As a counter stealth mechanism, LiDAR has rapidly emerged as a viable means to defeat the contemporary stealth technologies
The eternal duel between the prosecutors of the air threat and the defenders continues unabated. In the stealth domain, the same reflects in the two sides building rival technologies. While the attackers are revamping their stealthy platforms by exploiting the enabling wings of the cutting-edge technologies, the humble air defence warriors are taking solace in the fact that the technologies in the field of counter-stealth are also growing exponentially. Of course, the added joy of the defender resides in the fact that in the race of dollars vs. cents, he will have the last laugh because it would invariably be possible to field a cutting-edge counter-stealth shield at fractional costs compared to the costs of fielding stealthy offensive platforms. It is no wonder, therefore, that multiple technologies are at play in the defender’s domain to detect the threat that is fast revamping towards total opacity.
While the baby steps in fielding the counter-stealth muscle saw the conventional sensors move towards bi-static and then on to the multi-static solutions for detecting the stealthy targets, the stealth threat, using state-of-the-art solutions in the form of latest RAMs (Radar Absorbent Materials) and radar energy deflectors, complete with their di-electric composites, metal fibres and nano-materials, acquired the capability of near complete absorption of the incident radar energy or deflection of the same in multiple directions, randomly and unpredictably.
As the problem to detect stealth became more and more demanding, the counter stealth arsenal moved on from the bi-static or multi-static sensor architecture towards embracing more enabling technologies. The first technology to surface was the PCL or the ‘Passive Coherent Locators’. In this, the passive radars endeavoured to detect stealth by exploiting multi- bands, namely FM, Digital Audio Broadcasting (DAB) and Digital Video Broadcasting-Terrestrial (DVB-T). The visible products in the PCL domain were Lockheed Martin’s ‘Silent Sentry’, which debuted with this technology in the 1990s. There has been no looking back ever since, as PCL-based sensors with dual-mode functions could be prominently seen in the 2014 Paris Air Show, EADS Cassidian being the frontrunner. Cutting-edge research in this field is now focused on the enhancement of range, minimal RCS and locational accuracy, as also, to rope in the TV and the FM radio transmissions in the PCL fold. This is largely due to the near vertical climb of the mobile telecommunication technologies over the last decade.
While the PCL may be growing in its own vertical, the real front-runners in the counter-stealth domain are the laser-based technologies. Counter- stealth radars based on laser technology are called the LiDAR, which stands for Light Detection and Ranging as extrapolated from RADAR (Radio Detection and Ranging). Some Subject Matter Experts (SMEs) also refer to them as Laser Infrared RaDar or Laser Integrated Detection and Ranging.
What makes LiDAR a good fit
Many technologically-enabled advantages, essentially precipitating out of the beam signatures of LiDAR make them a good fit to defeat the contemporary stealth technologies. The objective of this article is to highlight the nuances of this good fit.
While most of the stealthy platforms are either designed to attenuate through absorption (using a variety of RAMs) or deflect the incident radar energy lying in the electromagnetic (EM) radio frequency (RF)range, these are not so optimised for the light waves lying in the laser range. Typical laser waves have extremely short wavelengths. For example, while a Nd: YAG laser normally lases at 1.06 μm to 1.4 μm, there are lasers which can operate in the Deep Ultra Violet (DUV) region of < 250 nm to long wave IR (11μm). On an average, the familiar laser wavelength bands lie between 0.532 μm through 1.064 μm to 10.6 μm. At such low wavelengths and hence very high frequencies or extremely high frequencies, the typical stealthy aerial threat vehicles which are optimised for opaqueness or near opaqueness to RFEM waves, give away their location owing to higher reflectivity.
While most of the contemporary stealth platforms have a low/very low reflectivity with respect to RF EM waves, the same gets highly enhanced against the light (laser) waves. Typical aircraft materials like the aluminium alloys have a reflectivity of 55% and titanium alloy has 47-48% with reference to a laser wave, lasing at 1.064 μm. In order to further increase detection, contemporary LIDAR systems employ multi-band lasers covering the entire laser-DUV spectrum. Against these laser based systems, most of the current stealthy platforms are likely to become visible in varying degrees.
