Surface tensions
Dr Thomas Withington
The advent of metasurface technology could herald major benefits for platform protection in the air domain, particularly as a means of reducing visibility to radar.
You might not have heard of metamaterials, or indeed metasurfaces, but do not worry, you are not alone. The science of metamaterials of relatively new. In a nutshell, metamaterials exploit the composition of certain materials to manipulate electromagnetic waves. The story of metamaterials dates back to the 19th century. The Scottish mathematician and physicist James Clerk Maxwell (1831 to 1879) helpfully devised several equations. Named eponymously, these equations consolidated existing observations on electricity and magnetism into consistent theories. Maxwell’s equations formed part of physics’ second great unification with the first unification being from the work of the polymath Sir Isaac Newton on terrestrial and celestial mechanics.
In 1865, Maxwell published his paper ‘A Dynamical Theory of the Electromagnetic Field’ which posited that light, electricity and magnetism were all the results of the phenomenon of the electromagnetic field. The work of Maxwell and others including Indian polymath Sir Jagadish Chandra Bose (1858 to 1937) reverberates today. These scientists were instrumental in discerning that the interaction of certain materials and physical shapes could have a profound effect on the behaviour of electromagnetic waves. Usefully, the science of radio was developing towards the late 19th century. Maxwell’s theories helped pave the way for the work of Italian physicist and engineer Guglielmo Marconi (1874 to 1937), who pioneered radio as a practical technology, transmitting the first transatlantic radio signal in 1901.
Metasurfaces defined
The industry journal Photonics provides a good definition of a metasurface as a “two-dimensional array of subwavelength-scale artificial structures … arranged in a specific pattern to manipulate the propagation of light or other electromagnetic waves at subwavelength scale.” What does this mean in practice?
Metasurfaces and EW
An instructive film by the United Kingdom metamaterials network; facilitated by the UK’s Engineering and Physical Sciences Research Council offers a good overview of how metasurfaces work. Metamaterials can slow, stop or redirect electromagnetic waves. These attributes could make a significant contribution to electronic warfare (EW) in the coming years. EW includes three subdisciplines: electronic attack (EA), electronic protection (EP), and electronic support (ES). The United States Department of Defense Dictionary of Military and Associated Terms provides a standard definition of electronic warfare as “military action involving the use of electromagnetic and directed energy to control the electromagnetic spectrum or to attack the enemy.”
- EA is the subdiscipline which involves “the use of electromagnetic energy … to attack personnel, facilities or equipment with the intent of degrading, neutralising or destroying enemy combat capability.”
- EP focuses on “actions taken to protect personnel, facilities and equipment from any effects of friendly or enemy use of the electromagnetic spectrum that degrade, neutralise, or destroy friendly combat capability.”
- ES involves actions to “search for, intercept, identify and locate or localise sources of intentional and unintentional radiated electromagnetic energy for the purpose of immediate threat recognition, targeting, planning and conduct of future operations.”
These definitions are important in the world of metasurfaces, as their properties are directly relevant to the EP mission. A key tenet of airpower is to perform Offensive Counter Air (OCA) and Defensive Counter Air (DCA) efforts against one’s adversary. As US Air Force definitions note, OCA focuses primarily on attack. Blue forces adopt an offensive posture to win and maintain control of the air at the tactical, operational and strategic level from red forces. OCA depends on denying, degrading, dislocating and destroying red force means to counter blue force efforts to win this control. Attacks on ground-based air defence (GBAD), integrated air defence systems (IADS), airfields, command and control (C2) facilities and industrial targets germane to red force airpower all fall within the OCA remit. Offensive counter-air may be fought for using blue force fighter escorts, combat air patrols and suppression/destruction of enemy air defence (SEAD/DEAD) missions. DCA, on the other hand, focuses on using blue force GBAD, IADS, accompanying C2 and fighters to protect friendly airspace against red force OCA efforts.
Suppose we have a radar that transmits a signal with a frequency of 1.25 GHz. It takes the signal 0.26 milliseconds to make the trip from the radar’s antenna to the target, and to be repropagated by the target towards the antenna. This is a total distance of 78 km (42 NM). By dividing this distance in half, we determine the target is 39 km (21 NM) from the radar. The frequency of a repropagated radar signal will change slightly if the target is moving towards or away from the antenna. If a target is moving towards the antenna, the frequency of the waves will increase, because waves take progressively less time to reach the antenna, and vice versa if the target is moving away. This phenomenon is known as the Doppler effect, and you have likely experienced it without even realising it. Take for instance the following scenario: If you are standing by a road and a police car drives past with its siren on, the pitch of the siren will increase. Conversely, the siren pitch will decrease as it drives away. The frequency of the siren is always the same, but for an observer it appears to change. To turn this back to our radar scenario, our target, which is 39 km from the radar, is flying at a speed of 481.5 km/h (260 kn). The re-propagated signal will have a frequency of circa 1.2499 GHz. Not much of a change, but enough to tell us the target is moving away from the radar.
The strength of the repropagated signal can tell us more about the target. Signal strength is measured in decibels/dB. Suppose our radar transmit signals with 56.3 dB of power. The radar’s antenna has a gain of 32 dB. Gain is a measurement of how much power a radar can focus in a desired direction. Our target in question is a large airliner with a radar cross section (RCS) of 100 m2. RCS is a measure of how large an object will appear to a radar and can be converted into decibels, in this case 20 db. The strength of the repropagated signal will be -0.2dB when it reaches the radar antenna. This is an 88.5 dB reduction in amplification from the original combined strength (transmit power and gain) of 88.3 dB. Despite the apparent weakness of this signal, the radar will be designed to detect and process this.
