Airborne radar has always been complex, given its weight restrictions, performance goals, and need for ruggedness and reliability. From today's cutting-edge military aircraft to the smallest unmanned aerial systems (UASs), these radar systems also have faced ongoing miniaturization. As a result, airborne-radar systems can fit more functionality into tighter spaces. At the same time, they have maintained and even improved their broadband performance levels. In the face of increased interference and other problems, for example, such radar has had to become impervious to "littered" environments. Airborne-radar systems also are leveraging the strengths of process technologies, adding cutting-edge imaging techniques and more to provide enhanced information for today's warfighters.

Brian Coaker, General Manager at e2v's Microwave Technology Centre, points to changes in radar sensor payloads for UASs in particular. "Alongside emerging work on higher-efficiency, lower-noise solid-state technologies," Coaker states, "e2v continues to address requirements for low-mass, compact sources of RF power, especially for use on small platforms such as UASs. Miniature, high-power magnetrons and traveling-wave tubes (TWTs)coupled with compact modulator packagescontinue to be developed and deliver multiple hundreds of watts from TWTs and kilowatt-power levels from magnetronsover 2 to 40 GHz."

In addition to leveraging more power from smaller packages, UASs must provide optimal performance in terms of their radar capabilities for intelligence gathering, terrain mapping, weather monitoring, and more. At the cutting edge are innovations like Lockheed Martin's Tactical Reconnaissance and Counter-Concealment-Enabled Radar (TRACER), which promises to provide intelligence beyond the capabilities of today's higher-frequency radars or electro-optical systems.

Last month, TRACER completed flight testing aboard a Predator B MQ-9 UAS (Fig. 1). This dual-band (UHF and VHF) synthetic-aperture radar (SAR) can detect and geo-locate objects that are buried, camouflaged, or concealed under foliage. Classified as a "queuing sensor," TRACER can immediately downlink captured images to multiple ground stations.

TRACER is predicated on Lockheed Martin's foliage-penetration (FOPEN) system, which was developed to detect vehicles, buildings, and large metallic objects in broad areas of dense foliage, forested areas, and wooded terrain. The radar's advanced-detection capability suppresses background clutter and returns from stationary objects. At the same time, it reveals the positions of mobile and portable targets.

Another prominent example of cutting-edge radar in a UAS is the Global Hawk from Northrop Grumman Corp., which just arrived at Grand Forks Air Force Base in North Dakota. The Block 40 Global Hawks are equipped with Northrop Grumman's AN/ZPY-2 Multi-Platform Radar Technology Insertion Program (MP-RTIP) sensor, which was built in partnership with Raytheon Space and Missile Systems. The MP-RTIP active electronically scanned array (AESA) radar is the first radar sensor to concurrently use synthetic-aperture-radar imaging while tracking moving targets simultaneously over large areas.

Of course, AESA radar's reach extends well beyond UASs to manned military aircraft and beyond. Raytheon Co., for example, recently received a contract from Boeing for the second procurement in the four-year Multi-Year III program. It will produce and deliver APG-79 AESA radars for F/A-18 Super Hornet tactical aircraft. According to Raytheon, the APG-79 AESA radar hardware has 10 to 15 times greater reliability compared to mechanically scanned array radars.

Given these and other attributes, AESA radar is being credited with historically altering the course of airborne radarand impacting the requirements for radar components, as a result. As stated by Larry Hawkins, RFG Business Development Manager at Analog Devices, "Through the years, radar designs have changed from passive electronically scanned arrays (PESA) to active electronically scanned arrays (AESA), the difference being that AESA radars have separate TRX modules, while PESA radars have one TRX module with separate phase shifters. This increased significantly the amount of active components in a radar and the amount of current/heat and size of the radar. Modern-day AESA radars continue to increase the amount of elements in the radar to increase its performance, impacting most of the RF components in the radar. The drive is for smaller, more integrated components using less power and yet providing the performance necessary for radar."

In addition to changing the requirements for components used, AESA radar demands some different types of components. An example is the amplifier. According to Dave Lester, RF Power Products Marketing at Freescale Semiconductor, "Radar systems are evolving dramatically. The AESA radar allows solid-state amplifiers to compete with TWT-based designs. They rely on amplifiers delivering comparatively modest power levels to drive antennas with high levels of gain to produce narrow pencil' beams at very high, effective radiated power levels. This is a significant departure from legacy radar systems, which required large tube-based amplifiers to generate massive amounts of power over wider beamwidths."

Brian Coaker from e2v also notes the changes in amplifier as well as antenna and component designs for AESA radar: "The migration of some radar platforms to active-phased-array (APAR) architectures has seen a shift from consolidated transceivers to distributed transmit-receive modules (TRMs) across an APAR array. In turn, the vacuum-power tube (magnetron, klystron, or traveling-wave tube) within a unitary transmit (Tx) chain is now augmented with solid-state power-amplifier (SSPA) options for many frequency and power bands."

