AESA – the next step forward

A couple of interesting articles on AESA and what we can expect next..

Small Aircraft May Become Large Sensors
David A. Fulghum, Douglas Barrie, and Robert Wall

An emerging arena for new competition in the defense industry will involve the melding of airframes and what they traditionally have carried internally.

The debate about whether platforms or payloads are more important will soon shift fundamentally as systems–particularly sensors, communications and weaponry as they merge into one–move to the outside of platforms and become their skins. Moreover, there are moves afoot to integrate the two more closely, even on more traditional designs.

Conformal arrays are being eyed for manned and unmanned aircraft, ships and ground vehicles. But they seem to have a special niche in unmanned aircraft. There the operational and low-cost advantages of small, stealthy, missile-size unmanned combat aircraft can be fused with the benefits of a sensor that uses the whole exterior of the platform as an aperture to communicate, image, jam or function as a weapon that fires high-power spikes of energy into enemy electronics.

Aerospace officials generally will not address this new industrial battlefield on the record because they don’t want to offend their commercial airframe partners. But some outline what they think is underway in the laboratories of major U.S. and European defense companies.

There’s also a race underway for international supremacy. Europe has trailed the U.S. in development of active electronically scanned arrays (AESAs). This time, there’s an all-out effort on to make sure that gap doesn’t emerge when it comes to the next-generation of arrays.

Some view what’s taking place–particularly in the U.S.–as a battle, with the sensor houses challenging the airframers for primacy.

In Britain, other fundamental question are under consideration. The Defense Ministry, for instance, sees no need to develop a manned, fast jet platform to eventually succeed the Eurofighter Typhoon or Lockheed Martin F-35 Joint Strike Fighter. What it does want to develop in the coming decade are high-end unmanned combat and reconnaissance air vehicles. Conformal multiaperture sensors, sometimes dubbed smart skins, will be central to this effort. Ownership of this technology would grant a contractor or nation, in an international competition or collaboration, significant input to the nature of the airframe.

U.S. industry observers believe there will be a continued effort to produce small numbers of large unmanned combat aircraft, based on designs like Boeing’s X-45 and Northrop Grumman’s X-47, because of the desire to conduct round-trip missions and avoid detection by enemy air defenses. At the same time, researchers are aware of the need for large numbers of sub-$100,000, missile-size unmanned aircraft that can perform missions with a single sensor. They see these aircraft decreasing in price while increasing in their range (due to more efficient engines) and payload capability. This would allow small unmanned aircraft to have operational parity with larger, more expensive designs.

Some companies are working toward networks of small, stealthy aircraft that provide many types of information that can be fused into a richer, more detailed image. Here, physics is maximized from both ends–with smaller, harder-to-detect platforms that carry antennas as large as the aircraft.

Moreover, researchers envision a number of aircraft working together as a single virtual array that would allow them to function at very low frequencies. They contend that experiments are already being conducted using Hunter- and Shadow-size unmanned aircraft.

So there’s a push to build lightweight antenna arrays that serve as aircraft skins and require very little electrical power. While initially these arrays would be single function, there is some promise that eventually they could be designed as multifunction sensors. Even a small array with limited power could serve as a microwave weapon if it got close enough to an enemy emitter or other antenna arrays, they contend.

The U.S., Britain, Australia and Italy all have expressed interest in directed-energy weapons that can be produced through software modifications to the F-35’s electronically scanned array radar. That nonexplosive technology is popular with left-leaning governments, and it’s expected to be miniaturized enough over time to fit into much smaller unmanned aircraft.

Integrated arrays should produce vehicle designs that are optimized for low observability. But whatever the long-term road map for a transition to aircraft that are virtually flying antennas, the world’s military establishments will continue to fly conventional, manned aircraft for the foreseeable future. A profitable avenue for industry will be upgrading these older aircraft with the new antenna technologies, and prolonging their operational lives by making them formidable weapons and sensor platforms.

In particular, front- or nose-positioned AESAs are likely to remain the central element of the sensor package of manned fighter aircraft for decades. As a result, researchers are working on reducing some of those system’s disadvantages, since efforts to produce conformal arrays have in part been driven by the limitations in the conventional approach to building air intercept radars.

