Chinese Radar Claims: Facts or Fallacies?
Wednesday, August 29, 2018
Potentially major breakthroughs have been reported regarding the radar detection of stealth aircraft, but do they stand up to scrutiny?
“Quantum Radar Could Detect Stealth Planes” ran the headline in April telling the world that scientists from the University of Waterloo in Canada were developing so-called ‘quantum radar’ technology that could detect stealth aircraft. At first blush the headlines and articles which heralded this news around the world implied that almost fifty years of research and practice which has worked to reduce the Radar Cross Section (RCS), not to mention billions of dollars of development and procurement costs sunk into minimising an airframe’s visibility to Radio Frequency (RF) energy, could be rendered null and void. Yet as this article will illustrate, reports of the demise of ‘stealth’ aircraft maybe greatly exaggerated with such predictions the results of misunderstandings as to how low RCS technology works, the important difference between detecting such an aircraft; and detecting it in a meaningful and usable fashion to enable a successful engagement with a Surface-to-Air or Air-to-Air Missile (SAM/AAM).
The third law of motion of the natural philosopher Sir Isaac Newton stated that “for every action there is an equal and opposite reaction.” This dictum is as equally applicable to radar and Electronic Warfare (EW) as it is to a ball bouncing off the wall it has been thrown at. Radar was first used en masse in anger during the Second World War. Put simply this conflict saw two of the main belligerents, Germany and the United Kingdom, principally using ground-based air surveillance and fire control/ground-controlled interception, and airborne radar, to detect and aid the interception of aircraft. During the conflict, once the UK realised Germany was using radar for aircraft detection at the start of the conflict it devised countermeasures to frustrate this radar as riposte. Most famously, the British developed ‘Window’, later to be known as chaff which remains in service today, which involved the sowing of millions of strips of aluminium foil into the atmosphere surrounding a formation of aircraft. Radar pulses would be reflected by these strips causing a radar operator’s screen to be inundated with millions of radar echoes thus hiding the actual aircraft.
For every advance in radar technology, an eventual advance is made in EW, principally in electronic attack and electronic protection, to frustrate the ability of the radar to perform adequate detection. Thus the need to frustrate radar’s ability to detect an object is as old as radar itself. This explains the growth of low-RCS technology in the 1970s, particularly in the United States and, more specifically within Lockheed Martin. Research performed by the Russian mathematician and physicist Petr Ufimstsev in the 1960s developed equations which could predict how an object would reflect radar transmissions as echoes. This culminated in his Physical Theory of Diffraction which posited that the RF energy returned by an object as an echo related to the object’s edge configuration, and not its size. English translations of Mr. Ufimstsev’s work, which was not considered sensitive by the Soviet authorities, would come to the attention of Denys Overholser, a mathematician working for Lockheed Martin. Mr. Overholser realised that an aircraft constructed from a series of flat panels would possess a very low RCS compared to other conventional aircraft, given the ability of the flat panels to transmit RF energy away from the radar antenna. Conventional curved surfaces on aircraft reflect a significant quantity of RF energy back to a radar’s antenna enabling the radar’s detection of its target. This discovery would form the basis of Lockheed Martin’s Have Blue prototype aircraft, two of which were constructed which influenced the later development of the United States Air Force’s F-117A Nighthawk ground-attack aircraft.
In contrary to much of what is written in the general media “low observable does not mean no observable,” noted a 2014 paper entitled Low Observable Principles, Stealth Aircraft and Anti-Stealth Technologies written by three Hellenic Air Force officers and published in the Journal of Computations and Modelling. An aircraft with a low RCS will not completely disappear from a radar’s gaze. So far, such a capability is thought to be impossible as the very fact that an object is in the air, and thus presents a surface upon which RF transmissions can be reflected means that some degree of RF energy, no matter how small, will always be reflected back to the radar. The paper defines RCS as “a measure of the power scattered from a target to a certain direction, when the target is illuminated by electromagnetic energy.” The larger the RCS the easier it is for a radar to detect the target.