In terms of components, Coaker notes, "Duplexed single-antenna systems continue to drive the need for low-loss circulators and receiver protectors. The proliferation of radar systems around the globe and greater user awareness of electromagnetic environments continue to drive requirements for sensitive, fast-acting limiters and receiver protectors, with filter characteristics to ensure resilience to a combination of operational and environmental factors. Effective and efficient protection of receivers therefore remains a key requirement on new and in-service radar systems" (Fig. 2).

To provide the power and performance needed for AESA radar, many are looking beyond the components to high-power semiconductor technology. For example, Northrop Grumman announced that is has reliably operated its gallium-nitride (GaN)-based high-power transmit/receive (T/R) modules for more than 180 days during continuous high-power testing. The T/R modules were tested using high-stressing, operational, longpulse waveforms, which were designed to simulate the electronic activities of actual radar functions. The testing was performed in an environment that allowed the firm's engineers to understand how well the modules would perform in tactical operation. With the success of these tests, Northrop Grumman has proven that the next AESA generation is capable of reliable operation while producing much greater radar sensitivityat the same time, providing higher efficiency at lower cost.

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Brian Battaglia, Director of Sales and Marketing at Integra Technologies, Inc., also sees a growing place for GaN in radar and avionics applications. As he emphasizes, "Silicon technologies are very mature and are limited in the amount of performance jumps that can be expected in future generationsespecially at higher frequencies above 5 GHz. For increased performance at the C-band of operation, newer technology is required. GaN technology has superior power density producing higher-performance devices in smaller packages than their silicon counterparts."

Battaglia notes that phased-array-radar (PAR) systems in particular could benefit from the system cost savings brought about by more power in a smaller package, which also would enable physically smaller designs. He emphasizes, "These system use 100s if not 1000s of devices in parallel to produce very high power levels. You can imagine the complexity involved with such an endeavor and the advantage of having the more powerful device in the smallest physical footprint sizeThe tradeoff in miniaturization of RF power components is in heat dissipation. The smaller devices have a physically smaller area in which to dissipate heat. This is offset by the higher performance in efficiency and thus less wasted heat of the GaN technology itself. In all RF power-amplifier systems, heat is critical to performance and long-term reliability. So this issue must be accounted for in the design process." Integra is currently targeting high-frequency radar systems in the C- and X-bands using a hybrid GaN/ silicon-carbide (SiC) approach.

Of course, RF power in radar, avionics, jamming, and tactical communications applications is still being derived from LDMOS as well. Lester notes that Freescale sees increasing performance requirements for both radar and avionics applications. "Both share the need for higher power levels and efficiency," he states. "LDMOS devices are used extensively in both applications because LDMOS offers high performance coupled with proven reliability, proven with millions of devices operating for billions of hours. LDMOS delivers both high power and high efficiency over frequency ranges used in avionics and radar applications."

Lester points out that LDMOS devices have seen continual, dramatic performance enhancements over the years. Such enhancements were required to meet the demanding and constantly evolving requirements of the wireless base-station marketplace. When employed in avionics systems, for example, they now offer comparable performance at substantially lower system cost when compared to other compound-semiconductor technologies.

In addition to providing new levels of performance, aircraft radar also will allow features to be combined that could not be merged before. An example is the recent installation and testing of a multispectral intelligence sensorhoused in a new keel beam accessory bay (KAB)on a modified E-8C Joint Surveillance Target Attack Radar System (Joint STARS) aircraft. This installation and test examined the use of the MS-177 camera, a 500-lb. multispectral intelligence sensor, on the all-weather Joint STARS weapons system (Fig. 3).

The goal was to prove that the sensor could improve combat identification, helping Joint STARS provide "eyes in the sky for boots on the ground" and thereby reduce the sensor-to-shooter timeline to mere minutes. While in test flights off the coast of Florida, Joint STARS operators had the MS-177 sensor collect information and stream it into the existing battle management system. The operators could simultaneously exploit ground moving target indication (GMTI) and high-resolution imagery, thereby expanding situational awareness. Images also were transmitted to off-board SIPRNET elements using beyond-line-of-sight (BLOS) satcom-system capabilities.

Beyond traditional microwave components, many next-generation radar enhancements will probably come from the digital side. For example, processing power is at the heart of a contract received by Raytheon Co. for the continued production of ALR-67(V)3 digital radar warning receivers. The ALR-67(V)3 is the first deployed radar warning receiver to combine a fully channelized digital receiver architecture with the power of dual processors. It promises to detect emitters in high-density electromagnetic environments and relies on digital technology for improved reliability. It is made for installation on all frontline, carrier-based F/A- 18 E/F tactical aircraft and is an integral part of modernization programs for US and international customers.

In addition to exemplifying the features that such processing power makes possible, this announcement reminds radar system integrators that this is a changing world. Microwave components have formed the heart of radar sensors for decades. With seemingly everything that is engineered being enhanced through digital technologies, however, airborne radar is sure to follow suit.