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Over the Horizon

Radar technology is about to make its next big leap forward and could revolutionise the design of future unmanned surveillance aircraft

The next big step in US military radar technology will this week be put under test. The subject is a five-element-by-five-element subpanel of a low-band active electronically scanned array (AESA) radar, but it is unusual in that it also functions as a load-bearing part of an airframe structure, such as a fuselage panel or length of wing.

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Going forward from this test, the US Air Force has mapped a 15-year development path. The half-scale subarray will advance to flight test within three years. A full-scale system may not be available for another dozen years, but it would be used to justify the design of the next generation of unmanned surveillance aircraft.

This new vehicle, which today is named SensorCraft, would be a marriage of stealth, endurance and a low-band array nimble enough to provide 360° coverage while sensitive enough to detect targets under dense foliage, according to Jim Perdzock, head of the SensorCraft programme at the US Air Force Research Laboratory (AFRL) air vehicles directorate. The vehicle would be expected to remain in flight for 60h, and air-to-air radar performance by 30dB and air-to-ground performance by 12dB compared with existing systems.

Higher-frequency X-band variants of this new breed of structural load-bearing antenna could be fielded even sooner, perhaps within a decade, replacing what could soon be known as “dumb” aircraft skin panels on fighters and unmanned aircraft.

The presumed launch candidate is the Joint Unmanned Combat Air System (J-UCAS) programme, and either the Boeing X-45 or the Northrop Grumman X-47. But load-bearing structural arrays could also replace mid-fuselage panels on the USAF’s latest manned fighters, such as Lockheed Martin’s F/A-22 Raptor and F-35 Joint Strike Fighter (JSF).

Reshaping surveillance

If the technology fulfils its promise, the implications could be significant for both military and industry. US defence planners are already thinking about how to reshape surveillance operations doctrine on the premise that, with a fleet of SensorCraft, it would be possible to detect and track any target – moving or stationary – in a defined battlespace at any time.

In the past two decades, US air power has improved its ability to drop a single bomb within metres and perhaps centimetres of a target. This has created a new demand for intelligence, surveillance and reconnaissance assets to produce more targets, and with increasingly higher resolution. There is also the need for more powerful – therefore, larger – low-band radars on surveillance UAVs, for detecting targets obscured by forest or jungle canopies.

The problem with existing active radars is that they have to be mounted on to, within or underneath an aircraft, introducing a weight penalty and sometimes creating drag. Meanwhile, US warplanes are using ever-more precise and smaller munitions, which drive the need for more precise targeting co-ordinates. That increases the need for better resolution from the sensor. Increasing radar resolution means increasing the size of the sensor aperture, which again adds more weight and possibly more drag. Simply building a larger conventional antenna is perhaps not the solution.

To find another way, the structures division of AFRL’s air vehicles directorate started serious work on the concept of load-bearing structural arrays in 1993, says division leader Jim Tuss. The concept is founded on the same principles that allow most modern communications antennas to become part of the aerodynamic design of a military aircraft, he says. The SensorCraft vehicle concept took form years later, while the initial focus has been on developing this new kind of radar array that can serve as part of the airframe.

Melding an antenna into the aircraft skin has advantages. Since most of the airframe skin then can be used as a sensor, the size of the aperture is constrained only by the aircraft’s dimensions. Moreover, when the sensor is no longer attached as a bulky payload, more fuselage space is freed for other equipment or fuel. The aircraft’s aerodynamic integrity is preserved as far as the array structure can sustain loads.

But there is also no shortage of technical challenges to be answered, as Tuss concedes. First, the AESA algorithms that are responsible for phase tracking have to greatly increase in complexity. And it should be noted even planar active arrays are in a relative infancy, with only 18 US fighters fielded with an AESA capability since 2002.

One challenge is the lack of a flat surface, the standard for current AESA technology. An array that is a load-bearing structure is, by definition, conformal to the airframe, which often has a curved shape. New algorithms will be needed to manage the AESA beam-guiding technique, adjusting for the added complexity of a non-linear surface. To date, the AFRL’s sensors directorate has been able to map and estimate “first-order” performance differences between planar and conformal arrays, a key enabler for the next phase of tests.

Many aircraft structures, especially the wing, twist, wobble and vibrate during flight, movement that must be compensated for by the radar processor. In addition, the antennas must be strong enough to survive loads and other in-flight events, such as birdstrikes, gust-loads and, potentially, battle damage.

Above all, the array material has to be practical, according to the AFRL. That means being affordable, being as reliable in service as the aircraft skin it is replacing and capable of easy assembly on the manufacturing line. Operating costs for such arrays must be roughly equal to sensors on the Northrop Grumman RQ-4 Global Hawk, says Perdzock. The load-bearing antenna is composed of layers of composites, and an affordability initiative has been launched to reduce a $50-100 million sensor suite cost to $5-10 million.

Despite these challenges, the programme has continued to inch forward, achieving a steady but patient progress. By 1999, Tuss, of the AFRL’s structures division, and a Northrop Grumman research team had achieved their first success in a ground test of a prototype antenna. They made a load-bearing, active antenna array, measuring 1 x 1m (3 x 3ft).

