Since the radar was invented in the 1930s, it has been of major concern to aircraft engaged in offensive air operations. Radar was used extensively as a tool of warfare during the Second World War to detect and track enemy aircraft, by day and night, in all weather conditions. Later, employability of the radar to detect threats spread to missiles, satellites and other systems. Naturally, as no strike aircraft would want to be intercepted, technologists developed ways to avoid or at least delay detection by the adversary’s Air Defence (AD) systems. These evasive methods have become ever more sophisticated, culminating in stealth technology.
Stealth jets are now regularly in the news and it may seem that they are a recent development. In fact they are not. The Lockheed F-117A Nighthawk was the first operational aircraft to be designed and built around stealth technology. It entered service with the United States Air Force (USAF) way back in October 1983, and made a deep impression on military minds that were grappling with the problem of survivability and hence kill capability of strike jets in the increasingly lethal Air Defence and Anti-Access (A2AD) environment they were expected to encounter. Over the next few years, several countries including Russia and China joined the US in trying to develop stealth aircraft. More recently, the US has begun supplying Lockheed Martin F-35 Lightning II stealth fighters to some of its allies and other nations are hoping to be on the list.
This proliferation of stealth jets has understandably triggered alarm among military forces because they depend heavily on radar to detect and prevent air attacks. Counter-stealth technology is thus assuming growing importance and several countries have embarked on the quest.
Stealth – the Fifth Generation
Stealth or Low Observability (LO) is a combination of technologies that attempt to greatly reduce the distance at which an object can be detected. Stealth aircraft evade detection by reducing the reflection or emission of radar, Infra-Red (IR), visible light, radio frequency spectrum, and audio. While the main emphasis is naturally on reducing the Radar Cross Section (RCS) and avoiding pick up by enemy radar, there is growing stress on countering thermal, acoustic and other means of detection.
The US, where it all began, has remained in the forefront of low observable technology. Following the F-117A single-seat, twin-engine strike jet, the next stealth aircraft to be fielded was the Northrop Grumman B-2 Spirit strategic bomber that entered service in January 1997. Then came the Lockheed Martin F-22 Raptor single-seat, twin-engine tactical fighter that entered service in December 2005. It set a standard for stealth that has not been surpassed and brought the term “fifth-generation fighter aircraft” in fashion. The vaguely defined fifth-generation combines stealth, super cruise, super manoeuverability and sensor fusion into a single airborne weapons platform.
Although the F-22 proved its immense utility in operations, the programme had to be prematurely terminated due to rising costs and an absence of significant adversaries. It was replaced by the F-35 Lightning II, a single-seat, single-engine multirole combat aircraft, designed for ground attack and air superiority missions. With a planned production run exceeding 3,000 and billed as the “largest and most expensive military programme ever”, the F-35 has been plagued by numerous design and production issues.
Other nations that have set about designing stealth aircraft have also discovered that it is no easy task. The Russians have since 2002 been struggling with the Sukhoi Su-57, a single-seat, twin-engine multi-role fifth-generation fighter. Technical problems with advanced stealth and avionics components have seemingly been beyond the capability of Russia’s aviation industry to solve. The unhappy state of the Russian economy has not helped. The Russian Air Force is scheduled to take delivery of the first Su-57 this year, but its capability as a stealth fighter remains in doubt. For this reason, after much hand wringing, the Indian Air Force (IAF) has, for all practical purposes, abandoned the Sukhoi/Hindustan Aeronautics Limited Fifth Generation Fighter Aircraft (FGFA), an Indo-Russian derivative project of the Su-57.
China has fared better. Its Chengdu J-20 Mighty Dragon stealth fighter could be declared combat ready this year. The Shenyang J-31 Gyrfalcon twin-engine, fifth-generation jet fighter, also earmarked for export to its allies, may be close to entering service. And it is believed that China is planning to develop a stealth bomber that might carry both nuclear and non-nuclear precision-guided weapons.
Enter the Sixth-Generation
Modern combat aircraft take decades to be designed and built. Even the USAF, the world’s most advanced, realises that by 2030, it will have too many older aircraft with inadequate range, payload and survival capability for the intense A2AD environment its adversaries are creating. Consequently, the US Department of Defense (DoD) has started preliminary work on a couple of sixth-generation combat aircraft, targeting their operational readiness by the mid-2030s. Sixth-generation fighters would have all the characteristics of fifth-generation aircraft, including stealth and sensor fusion, with greater emphasis on Beyond-Visual-Range Air-to-Air Missiles (BVR AAMs). They would probably be armed with Directed Energy Weapons (DEW) that damage the target with highly focused energy, including laser, microwaves and particle beams. Several other countries that have neglected or failed to build fifth-generation jets are also hoping to catch up with their own sixth-generation fighters by around 2040. These include France, Germany, Spain, the United Kingdom, Russia and Japan. China is certain to join their ranks sooner than later.