Both the U.S. and Russia have looked at adjunct antennas to supplement the primary nose-mounted sensor. The F-22 has “chin cheeks” for two small conformal arrays. In the 1990s, Russian designers experimented with using a secondary passive e-scan antenna mounted at the rear of an aircraft to provide greater radar coverage.

There are also low-observable and jamming implications for AESAs. Forward-positioned arrays are potentially large reflectors, which could compromise radar cross sections.

However, frequency-selective radomes can now be designed to let only signals pass through that operate in the frequency of the radar itself. This makes it harder for an adversary’s sensor to penetrate the radome because it would have to be exactly in the same band.

Moreover, developers are working on tunable radomes that can change their filtering characteristics as the onboard array shifts frequency. Frequency agility provides added target-recognition benefits, while making it harder for an adversary’s electronic warfare suite to characterize the threat. Being electronically agile while masking the array is seen as a huge boon to survivability.

Also being examined is plasma-shielding using a generator within the radome to produce a comparatively small field to ward off enemy electronic penetration. The U.S., Russia and European states have active programs in plasma research for airframe applications.

Designers Plan Radar as Aircraft Skin
By David A. Fulghum

Radar will begin making its transformation by year-end from the large, heavy device inside aircraft to large, thin arrays that become part of the wings of long-endurance unmanned vehicles or the skin of a high-altitude airship.

Within a few more years, radar will be available in shapes that fit one-dimensional curves like reconnaissance pods or the fuselages of large, manned reconnaissance platforms. These include the E-10 surveillance aircraft, E-3 AWACS and RC-135 Rivet Joint (signals intelligence) follow-on designs.

And in about five years, with adequate funding, the new antenna arrays will be built in complex two-dimensional curves to fit onto almost anything flying, from the newest unmanned combat vehicles to retrofit packages for basic cargo aircraft, according to Charles Engels, deputy director for advanced RF programs at Raytheon Space and Airborne Systems.

In addition, there’s the potential for a single, large array to track targets, image and communicate. Over time, new features could be added, including the ability to jam enemy electronics and mount electronic attacks. This array could even serve as the launch site for communications/computer network attacks (although electronic attack from long range creates big power and cost problems). Some radar and electronic warfare specialists contend that a better solution than long-range electronic attack would be to wrap conformal arrays around missile-size unmanned combat aircraft–possibly cheap enough to be expendable–and fly them to within a short distance of their targets. At short range, less power would be needed to create a damaging spike of energy.

Some aerospace analysts see an industry shootout between the large airframers that specialize in platforms and the systems houses that don’t really care about the platform, only what goes inside them. Advocates of the systems approach contend that small, missile-size aircraft stuffed with the latest electronics may take the unmanned combat aircraft market away from the larger X-45 and X-47 designs.

A longtime Pentagon UAV specialist says the answer is a combination of the two: The military wants aircraft, both manned and unmanned, that it can use over and over, he says. But when the targets are heavily defended, they want the smaller, one-way aircraft to take over. What may upset that formula is a new generation of small, powerful, fuel-efficient engines that may give small unmanned aircraft a round-trip range.

However, in the near term, the goal is to build a 0.4-2-in.-deep radar array that weighs about 8 lb. per sq. ft. and can be expanded to fit the platform. In fact, the Defense Advanced Research Projects Agency has awarded Raytheon an $8-million contract to build a football field-size antenna that weighs about 2 tons.

The active electronically scanned array (AESA) radar antenna is to be bonded to the hull of an unmanned airship. From an altitude of 65,000-70,000 ft., the array would transmit both on UHF (for low-bandwidth communications and long-range surveillance) and X-band (for high-resolution radar capable of finding very small targets) to the horizon, about 400 naut. mi. The antenna would be associated with Darpa’s Integrated Sensor Is Structure (ISIS) program, which also is studying saucer-shaped airships.

Darpa wants to look for airborne and ground targets, and communicate directly to units on the battlefield using a single, electronically steered antenna. The hull would be the support structure for the antenna. The bonding itself will be critical, because it will have to withstand temperatures of -112F.