Matters become more problematic when aircraft are designed to have as low an RCS as possible. The US Air Force’s Lockheed Martin F-117A Nighthawk combat aircraft had a reported RCS as low as -30 dB. Using the same radar signal criteria as those above, the repropagated signal from an F-117A will be -5.2 dB. This is 26 times smaller than the strength of the airliner’s repropagated signal. Some RCS levels maybe so low that they are comparable with those of birds. This is deliberate. Ground-based air surveillance radars may not need to see and track birds. Such targets may be superfluous to the radar’s core tasks, and the radar’s processor may be programmed to simply ignore targets with such low RCSs. From a camouflage perspective, it makes sense to design combat aircraft to have as low an RCS as possible.
Another useful RCS reduction technique is to mask parts of the aircraft which could enlarge the RCS. Spinning engine fan blades are good RF reflectors which a radar can exploit to determine that it has detected an aircraft. Unlike the wings of a bird, engine fan blades move in a predictable fashion. A standard approach in RCS reduction is to use S-shaped engine inlets which help shield the engine fan blades from the radar’s line-of-sight.
Nonetheless, it is the advanced coatings and paints that first-generation ‘stealth’ aircraft like the F-117A used which were indicative of early metasurface adoption. These coatings were designed to absorb incoming radar signals rather than reflect them back to the antenna. Some coatings use tiny iron balls which resonate when hit by the incoming radar signal. This resonance creates heat which then dissipates, causing very little energy to be repropagated.
Advances in metasurface technology constitute the next step in ongoing work to help protect targets from radar detection. In a recent lecture given by Dr Rafael Licursi, author of the book Metasurface-Driven Electronic Warfare, to the Association of Old Crows global electronic warfare advocacy organisation, he said that metasurfaces could be used to scatter incoming radar signals, or to change the signal’s polarisation. Much like the RCS reduction technique of using flat surfaces, metasurfaces could be employed to repropagate signals away from the radar antenna. Dr Licursi stated in his lecture that scattering can produce a signal reduction as high as 10 dB in wide RF bandwidths.
A standard technique used by contemporary combat aircraft to reduce radar detection is to employ a Digital Radio Frequency Memory (DRFM) in their integrated self-defence systems. DRFMs listen for offending radar signals which are then sampled so that these signals can be manipulated. To return to our example, the radar is transmitting on a frequency of 1.25 GHz. The frequency of the signal returned by the target is 1.2499 GHz as it is moving away and causing a phase shift in the returned signal. If the repropagated signal had a frequency of 1.25 GHz, the signal would be in phase, meaning the peaks and troughs are occurring at the same moment. The different frequencies of the incoming and repropagated signal means the latter is out of phase.
After sampling the incoming radar signal, our DRFM creates a false signal with a frequency of 1.250 GHz which will have the same -0.2 dB strength as the real 1.2499 GHz signal. The radar now receives both signals, causing the processor to assume two targets are in the sky. By continuing to alter the fake repropagated signal, the DRFM starts to convince the radar that the fake target is moving at certain speeds and in certain directions. Such techniques pose a dilemma to the radar operator: Which target is genuine, and which is fake? In fact, DRFMs are capable of mind-bending levels of radar signal manipulation.
Advancing the technology
As Licursi noted, even with the onward march of electronics miniaturisation, it may be impractical to outfit size, weight and power-constrained platforms such as small UAVs with DRFMs and accompanying antennas. Instead, a metasurface could collect the energy from the incoming radar signal. The signal is then amplified, and a similar phase shift imposed to the one discussed in the DFRM example, before this new signal is transmitted to the radar. The advantage of this approach is that the metasurface material can be under 1 mm thick. Moreover, it does not depend on any digital technology or complex processing.
There may well be an unbelievable number of applications in the EW domain for metasurfaces that we have yet to foresee as the technology remains in its infancy. Dr Licursi told this correspondent that metasurfaces have been tested in laboratory conditions, but not yet in operational, real-world scenarios. He says that the technology is probably at about TRL (Technology Readiness Level) three or four. According to European Union definitions, TRL-3 denotes that a design proof of concept has been reached. TRL-4 indicates that the proof of concept has been demonstrated: “The basic components have been tested in laboratories,” he says, “but we have only tested the materials, and not yet tested the metasurfaces on platforms.” He added that there are already some products available based on metasurfaces, such as those produced by a company called Greenerwave. Furthermore, metasurfaces for stealth aircraft have probably been tested in real-world scenarios by some governments. Information about such endeavours is understandably sparse.
The current state-of-the-art uses Printed Circuit Boards (PCBs) for the construction of metasurfaces. Dielectric laminates have copper on both sides, and Dr Licursi added that “we can etch the copper in a particular way to give the metasurface a specific configuration or property. All we then need to do is to add whatever electronics we might need to produce a particular effect.” As discussed above, amplifiers could be added to metasurfaces to control the strength of a repropagated signal. As well as using PCBs, Dr Licursi noted that 3D printing can be employed to build conformal metasurfaces. Bespoke surfaces could be printed to cover the intricate airframe of a small UAV, for example.
Dr Thomas Withington