RCS can be reduced by the shape of the airframe (see above), the employment of Radar Absorbing Materials (RAMs), passive radar cancellation and its active counterpart. Fundamental to designing an aircraft with a low RCS is ensuring that its airframe surfaces and edges reflect RF energy away from the radar. RAMs use an ingeniously simple principle: they absorb RF energy and convert much of it to heat, reducing the quantity of RF which echoes back to the radar. As the paper states, RAMs cannot absorb all the RF and a small proportion will always be reflected back, and may not work across the entirety of the radar segment of the electro-magnetic spectrum. Hence aircraft designers have to decide on the radar frequencies which present the biggest threat and use RAMs configured accordingly. A third technique is brought into play in the form of passive cancellation. While airframe design techniques and RAMs exploit the aircraft’s physical characteristics, active cancellation exploits the radar’s transmitted pulse to reduce the visibility of the aircraft. As the paper explains, aircraft equipped with active cancellation systems will detect a radar pulse, measure its characteristics in terms of pulse length, level of amplification, phase, frequency and angle of arrival and then create an RF waveform which will replicate these characteristics and retransmit them to the radar. The idea being that the pulses the radar will receive will crowd out the echoes which will still be received but subsumed under the much more powerful signals from the active cancellation system. This is because radar echoes return to the radar at greatly reduced levels of amplification compared to the original pulse. The principle of active cancellation works to persuade a radar that an aircraft is effectively invisible. Nevertheless, active cancellation has pitfalls. Get the waveform of the radar incorrect and the system risks transmitting an RF signals sufficiently distinct to announce the presence of the aircraft. An analogy would be that a person wearing a light shade of orange will be highly visible in a room painted a dark shade of orange; the shades of orange must be exactly the same if they are to blend together. Open source information regarding active cancellation systems remains scant, although it is strongly suspected that the Dassault Rafale-F3B/C/M combat aircraft employs active cancellation in its Thales Spectra aircraft self-protection system.
A variant on a theme of active cancellation is a principle called ‘Plasma Stealth’. Plasma Stealth employs an onboard generator which produces a cloud of highly ionised gas around the aircraft. Much like active cancellation, much regarding this technology is shrouded in mystery. This could be because the technology is still some years away from maturity, or that the technology has already entered the ‘black’ world. The fascinating world of plasma stealth would merit an article in its own right, but there is insufficient space here to explore it in more detail. Open sources have stated that Russia’s NPO 3M22 Ziron (NATO reporting name SS-N-33) anti-ship missile is fitted with a plasma stealth masking device, although this is yet to be independently verified and Roger McDermott, an expert in the Russian armed forces based at the Jamestown Foundation, a think tank based in Washington DC believes that such claims such be treated with caution.
To be frank no aircraft is impossible to detect by radar. The design of fifth-generation fighters such as Lockheed Martin’s F-35A/B/C Lightning-II indicate the trade-offs that all designers must make regarding their airframes. Open source information indicates that the aircraft has a very low RCS in X-band (8.5GHz to 10.68GHz) and Ku-band (13.4GHz to 14GHz/15.7GHz to 17.7GHz), yet the aircraft’s RCS increases in size as one moves down the electromagnetic spectrum towards lower frequency radar wavebands. Broadly speaking, Surface-to-Air Missile (SAM) fire control radar, combat aircraft radar, and active radar homing SAMs and air-to-air missile radars typically inhabit these higher bandwidths as they offer very sharp target resolution at the expense of range, and vice-versa for lower frequency radars. This is necessary as SAMs and AAMs require highly accurate radar information to effect a kill. The emphasis for the F-35 is not to render the aircraft invisible to all radars, but to frustrate the ability of an adversary to generate a track accurate enough for an engagement. That said, the need to improve radar technology to detect low RCS targets will not diminish, and has arguably assumed yet more urgency with the service entry of a number of low RCS combat aircraft such as the Lockheed Martin F-22A Raptor, the F-35 series and the Rafale-F3A/B/C to name just three. Two technologies have recently hit the headlines, both of which it is claimed can low RCS targets, namely terahertz radar and quantum radar.
In September 2017 reports emerged that the China North Industries Group Corporation (CNIGC) had tested a radar capable of transmitting terahertz radiation for the detection of low RCS targets. The technology for the radar was reportedly developed by the China Academy of Engineering Physics (CEAP). Terahertz (THz) radiation employs RF transmissions with exceptionally short wavelengths of between 0.3THz up to three terahertz, corresponding to wavelengths of between one millimetre to 100 microns. Terahertz radar uses photon energy which is less than the band gap of non-metallic materials which allows the photons to penetrate such materials and see inside an object in a similar fashion to an X-ray. The logic for its use in radar is to penetrate RAMs covering an aircraft and hence see objects inside the aircraft which will thus betray its presence and position. Nevertheless, Terahertz radiation has disadvantages in being unable to penetrate metal and is challenged by fog and clouds, although it can penetrate plastics and ceramics. This may give it an advantage in detecting low RCS aircraft employing a significant quantity of carbon fibre or composites in their skins. Moreover, existing research states that the composition of the Earth’s atmosphere is a good absorbent of Terahertz radiation which can limit its range to just tens of metres. The report commented that the Chinese company had developed a Terahertz radar boasting “unprecedented power levels”, of circa 18 watts, with a peak power output of one megawatt. Such power levels would be necessary if the radar was to be capable of detecting any aircraft at a tactically useful range. One CNIGC executive stated that the RAMs used in the construction of the F-35 <I> “will look at thin and transparent as stockings.” <P> Media reports continued that, as of September 2017, CEAP was working to reduce the physical size of the radar, and increase its power output to develop a militarily useful system. Given the technical challenges implicit in Terahertz radar, how long this will take is anyone’s guess.
