Making AESA Radar More Flexible

December 1, 2015

A modular, building-block approach brings greater design flexibility to Active Electronically Scanned Array (AESA) radar technology, simplifying system integration, and permitting rapid first-line repair with no downtime using standard off-the-shelf components.

Until recently, AESA radar was utilized almost exclusively by prime aerospace contractors within their own proprietary systems. These customized solutions were relatively costly and time-consuming to manufacture, and not reconfigurable to alternative uses.

Figure 1. The “roadmap” to an Active Array Antenna Unit (AAAU).

Meanwhile, growing requirements such as border security have punctuated the need for a modular, building-block approach that expands the use of AESA radar technology to a wide array of applications including naval, airborne, vehicle-mounted, and ground-based systems; coastal and harbor security; air traffic control; foreign object detection (FOD) for airport runways; satellites; and data links.

AESA radar systems contain multiple 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. By generating unpredictable scan patterns, AESA radar systems can track multiple targets simultaneously. These scan patterns are also difficult to detect by radar warning receivers (RWRs) – particularly older systems – thus providing high jamming resistance. These systems can also operate in 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 able to support peer-to-peer networking by combining data from multiple platforms while also delivering expanded radar coverage and enhanced resolution.

AESA radar does have its limitations; the highest field of view (FOV) achievable for a flat phased array antenna is generally between 90 to 120 degrees. Wider coverage can be obtained through multiple antenna faces or two rotating antenna faces. Similarly, an X-band array mounted onto the nose of an aircraft can expand its FOV through the use of a mechanical gimbal.

Thermal management is also required to dissipate heat generated by the power amplifiers (PAs) that are distributed across the antenna face. The cooling system must fit within the limited space envelope between the elements.

A Modular, Stackable Approach

A recently introduced Active Antenna Array Unit (AAAU) consists of modular Quad Transmit Receive Module (QTRM) sub-arrays (Figure 1), which are also available as a standalone product or as scalable planks. The typical plank is constructed from four QTRMs, along 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. The Planks are designed to plug into slots in the main array structure to create a 2D array solution.

Each QTRM module (Figure 2) consists of four T/R channels, each containing a power amplifier (PA), a low-noise amplifier with receiver protection, along with digitally controlled phase and gain control elements to reduce undesirable sidelobes. QTRM modules also feature local DC power supply conditioning, a built-in logic interface for serial control and BITE power supply monitoring, and a protective thermal shutdown facility.

The QTRMs are supplied factory-calibrated and individually addressed for plug-and-play installation and rapid integration. The system integrator simply programs in adjustments for external system loss, antenna offsets, and phase offsets. A key performance attribute is graceful degradation, as each T/R channel is individually controlled, so the failure of any individual T/R channel will not impact the rest of the module. By contrast, legacy radar systems can become inoperable due to a single Point of Failure (PoF), such as the loss of the travelling wave tube (TWT) power amplifier.

Modular, stackable QTRMs use standard commercial off-the-shelf (COTS) components, and are designed as Line Replaceable Units (LRUs) to reduce first-line repair costs. Individual QTRMs have unique address codes so individual modules can be swapped out anywhere within the overall array without incurring any system downtime. By contrast, with older, non-modular AESA systems, the entire platform needs to be taken off-line in order to perform routine repairs, maintenance, and upgrades.

Choosing the Right Frequency Range

Modular, stackable QTRMs are available at X-band and C-band, along with a Dual Transmit Receive Module (DTRM) at Sband (Figure 3). Dual-module S-band systems are ideal for long-range applications such as seaborne surveillance and tracking, where higher output power per element and lower atmospheric attenuation must be achieved. S-band systems utilize Silicon LDMOS or GaN discrete transistors for the output stage of the PA.

C-Band radar is most commonly utilized in short- and medium-range mobile battlefield surveillance and missile control applications where rapid relocation and deployment are required. Higher-frequency C-band radar systems permit the use of a smaller antenna while also improving accuracy and resolution, thus enabling radar systems to be mounted onto mobile platforms.

Figure 3. A Quad Transmit Receive Module (QTRM)

Use of the X-band frequency range permits even greater miniaturization and resolution enhancement, as over 1,000 elements can be concentrated within a square meter. These miniaturized systems are commonly used by aircraft for intercept and attack of enemy fighters and ground targets, and also as high-speed data links. X-band systems are also ideal for short-range applications, including border surveillance, as their compact size permits man-portability and fast deployment.

Additional Technical Considerations

Application-specific requirements dictate the frequency, which, in turn, influences the available space envelope, the circuit topology, and the circuit technology.

The available space envelope is dictated by the need to maintain a halfwavelength (or less) antenna spacing in order to reduce undesirable grating lobes, and by the array configuration, with total power output limited by the module’s size, frequency, and heat dissipation requirements. Multiple modules are packaged into a single housing – typically four per module for the higher frequencies (C-band and above), and two per module for S-band – providing sufficient space for full digital functionality, local power supply conditioning, and a single, all-encompassing environmental seal rather than multiple channel-to-channel seals.

For circuit topology, the functional building blocks of a typical T/R channel remain the same regardless of overall system requirements. For C-band frequencies and above, a MMIC core chip is likely utilized along with a low-noise amplifier MMIC in the receive path, and a power amplifier MMIC in the transmit path. The MMICs are typically designed as a chip set, with the power amplifier being driven directly from the core chip.

The typical core chip consists of a digital phase shifter and an attenuator, along with low-noise and medium-power amp - lifiers that interface directly with the receive and transmit path MMICs. Switches within the core chip allow the attenuator and phase shifter functions to be utilized in both transmit and receive paths, thus forming a common leg circuit. The system’s minimum detectable range (MDR) can be reduced by minimizing T/R switching speed, limiter recovery time, and DC supply gating circuit requirements.

A limiter circuit in the LNA protects the device from high-power RF signals generated 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. Combining a low-noise receive channel with high output power extends the signal transmission range. Adjacent T/R channels always need to be isolated using channelized grounded cavities or metal covers.

The circuit technology is influenced by the frequency band, which, in turn, dictates the available space envelope. Lowerfrequency designs lend themselves to a single-layer RF PCB design mounted onto a backplane, with SMT packaged MMICs, and drop-in devices such as circulators or packaged discrete transistors. Higher-frequency designs have a smaller space envelope, making it difficult to fit all the required RF functionality and associated interconnects onto a single layer, and prohibits the use of packaged devices. Therefore, a chip and wire approach is required using a highly integrated MMIC chip set. A multilayer approach can also be considered using either LTCC packaging or a mixed-media multilayer board.

Conclusion

The development of a modular, stackable approach to AESA radar enables this technology to be quickly and cost-effectively adapted to a wide variety of applications. This modularized approach reduces the total cost of ownership by using COTS components and MMIC technology, and by simplifying installation and integration. Once installed, these modularized systems are also relatively inexpensive to maintain, offering graceful degradation reducing single points of failure (PoF), and permitting in-field TRM replacement (LRU) without having to take the entire system off-line.

This article was written by Mark Howard, Chief Engineer at API Microwave Ltd., Philadelphia, PA. For more information, Click Here .