However, even the most fervent advocates of stealth recognise that advanced sensor technology may ultimately defeat low observable airframes and there would be no way to upgrade them. Hence sixth-generation fighters need to be capable of engaging in all types of Electronic Warfare (EW) and possess IR obscuring devices too. These would combine the air combat prowess of an F-22 with the EW capability of the Boeing EA-18G Growler aircraft. Their aim would be to delay detection as long as possible, ideally till it is too late for the defender to launch a weapon.
Countering Stealth – Current Radars
Quite clearly, stealth aircraft will be the mainstay of any major strike force for years to come. From the defender’s point of view, stealth affords an aggressor two overwhelming advantages. First, the defender’s interceptors and ground-based defences would find it extremely difficult to detect, acquire and engage a stealthy intruder. Second, if the defender were to deploy a non-stealth interceptor jet against the intruder, the interceptor would itself be vulnerable to being detected and shot down by the enemy aircraft perhaps without even realising that it is under threat.
That is why all major military forces are striving to develop effective countermeasures to stealth. And some currently available radar systems could prove useful for the purpose.
- Low Frequency (LF) Radars. Low observable aircraft are usually optimised against particular radar frequency bands, especially those in the high-frequency range such as C, X, Ku and S band, where the radar accuracy is higher. They are also designed for the lowest RCS in the head-on approach aspect. Once the radar frequency drops below a certain threshold, it may cause a resonant effect making the stealth jet more visible on radar. Typically, such resonance occurs if some portion of the target is less than eight times the wavelength, as happens with the tail fin of a fighter. Radars that operate at frequency bands below 300 MHz (lower UHF, VHF and HF) are particularly useful to counter stealth. Even ordinary Air Traffic Control (ATC) radars that operate at lower-frequency bands, will possibly detect stealth fighters whose shape has portions that can cause resonance. The problem is that such radars require large antennas rendering them vulnerable to attack. They also have low precision – perhaps enough to pick up a stealthy target, but inadequate to guide a missile within its kill range. Besides, the US Navy is already considering installing low-band RF jammers to enable its EA-18G Growler aircraft to foil such anti-stealth radar systems.
- Over-the-Horizon (OTH) Radar. The US, Russia, China and others have been experimenting with OTH early warning radar since the mid-twentieth century. OTH systems generally work by bouncing shortwave (HF) signals off the ionosphere. They have very long detection ranges sometimes extending even to three or four thousand kilometres. Another OTH system relies on the properties of surface waves. It produces long-wave emissions that follow the curvature of the Earth beyond the horizon for a detection range of up to a couple of hundred kilometres, although less against stealth jets. However, OTH systems have major shortcomings including low resolution and inability to provide a real-time missile-grade track. They could conceivably work in conjunction with other radar systems and guide them regarding where to look for the target. They are very vulnerable to attack being large fixed installations with huge antennas.
Countering Stealth – the Chinese Way
Over the last few years, China has been making a series of extraordinary claims about its anti-stealth capability, including a metre-wave radar system which it says can detect stealth aircraft and even guide missiles towards them. The reason is clear. It knows that the US has an overwhelming advantage in stealth aircraft and the only way it can try and obtain a level playing field is by breaching the American stealth curtain.
China claims to have developed two futuristic radars as under:-
- Terahertz Radiation Radar (TRR). Terahertz radiation has wavelengths between IR rays and microwaves. It can potentially penetrate the composite materials that form the skin of stealth aircraft and expose the metallic parts within. The Chinese say that their TRR can clearly trace the shape even of a stealthy object and identify it as a fighter jet or otherwise. However, while terahertz radiation may indeed be highly effective at short range it decays rapidly in the air. The Beijing-based Global Times admitted as much: “Experts said the terahertz radiation decays very fast in the air, meaning the effective range of the radar is likely to be low and not sufficient for detecting an advanced stealth fighter jet before it launches attacks from beyond visual range.”
- Quantum Radar. This is another exotic device almost in the realm of science fiction. A quantum radar system employs a source of strongly correlated “entangled” photons that work as one quantum system even when separated by large distances. A beam of photons generated at the base station is, therefore, split with one half transmitted and the other half retained as reference. If the beam hits a distant object and returns to the base station, it can be compared with its other half to gain useful information about the object, including its shape, location, speed, temperature and even the chemical composition of its paint. Quantum radars need only a tiny echo to detect a target and they provide good results even with low power transmissions. Therefore, they are more difficult to detect and jam. However, experts opine that while a laboratory demonstration of such capabilities may be truly spectacular, quantum information is very fragile. It can be destroyed by the slightest noise or atmosphere disturbance, a process known as “decoherence”. Still, these problems may not be insurmountable. China has made known its intention to become a global leader in quantum technology by 2030, and is spending enormous sums of money to pursue its dream. It is quite likely that it will be the first to solve the quantum radar riddle.
Electro-Optical Infrared Search and Track (IRST) Systems
While stealth aircraft may elude radar detection, they need powerful engines that unfortunately produce large amounts of waste heat. Hence they might not be as successful in evading advanced IR sensors. Russians sources claim that the F-22 Raptor has been captured in air-to-air IR footage by the OLS-35 electro-optical IRST system of the Sukhoi Su-35S. The claim appears plausible since the Dassault Rafale, the Eurofighter Typhoon and the Boeing F/A-18 Super Hornet are all known to have acquired F-22 jets using similar IRST equipment.
