are critical requirements. Fighter aircraft can utilize X-Band systems for intercept and attack of enemy fighters and ground targets. In their multi-function role, these radar systems can also serve as a high-speed data link. The compactness of an X-band system is also ideal for shorter-range applications, including border surveillance, where the need for man portability and fast deployment are essential. Figure 4 shows an X-Band sub-array face.
Compactness of an X-band system is ideal for shorter-range applications, including border surveillance, where the need for man portability and fast deployment are essential.
Benefits in Agility, Flexibility
The growing need for systems that offer higher levels of agility, flexibility, and functionality over a wide range of radar and data link applications will stimulate ongoing demand for modular, stackable AESA radar solutions that are configurable to both retrofit and new system design. Modular, stackable AESA radar systems provide a cost effective alternative that reduces the total cost of ownership: accomplished through the use of COTS components and MMIC technology, ease of installation and integration, graceful degradation without a single point of failure (PoF), and in-field TRM replacement (LRU) without having to take the entire system off-line.
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