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See the invisible. Counter-furtivity technologies
As Beijing and Moscow focus their budgets on the development of interdiction bubbles, hypersonic weapons, and space-based weapons, Western military projection capabilities are being dangerously challenged. This context only reinforces the role of operational stealth platforms (F-22, F-35, B-2, J-20, Su-57) and especially bombers (American B-21, Russian PAK DA, Chinese H-20) to destroy adversary strategic sites by surprise. However, several countermeasures have already been put in place to neutralize them.
Myths and realities of stealth
Symbolizing Western technological superiority, the first American F-117 stealth aircraft, which entered service in the greatest secrecy in 1983, was revealed to the general public in 1991 during the Gulf War, when it was presented as the ultimate weapon. Its angular and faceted profile, combined with the use of composite materials and a coating that absorbs electromagnetic waves, divides its radar signature by a thousand, reducing it to the equivalent of a golf ball.
However, afterwards, two events will put the chip in the ear of certain military experts. During the first hours of the offensive on Iraqi territory in 1991, a deep raid by a squadron of Apache AH-64s destroyed three Soviet ground radars considered totally obsolete: a P-18 Spoonrest and two P-15 radars. The intended effect of this raid, when the Iraqis were equipped with far more powerful devices, was never justified. But seven years later, in the middle of the campaign against Kosovo, Serbian forces, equipped with the same P-18 radar, managed to destroy an F-117, the debris of which was shown to the press. In fact, the Americans had been aware for several years that their stealth aircraft had a major vulnerability, and were informed that the Russians had also discovered it. In the summer of 1991, just after the election of Boris Yeltsin, the Americans abruptly interrupted the flight test campaign of the first two prototypes of the B-2 Spirit strategic bomber, to make major modifications that would significantly delay the program.
Relying on the immutable laws of physics to counteract Western technology, Soviet researchers soon discovered the flaw in the F-117. When the wavelength used by a radar is a multiple of the wingspan of the plane, the energy transmitted by it creates an induced current which will then radiate in all directions without being able to be controlled. However, these wavelengths are located between 1 m and 3 m, i.e. those of the Russian radars of the 1950s and 1960s operating on the VHF frequency band (30 to 300 MHz), and massively resold second-hand to developing countries.
The return of UHF-VHF radars...
These frequencies were the first to be used when military radars appeared. However, their poor angular resolution and poor detection of low-flying targets led the industry to move to higher frequencies to increase the accuracy needed to engage aircraft. Thus, when stealth work began in the United States in the 1970s, the Soviets had search and acquisition radars operating in S-band (1.5 to 3.9 GHz) or X-band (6.9 to 10 GHz) to equip the SA-2, SA-3, SA-4, SA-5, SA-6, SA-8, SA-10 and SA-12 ground-to-air systems. For stealth as well as for electronic warfare or anti-radar missiles, the Westerners focused only on these frequency bands. After their discovery, the Russians equipped their old radars with modern electronics in order to detect and then pursue a stealthy aircraft at a minimum distance of 100 km, to serve as mid-course guidance for ground-to-air systems or interceptor aircraft. They install them on crossing vehicles in order to have a multi-layered system in constant motion, which is thus able to evade enemy strike plans.
Over the years, the NNIIRT group has developed a whole range of systems designed to cover the entire threat. Let us mention the Rezonans-NE with a range of 350 km, the 2D Vostok-E radar with its antenna in folding panels, the imposing 3D Nebo-UE radar which supports the S-400 system or the Nebo-SVU with a shorter range; but also the Barrier-E often deployed on the edge of the forest to track aircraft and cruise missiles flying at very low altitude (up to 30 m) to escape radar coverage, and this, according to the manufacturer, with an accuracy of 80 m up to a distance of 450 km, as well as, since 2006, the Voronezh family of UHF early warning radars. With a fixed antenna 100 m long, they are capable of tracking more than 500 targets (satellites, ballistic missiles, stealth bombers) at more than 6,000 km. Four are now operational and six others are under construction.
The Chinese have the densest network of VHF radars in the world along the entire coast of the China Sea. This includes models such as the 2D JY-26 radar with its active modules specifically designed to counter the F-22, or the 3D JY-27A radar derived from the Russian Nevo-SV and deployed near Chengdu. Other models complete this system, such as the YLC-8B, the SLC-7, the SLC-12, the JYL-1A, and the HK-JM range. Iran has also acquired this capability with the Russian Kasta-2E2 radar, but also by developing the Matla-ul-Fajr 3D radar presented in 2010 and with a range of 480 km up to 20,000 m altitude. The jamming of these radars has become increasingly complicated due to their massive use of frequency evasion tactics.