IRST depends on the thermal signature emitted by the target, particularly its engine exhaust. The advantage of an IR system over radar is that it is passive and the enemy aircraft has no indication that it is being tracked. Targets can be tracked at very long distances, but IRST is adversely affected by both clouds and humidity. Hence its performance may not be adequate to generate range data for a weapon-quality track against stealth jets. Also, IRST’s success depends on “cued search”, that is where the system knows exactly where to look for the target. Only radar can provide such a cue and if the linked radar cannot pick up the target, the IRST would be looking for a needle in a haystack. However, if two or more IRST systems obtain lines of bearing on the same stealth intruder from different angles, a fusion algorithm could conceivably compute a useful track on the target.
Of course, the first choice of any force assigned the unenviable task of defending against an adversary armed with stealth jets would be a weapon system capable of shooting down the intruding aircraft. That is what Russia’s Almaz-Antey claims for its S-400 Triumf multi-missile air defence system. Independent analysts believe the Triumf is among the most advanced AD systems in the world today. Russia has already exported the S-400 to China and is scheduled to deliver it to Turkey this year and India in late 2020. However, the US has reacted furiously to these moves perhaps because the Triumf could jeopardise the secrecy and security of its F-35 fighter. It is employing a carrot-and-stick approach to persuade both countries to cancel their deals with Russia.
The S-400 supports four different Surface-to-Air Missiles (SAM) with ranges of 400km, 250km, 120km and 40km. These missiles fly at high speed, have excellent manoeuverability even at high altitude and are resistant to stand-off jamming. The system could, therefore, threaten an attacking aircraft through much of its ingress to the target. A successor to the S-400 is also under development. Known as the S-500 Triumfator-M, it is specially designed to take on stealth aircraft. It will come with new radars and even longer range missiles.
However, the claimed ranges of these missile systems are against large, slow-moving aircraft such as aerial tankers, transport aircraft and Airborne Warning and Control System (AWACS) aircraft flying at high altitude. Stealth fighter jets would only become vulnerable much closer to the SAM site. The kill chain for shooting down an intruding aircraft is also complex. First, the defender who may already be experiencing jamming and other electronic interference must detect the intruder. Thereafter, a track needs to be developed and refined to include range, bearing, altitude and velocity information. The track must then be identified as friendly or hostile. If declared hostile, a weapon system may be assigned to destroy the intruder. The weapon system must itself acquire and lock on to the target and fire only when it comes within range of its specific weapon. For a stealth jet, the entire process will be delayed and the defender may ultimately fail to bring it down.
Yet it is not impossible to shoot down a stealth jet. On March 27, 1999, three days after the North Atlantic Treaty Organisation (NATO) commenced the bombing of targets in the Federal Republic of Yugoslavia, a Serbian anti-aircraft battery using long-wave radar succeeded in detecting and tracking a USAF F-117A Nighthawk in egress mode. The missile crew made no mistake in launching their Soviet S-125 Neva missile based on 1960s technology and shot down the jet. According to Serbian sources, Belgrade’s AD operators had discovered that their modified Soviet radars could detect the stealth jets at relatively short range when the bomb-bay doors were opened to release their 2,000 lb bombs.
For the record, it was not just the radar equipment that succeeded in piercing the F-117A’s invincibility bubble. The USAF crews seem to have been convinced that their stealth jets were immune to detection. They rather carelessly followed the same route again and again, making their position easier to guess. The Serbs were also able to monitor NATO radio communications since they were mostly unencrypted.
The cat-and-mouse game between attacker and defender will of course continue as long as the world indulges in armed warfare. However, in the absence of actual engagements between two evenly matched adversaries, it is hard to tell either how successful stealth aircraft might be in evading enemy AD systems or how effective counter-stealth systems might be in engaging the intruders. The waters are further muddied by claims and counter-claims, some simply to scare the opponent, others to try and increase the marketability of military systems. Both Russia and China are known to make tall claims about their equipment. The US too has made some exaggerated interception claims for its Patriot SAM system during the Gulf Wars.
It is however safe to say that stealth is now a fairly mature technology and even huge investments can yield only limited increase in capability. Stealth jets are losing the edge they enjoyed for many years and will continue to do so and the US knows this. Russia and China may not yet have succeeded in mounting a credible defence against American stealth jets, but they eventually will. Given the enormous sums China is investing in anti-stealth capability, it is quite likely that it will taste success someday. On the other hand, the stealth warriors claim that LO relies on more than one type of technology and a single breakthrough will not see through their invisibility cloak. They also say that a stealth jet in strike mode will always have far greater chances of survival than a non-stealth one.
If and when a viable system to counter stealth aircraft emerges, it may involve V/UHF Active Electronically Scanned Array (AESA) radar systems, other types of radars, long-wave IRST systems and visible-band imaging devices. All these systems would have to be connected using modern high-speed data networks and advanced computer algorithms for multi-point sensor fusion. This is a process that would take time and money to design, develop, build and integrate into the defending force.