The Concept of LRCS
Connected with the above, the game of stealth is based on the reduction of the Radar Cross Section (RCS). This RCS has different values for both RFEM waves and lasing waves. In the context of the latter, the RCS is called LRCS (Laser RCS), which is the measure of the laser scattering ability of the target. It is the ratio of the incident power in the unit area of the target to the scattered power per unit area when the target is isotropic scattering. Since the lasing waves impinging on the targets at tremendous frequency have very high beam quality, strong direction ability and very high measuring accuracy, the LRCS signatures (for lasing waves between 0.532 μm to 10.6 μm) are invariably higher than the RCS, making detection of stealth possible. Experiments have shown that regardless of the shape, a stealth type of an aerial vehicle cannot effectively scatter EM energy lying at the smaller wavelengths of light emitted by LiDAR systems.
In order to increase detection probability of stealthy platforms, typical LiDAR systems employ plurality of laser transmitters mounted on the support in sets of different angles. Each of the laser transmitters is adapted to transmit a coherent beam of light along an axis. The transmitters are so paired and oriented so as to achieve a grid of parallel coherent beams in the atmosphere. This plurality has two advantages. Firstly, it amounts to ‘many eyes’ capable of being trained on one target, each returning its own signal hence enhancing detection. Secondly, in cases where laser-kill is intended and not only mere detection, a target passing through the array of coherent laser beams will suffer the same effect as one laser targeted directly on to it for a longer period of time. Even, if the cumulative beam energy is not sufficient to defeat the target, the same is likely to become manoeuvrable because of the likely blinding/dazzling of the pilot/ cockpit. This is something to which a stealthy UAV platform will probably be immune to.
Wake signature as a key advantage
Based on its high beam coherence, strong direction ability, high measuring accuracy, laser beams have been used traditionally (right from the 90s) to detect atmospheric turbulence. This has been achieved through the measurement of Doppler Shift in the frequency of laser-emitted energy scattered from the natural aerosols present in the atmosphere, such as dust particles and water vapours, etc. Pulse Doppler LiDAR having wavelengths below 10 μm have typically being used as turbulent detectors. Since the wind speed is relatively small in the atmosphere, the corresponding Doppler Shift is also very minimal. This is made detectable by using heterodyne detection technology, in which, the weak return signal is mixed with a stronger signal close to that of the return signal, thus making the Doppler Shift measurable. The measure of this quantity in an ambient situation and a clear atmosphere is referred to as CAT or Clear Air Turbulence. This becomes a reference signature.
Wave turbulence is the turbulence that forms behind an aircraft as it passes through the air. This turbulence includes various components of the aircraft such as the wingtip, vortices and jet wash. Irrespective of the stealth signature of an aerial threat vehicle, it is impossible for it to conceal its wake signature.
Besides this, two other advantages also accrue. First, since the turbulence detection system is not illuminating the aircraft but its turbulence wake which is tracking well behind the aircraft, the on-board Radar Warning Receivers (RWRs) are unlikely to detect the fact of being tracked. Second, a turbulent-sensor system would also be less likely to be defeated by the ‘Doppler Notch’, a tactic being used by the attacking aircrafts to defeat the early warning/fire control radars operating on the Doppler Shift.
Going further, since every aerial platform will have specific wake signatures with respect to a given CAT, this quantum of measurement can be built up as a threat library to detect a friend from a foe, thus assisting in solving the abiding riddle of the IFF. SMEs refer to this threat library as TWDD (Turbulent Wave Detection Database). Experiments have shown that such capability can detect aerial threat vehicles in terms of range, altitude, heading and air speed. Also, open source unclassified data on experiments have shown the TWDD capability to distinguish platform such as F-15, F-16, F-18 and B-52.
The use of LiDAR in anti-stealth domain offers higher accuracy, weather independence and unparalleled flexibility both in terms of offense and defence. Research and technology development in this area will surely unfold new advances in the future.