Some Western industrialists then explored the track of offensive jamming. But the Russians, in response, imagined bi- and multistatic radars whose mobile and redundant (and therefore identifiable) transmitting antennas are separated from the receiving antennas (passive and therefore undetectable) by distances which can reach hundreds of kilometers. One understands better the reasons which led to stop the program of the B-2 bombers during several years. The wings have in fact been completely redesigned to offer a large section (the famous "Batwings") allowing the installation of pylons equipped with insulators inside the wing, in order to prevent any resonance phenomenon. This device is now integrated into the F-22, J-20 and Su-57, but another method is possible to detect them.
... and short-wave radars
In order to increase the range of radars, low frequency bands have been explored. Indeed, the short waves (0-30 Mhz) have the characteristic to be reflected on the ionosphere, and thus have a range much higher than the other frequencies which, them, propagate in straight line. Just after the Second World War, Russians and Americans experimented with prototypes that could reach 3 000 km. But the American-British Cobra Myst or the Russian Duga-1 had mixed results. Sensitive to interference and with a resolution that was too low, they often confused icebergs and surface vessels. But, in the mid-1990s, the computing power of computers allowed efficient signal processing to make these devices more accurate. The movements of the ionosphere generate a considerable amount of noise that significantly affects the quality of the signal received. A new generation has thus been created with the American ROTHR and the Russian Container (1). But above all, the French prototype built by ONERA, the Nostradamus, managed to detect in June 1998 a B-2 stealth bomber flight on a mission towards Kosovo and demonstrated that short waves were able to keep in check the American aircraft's devices, developed to escape UHF/VHF radars. These performances have pushed Singapore to acquire it.
The Australians also relied on this technology at the request of the Commonwealth. Launched in the early 1970s, the project was delayed several times until Lockheed Martin and the German company Rhodes & Schwartz joined forces in the late 1990s. The Jindalee is a multistatic radar system controlled from the RAAF base in Edinburg, South Australia, and has three sites spread over the Australian territory (Longreach, Leonora and Alice Springs). Thanks to interferometry, it has a virtual antenna several hundred kilometers wide, whose performance is boosted by the quality of reception provided by the desert expanses, as well as by its ability to compensate for ionospheric movements in real time. A range of over 4,000 km to detect the arrival of air and naval platforms from the north, and a surveillance area estimated at 1.7 times that of Australia, make it the most efficient OTH radar currently available. Its accuracy is such that it would be able, according to some former operators, to classify detected objects by evaluating their size (2), or to identify the take-off of air platforms. All the receiving antennas are synchronized thanks to an atomic clock, and the system's supercomputer can measure with extreme precision the delay with which each of the antennas receives the echo sent back by the target. A delay that allows to deduce the location of the target by triangulation. This strategic system has a new budget of 1 billion euros allocated to BAE Systems to further increase its performance.
China has also had such a capability on its coastline since 1967. There are at least five OTH radar sites (Doumen, Shanquian, Qian Sanzao, Shayuan, Cuarteron) as well as two others for direction finding (Zhang Jiayingcun, Fiery Cross). In Iran, experiments began in 2004. From 2009, the Pasdaran in charge of anti-ballistic warfare and surveillance of the air identification zone got closer to the Air Defense Force (IRIADF). After a first prototype built at the end of 2009 on the Qods air base for surveillance of the Persian Gulf, a second OTH, named Ghadir and covering more than 1,100 km, was erected in 2012 near the city of Garmsar. Equipped with four 39 m long antennas and a central Yagi mast, it provides 360° coverage towards Iraq, south-east Turkey and north-east Saudi Arabia. In 2014, a third appeared near the city of Ahvaz in Khuzestan province. The influence of the Russian Rezonans-NE radar is giving way to a new generation called Sepehr ("Cosmos"), the first of which was installed in 2013 near the city of Bijar, in Iranian Kurdistan. With a range of 3,000 km, it radiates as far as Tallinn, Sicily and Djibouti.
In September 2018 Iranian television broadcast images of a radar of a form hitherto unknown in this country, but close to Chinese developments, and consisting of 14 masts of several tens of meters high. The images clearly show "log periodic" antennas arranged in phase and in a circular arc to increase the gain and allow direction finding, but also to increase the bandwidth and thus promote frequency evasion to resist jamming. The vertical polarity of the antennas makes it suitable for the detection of vertical objects such as aircraft fins, surface ships islands, and especially ballistic missiles.
However, despite the lightning progress of OTH radars, in terms of both range and resolution, short waves require a distance of at least 100 to 150 km from their target to be reflected on the ionosphere. Below this distance, these radars are plunged into a so-called "blind zone". Moreover, the future American B-21 would seek precisely to make itself invisible in front of short waves. However, other systems can be used to engage stealth targets and above all to ensure the guidance of weapons systems in their terminal phase.
