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With features to reduce acoustic and visual signatures a key part of the current crop of next-generation fighter jets preparing to deploy around the world over the next ten years, it’s an important time for stealth technology.

As well as playing a central role in current aerial operations and next-gen fighter development, stealth technology is also bleeding over into cutting-edge unmanned aerial vehicle (UAV) designs such as Boeing’s in-development Phantom Ray.

The importance of stealth

Several recent events highlight the priority being assigned to stealth technology by the world’s military powers and defence commentators.

Earlier this month, for example, the US’ temporary lack of stealth-capable aircraft received widespread media coverage. The country’s 150 frontline F-22 Raptors were grounded after concerns were raised about the aircraft’s oxygen supply system, while the in-development F-35 Joint Strike Fighter (JSF) had testing suspended due to a valve problem in its integrated power package. This left the US Air Force with virtually no combat-ready stealth aircraft, sending many media commentators into a tailspin.

“Stealth technology is considered important enough to set off an international crisis.”

Stealth technology is also considered important enough to spark an international diplomatic crisis. Relations between the US and both Pakistan and China were strained further this month when it was alleged that Pakistani secret services had allowed Chinese officials to study a sample of the UH-60 stealth helicopter downed during the US mission that killed Osama Bin Laden.

Although China’s Defence Ministry strenuously denied the rumours, calling them “entirely groundless and very ridiculous”, US Naval War College professor of strategy Timothy Hoyt told The Diplomat how valuable such a sample could be to a rival power like China.

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“Since they are deploying the J-20 ‘stealth’ fighter, access to other examples of stealth technology might be very useful for their own domestic designs,” he said. “In addition, they would probably want to know specifics for their own air defence efforts in the future.”

Stealth fighter projects: the F-35 and its rivals

Stealth capability features heavily in the designs for virtually all next-generation fighter platforms currently under development. The most high-profile of these is the US-led, troubled production of the JSF.

Despite the controversy surrounding the ongoing costs of the international development project, the F-35 and its variants are loaded with stealth technology.

Creating a stealth-capable fighter jet requires striking the right balance between stealth, manoeuvrability and affordability. Central to the F-35’s design is the application of composite materials to reduce observability as well as ongoing maintenance costs. A substance called fiber mat has been cured into the composite skin of the aircraft, avoiding the need for the application of stealth coats. Lead developer Lockheed Martin’s executive vice president of F-35 programme integration last year told Aviation Week this also makes the aircraft more rugged. “You literally have to damage the airplane to reduce the signature,” he said.

Other efforts to reduce the F-35’s radar signature include the lack of bulky external fuel tanks and diverterless supersonic inlets (DSIs) to provide airflow to the engine without compromising the fighter’s low-observable (LO) characteristics.

The F-35 incorporates features to make it stealthy against visual and infrared detection as well as radar. It should be noted, however, that some observers have noted that while the F-35’s conventional fighter jet shape might make it a more manoeuvrable in aerial combat, it could compromise its stealth profile.

This fact led Dr Carlo Kopp to deem the JSF “not a true stealth aircraft” in a 2009 report on the F-35’s stealth capabilities. “The Joint Strike Fighter SDD design departs strongly from key stealth shaping rules employed in the development of the F-117A, B-2A, and F-22A, or the never built YF-23A and A-12A designs,” the report states. “As a result the tactical options available to Joint Strike Fighter users when confronted with penetrating modern Integrated Air Defence Systems (IADS) are mostly those necessary to ensure the survival of non-stealthy legacy aircraft types.”

“Creating a stealth-capable fighter requires striking the right balance between stealth, manoeuvrability and affordability.”

These factors have given rise to concerns that the hugely expensive F-35 programme might be losing its competitive edge against rival projects like the joint Russian / Indian PAK FA T-50 design or China’s secretive Chengdu J-20 programme.

While solid information is understandably more difficult to come by with these projects, various reports hint they could rival or even outdo the JSF in the stealth department.

In a 2011 assessment, Kopp and Peter Goon suggested the J-20 “stealth shaping is without doubt considerably better than that seen in the Russian T-50 PAK-FA prototypes and, even more so, than that seen in the intended production configuration of the F-35 Joint Strike Fighter.”

Even former US Secretary of Defense Robert Gates recognised in a statement that Chinese military technologies, the J-20 included “have potential to put some of our capabilities at risk, and we have to pay attention to them, we have to respond appropriately with our own programmes.”

The evolution of anti-stealth detection

Even with the huge investment being poured into stealth aircraft, the technology hardly provides fighters with the power of invisibility. Technologies, both highly advanced and surprisingly low-tech, exist that are able to spot stealth aircraft.

That much was proved back in 1999 when a US F-117 stealth ground-attack aircraft was shot down over Yugoslavia, having been spotted by long-wavelength radar after its radar signature was briefly raised when its bomb-bay doors opened.

The availability of information on counter-stealth technologies remains low, but it’s clear that adaptations of generations-old radar technology have the potential to turn the invisible visible.

Very high frequency (VHF) and ultra high frequency (UHF), pioneered in the 1940s, is still used today for long-range aerial surveillance. These frequencies, as explained by Arend G. Westra in a 2009 issue of Joint Force Quarterly, can confound stealth techniques by operating on decimetre to metre-long wavelengths.

The meeting of wavelengths between radar and aircraft causes resonation between the two, significantly raising an aircraft’s reflection in the radar spectrum, making it much more visible. VHF radar has been incorporated into the Russian military’s 1L119 Nebo SVU, its first VHF-band active electronically steered array (AESA); although detailed analysis of this vehicle-mounted array, Russian sources report it has achieved excellent results in spotting stealth aircraft.

“Adaptations of generations-old radar technology have the potential to turn the invisible visible.”

Passive radar is another well-established, relatively inexpensive technology that has potential against LO aircraft. This system uses multiple transmitters of opportunity to collate data, estimating aircraft positions by calculating the intersection of the receiver-to-target bearing and the bistatic range ellipse. In the past, these estimates have been too inaccurate to be useful, but modern advances in signal and digital processing, along with the availability of sophisticated, low-cost hardware, make passive radar a viable way to detect stealth targets. Passive radar systems in the defence market include Lockheed Martin’s Silent Sentry passive coherent location (PCL) system.

The game-changing advance for passive radar technology will be the ability to identify targets as well as track them, allowing passive radar to integrate with surface-to-air missile defence systems.

While the concurrent development of the latest generation of stealth-enabled fighters constitutes an arms race in itself, this race sits within a wider technological contest between stealth and counter-stealth technologies and techniques. Governments around the world are pouring investment into stealth aircraft development programmes, but it remains to be seen if these costly paragons of modern military hardware will end up undone by the evolution of comparatively modest radar systems.