The key to building these large antennas without driving up weight and cost is to make them capable of carrying aerodynamic skin loads, but not make them part of the aircraft’s load-bearing structure. That means integrating the antenna without making a hole in the airframe that has to be reinforced.

“There are several big payoffs” that come from using conformal radars attached to an aircraft’s exterior, Engels says. “The first is ubiquitous sensing. Virtually any aircraft could easily be converted for gathering intelligence. A wing, door or fuselage could become the radar antenna so that more interior space is available for other payloads.” For decades, the U.S. Air Force and Army have used aircraft designed for innocuous transport roles as intelligence collectors. Now virtually any aircraft, manned or unmanned, could conduct surveillance missions.

Larger apertures mean better radar performance. For example, the long wings of a Predator UAV could be reskinned with X-band antenna arrays. X-band offers a good compromise between range (lower frequencies travel farther) and increased resolution (high frequencies resolve smaller objects). “Normally the wing is only used to carry fuel. But a large, wing-size antenna produces more gain, which equates to longer standoff ranges and better discrimination of small targets. However, work is moving forward on arrays in a wide range of frequencies, including VHF, UHF, S, X, Ku and Ka bands.

In addition, putting radar on the aircraft’s exterior leaves much more room for larger apertures. It also eliminates the need for a radome, which increases aerodynamic drag. That, in turn, improves loiter time over the battlefield, and it allows the manufacturer to design the aircraft for a reduced radar signature. Here again, the shape of the antenna will dictate how difficult the signature is to control. A flat conformal array is simple to build, and its transmission and reflective characteristics are easy to predict. “That technology is here, today,” Engels says. An aperture with a one-dimensional curve is considered appropriate for smaller UAVs or in externally carried pods. Raytheon is building one-dimensional curved antennas and is testing them now.

“The hard nut is a conformal aperture with two-dimensional curves,” Engels says. “It’s a more complex surface, so it’s more difficult to manufacture.” Another problem is producing regular spacing of the antenna transmitter and receiver elements. Nonetheless, the effort is expected to enable “ubiquitous sensing for everything from the striker to the trash hauler.” For cost and weight to remain low, he says the radars will be built from a family of components that include radiators, cooling innovations, radical chip designs, chip attachments, interconnections and power distribution systems–all of which are required to make panel arrays a reality.

Projecting the capabilities of such devices in a decade or so, researchers believe these large antenna arrays could be used for producing pulses of high-power microwaves to jam enemy electronics or even damage their components with sharp, powerful pulses. Using advances in software and hardware, a single array could be capable of electronic attack, jamming, transmission of large imagery packets, cooperative combat identification and long-range sensing.

For example, the Army contemplates using electrically large arrays operating in the Ka band, where they can both sense and communicate. Communication rates could vary from peaks of 100 Mbps. to a sustained average (when operating on a time-share basis) of 10 Mbps. over ranges of 80 km. (50 mi.). That would allow the radar to serve as a data link for the aircraft’s other sensors, including electro-optical, infrared video, ground moving target indicator and synthetic aperture radar images–all at the same time.

What’s less certain is whether the electronic attack capability could be practical for large arrays. The capability would require the generation of large amounts of electrical power on the aircraft as well as the ability of the radar to produce a narrow, powerful and steerable beam. Once again, weight becomes an issue because it’s harder to build large, light antennas in the higher frequency bands, and the cost of the system goes up as the distance from the target increases.

For now, the technology is being demonstrated in the laboratory and is considered, in some forms, to be about three years from an operational capability. The nearest-term use of conformal antennas is thought to be for unmanned applications–the Global Hawk unmanned reconnaissance aircraft, the Predator A and B, and the A160 unmanned helicopter. Manned uses are expected to include the E-10 (its follow-on or as an upgrade to the E-8 Joint Stars), an E-3 AWACS follow-on and an RC-135 Rivet Joint follow-on for the electronic surveillance mission. The last example also could involve a follow-on for the abandoned Army/Navy aerial common sensor aircraft.

According to Raytheon, a conformal 1D array will be demonstrated around the end of this year, and the 2D complex curve will follow in about five years.