This April reports circulated that Canada’s University of Waterloo was working on developing Quantum Radar as a result of a $2.7 million investment by the Canadian government. The reports continued that such technology could eventually enhance the radars equipping the North Warning System (NWS). The NWS is a joint Canadian-US chain of radar stations that provides early warning of air attack to the joint North American Aerospace Defence Command. The NWS includes both Lockheed Martin AN/FPS-117 and AN/FPS-124 L-band (1.215 to 1.4GHz) ground-based air surveillance radars with instrumented ranges of 250nm (470km) and 59.4nm (110km) respectively. Open sources state that the AN/FPS-117 radars are used for long-range air target detection, with the AN/FPS-124s used as gap fillers, and to detect targets with a small RCS such as cruise missiles. These radars are expected to be replaced from circa 2025. Quantum Radar exploits a phenomenon known as Quantum Illumination. According to the University of Waterloo’s Institute for Quantum Computing, Quantum Illumination employs the principle of Quantum Entanglement where two photons form a connected pair. This creates a strange phenomenon where if something changes in one photon the other entangled photon will change too, even if the two particles are light years apart. In theory the radar would work by passing clusters of photons through a crystal cutting each in two, causing it to become an entangled pair. One of these photons will be retained in the radar, and the other transmitted. When that photon hits a target it would bounce off and thus any change to the state of this particle would be instantly reflected in other particle at the radar. By filtering out the ambient noise of the other particles in the atmosphere, it will be possible to only retain the changed particles, thus revealing the presence of a target. Interestingly, as many aircraft with a low RCS are configured to frustrate detection with RF, their technology maybe greatly degraded in this regard as Quantum Radar employs photons, i.e. light particles, which stealth aircraft have not been traditionally designed to. Quantum Radar could not only increase the chance of low RCS target detection, but it could help to outflank the challenging electromagnetic characteristics of the Arctic which, given the magnetic North Pole, attracts sunspot and solar flare activity creating significant EM background noise which can inadvertently mask low RCS targets.
The need to detect air-breathing targets with a low RCS is highly relevant for the North American Arctic regions, where such technology could eventually be applied. The Russian Air Force’s (RUAF) new Sukhoi Su-57 fifth-generation fighter reportedly has an RCS of 0.5 square metres (5.3 square feet). Similarly, the RUAF’s new Tupolev PAK-DA strategic bomber which could enter service in circa 2023 may also have a minimal RCS. Any attack by the RuAF on targets in North America could see RUAF aircraft traversing the North Pole to reach their targets, hence the need for the NWS to detect targets with a low RCS.
Clearly, both Terahertz and Quantum radar offer promise in reducing the tactical advantage offered by low RCS technology. However, the big question is whether either technology will provide target detection and tracking to the level of accuracy required to directly support an AAM/SAM engagement. General media reports covering such radar technology often miss the point that detecting an aircraft with a low RCS is one thing, while obtaining detailed enough information to perform an engagement is another. The past 30 years of airpower history indicates that some radars can already detect stealth aircraft. Radars transmitting on comparatively low VHF wavebands contributed to the downing of a USAF Lockheed Martin F-117A Nighthawk ground-attack aircraft on 27 March 1999 by a missile shot from a Serbian Army Almaz S-125 Neva medium-range SAM battery during the NATO-led Operation Allied Force air campaign over Serbia and Kosovo. This was in part the result of local modifications to the P-18 VHF ground-based air surveillance radar which reduced the frequency of this radar to ease the detection of the F-117A which was designed to outfox radars operating in centimetric and millimetre wavebands i.e. fire control and missile guidance radars. Clearly, advances in radar technology are a cause for concern for military aircraft designers, worries which will deepen if such counter-stealth technology is adapted for radars of increasingly higher frequencies.