The curved array was subjected to loads of 280kg/cm2 (4,000lb/in2), a condition comparable to the in-flight loads on a mid-fuselage panel of a Boeing F/A-18 Hornet. The array’s structure not only had to survive the loading over a fatigue life cycle, but also perform its surveillance function. The experiment was marked as a success, says Tuss, with the array achieving a 150% design limit load before failure. Northrop claims to date that the array has survived loads of more than 700kg/cm2.

The Northrop-funded effort con­­tinued after 1999 as the SensorCraft conformal low-band antenna structures (S-CLAS) programme. That produced a 25-element array, measuring 7.6 x 2.7m, half the scale of the 15m-long array projected needed for the SensorCraft vehicle. However, the programme did not include funding for a ground test, but that was appropriated finally in 2003 as a Congressional add-on to the US defence budget.

Now renamed the low-band structural array (LOBSTAR) programme, the AFRL/Northrop sensor faces a defining, month-long series of ground tests starting this week. It marks the first government-funded pursuit of a structural array, 12 years after the AFRL began to study the feasibility of transforming common aircraft structures into active sensors.

Scaling up

LOBSTAR is a five-year $12 million effort that is a major scale-up of research activity, starting with the ground test in September of the array produced under the S-CLAS programme. A flight test is scheduled for 2008 using Northrop’s BAC One-Eleven testbed aircraft, potentially against real ground targets. The low-band array is focused on detecting and tracking slow-moving targets hidden under foliage, which is not possible using conventional ground moving-target indicator (GMTI) radars. The sensor also can be used for air-to-air tracking.

While LOBSTAR will continue to evolve over the next decade, the more recent X-Band programme has been put on the fast track. In April 2004, the AFRL sensors directorate awarded contracts to Raytheon and Northrop to compete for a new development programme called X-Band thin radar aperture (XTRA), which is focused on producing a synthetic-aperture radar and GMTI sensor for tactical aircraft. The AFRL downselected to the Raytheon proposal in December 2004.

XTRA is another conformal, load-bearing antenna structure that would become part of SensorCraft, but there may be more immediate applications. Perdzock says XTRA “represents more of a transition opportunity”, compared with LOBSTAR, for interim upgrades J-UCAS and manned fighters for which a conventional SAR and GMTI sensors are deemed impractical. These could have access to the new technology after 2012, he says.

The $4 million XTRA contract provides funds to carry out a demonstration of the array in September, according to Raytheon officials. This first demonstration will focus on testing a miniaturised version of an X-band radar processor, while the front-end antenna will be an off-the-shelf model.

The concept for the SensorCraft vehicle is a high-aspect ratio joined-wing design, of a kind that has yet to be tested in flight. The joined-wing shape, in which the forward wings are swept back to join wings extending from the aft fuselage, is considered ideal for a platform attempting to provide 360° radar coverage. Active conformal arrays can be embedded in the load-bearing structures of both sets of wings, with a pair of arrays each pointing forward and aft.

The vehicle would have unique propulsion requirements, being the first aircraft to combine both air MTI and ground MTI radar modes. That combination had been planned for the Northrop E-10A Multi-sensor Command and Control Aircraft (MC2A), but had to be discarded due to the constraints of the power capability of the Boeing 767-400ER testbed. For SensorCraft, the AFRL estimates that existing aircraft power-generation capacity must be doubled to meet the needs of the sensor. The engine also must be rated to operate at such levels continuously for 60-80h, the projected length of an average flight.

A Boeing/Raytheon design team has a lead position in early design studies, proposing an airframe that most closely matches the USAF’s design goals. So far, the vehicle exists only in conceptual studies. Both companies have the challenge of designing not only one of the first joined-wing vehicles, but also the first to be autonomously controlled, requiring a whole new body of flight-control data for such an aircraft.

Northrop has proposed another concept vehicle design, which it is offering the AFRL as an alternative to the “joined-wing” approach, which the company considered to be highly risky. This vehicle has the classic flying-wing shape, with a wingspan of more than 30m. Northrop believes it can develop a single active array giving 360° target sweep. This would offer the advantage of allowing a more conventional airframe development and require less power-generation capacity.

Third candidate

The AFRL’s Perdzock has also presented a slide showing a third candidate vehicle, which he describes as offering the most conventional design approach, with a straight wing and traditional fuselage structure. Perdzock declined to comment on the source of the proposed design.

For the radar industry, which has entered a period of competitive stalemate on tactical aircraft programmes, there is pressure to avoid missing out on what could be the biggest engine of funding and technological growth for the next few decades.

Although AESAs will only start to come of age with the introduction of the Boeing F/A-18E/F Block II in November, it is a market that has been frozen for several years. Northrop produces the APG-77 and APG-81 radars, to be fielded on the F/A-22 and F-35, respectively. Raytheon makes the APG-63 2 and 3 and the APG-79 for Boeing’s F-15C and the F/A-18E/F, respectively. An ongoing analysis by Pentagon planners to redraw spending plans for fighters may tweak the ultimate deliveries, but neither Northrop or Raytheon have much room to gain a competitive advantage.

SensorCraft, as a potential new production vehicle and a platform for the complex new antennas, has the potential to fill that void.

STEPHEN TRIMBLE / WRIGHT PATTERSON AFB, OHIO

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