Civilian telecommunications
In recent years, a new family of detectors has appeared on the market: "passive radars". Unlike traditional radars, these do not have transmitters, but use the waves emitted by civilian communication systems such as FM radio, digital television or cell phone networks. The idea here is to measure the time difference between the signal received directly by the source, whose location is precisely known, and that reflected by the target thanks to a network of totally passive antennas, and therefore undetectable by airborne jamming systems. The distance, azimuth and even the type of platform can then be deduced thanks to an associated signature database.
The ancestor of this device was the Kleine Heidelberg radar built by Telefunken in 1943, which used the waves emitted by British radars visible from the French coast to detect RAF aircraft. The American company SRC, a pioneer in this field, Leonardo with the Aulos, and the German company Hensoldt, now offer operational solutions that can be deployed on simple SUVs. But other players are using this technology to surpass the current capabilities of military radars. Like the Czech company ERA, which, with its Silent Guard, claims to be able to locate targets hidden under vegetation, or the Finnish company Patria, with its MUSCL system, which is able to identify and track a hundred stealthy targets at a distance of several hundred kilometers, including the brand new Russian KUB mini suicide drones, which are used to discreetly infiltrate the loopholes left by Western air defense systems in order to damage critical points.
SWIR infrared
But for countries with a significant coastline, these passive radars will not be of much use. For the past ten years, another technology has been offering spectacular results. In order to provide the U-2 reconnaissance aircraft with new optical capabilities, the Americans have explored "shortwave infrared," or SWIR (3). Between 1 and 2.5 μm in wavelength, the resolution and range of infrared sensors are increased tenfold. More importantly, around 1.4 μm, water ceases to be transparent and becomes opaque. All naval or aerial platforms, stealthy or not, by principle made of dry materials, appear here like white dots on a black screen.
Particularly vulnerable to attacks by B-2 bombers, the Chinese have invested considerably in this segment. For while the F-22 and F-35 have undergone significant work to reduce their infrared signature, this thermal stealth would only be effective between 7 and 8 μm... Since 2015, the company A-Star has been presenting its catalog of SWIR cameras at the arms fairs of "friendly countries" to equip the air, naval and ground platforms made by the Chinese BITD.
But the detection of U.S. stealth bombers is the subject of a networked approach. First, a wide-field geostationary reconnaissance satellite with a SWIR sensor, the Gaofeng-5, has been monitoring the China Sea since May 2018. Then, a mesh of surveillance ships and aircraft equipped with these sensors will take over to confirm the threat up to a range of 150 km (thanks to thermal radiation from the nozzles) and direct interceptor aircraft. China's new fifth-generation J-20 fighters are also now equipped with the EOTS-86, an IRST with a SWIR sensor system spread across the aircraft's fuselage to provide a 720° field of view. It will then be able to rely on PL-10 short-range infrared-guided missiles. The use of this "SWIR killing chain" allows the aircraft to avoid using its fire control radar to evade detection and enemy electronic warfare devices.
Terahertz and quantum tracks
The St. Petersburg Institute of Optics (ITMO) and several Chinese DTIB players (SASTIND, CNIGC, CEAP, CASIC, CETC) have been communicating over the last few months about the development of new airborne sensors that would be able to image the shape of stealth aircraft, reveal metal parts hidden under absorbent materials, and also penetrate vegetation cover and infrastructure to detect troops and equipment. Located between microwaves and infrared, this new generation of sensors emits photons (300 GHz to 3 THz) from a helium-neon laser. As with SWIR, this is a long-standing U.S.-funded technology. In 2012, a U.S. team from the Rochester Institute of Optics published in the journal Applied Physics Letters an account of laboratory experiments designed to demonstrate the feasibility of such an imaging system. The article was illustrated by an image of B-2 reconstituted thanks to this system (4). But the atmospheric absorption capabilities of these lasers and the energy required to operate them would make them suitable for integration initially on battlefield surveillance platforms with at least four engines.
Thus, in December 2018, one month after the "quantum radar" (5) produced by CETC was presented to the public at the Zhuhai Air Show, the CASIC Group's Research Institute 23 conducted the first flight tests of a terahertz SAR radar on a Y-8 aircraft attached to the 20th Division in charge of battlefield surveillance. But, in the long term, the goal is for both the Russians and the Chinese to equip their fighter planes with this targeting capability. The Russian electronics manufacturer KRET is already working on such a prototype for the Su-57.
The multitude of these innovations makes the new American B-21 strategic bomber a priority program for the Pentagon, despite a cost that could reach 80 billion dollars. This cost seems to be justified because of the hundred or so platforms that will be produced, but above all because of the scale of the technological challenges that its designer, Northrop Grumman, is already facing, as are the European manufacturers involved in the FCAS and Tempest programs.
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