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15 May 2006
A femtosecond laser's ability to generate an intense electric field makes it ideal for disrupting missiles that use infrared guidance systems. Emmanuel Marquis describes a ground-based solution that could protect civilian and military aircraft against missile attacks.
Shoulder-fired ground-to-air guided missiles known as MANPADS (man-portable air defence systems) have been widely used in conflicts around the globe since the 1960s. They account for a large number of aircraft downed in recent wars and now pose a major threat to military and civilian transport alike.
Such missiles are usually short-range devices, with engagement distances close to 3000 m, and operate on the principle of infrared detection. The first generation that appeared in the 1960s, such as the Soviet SA-7 MANPAD, used a cooled infrared detector operating in the 1-5 μm range to home-in on the hot jet plume of the aircraft engines.
New generations, such as the US Stinger, combine infrared and ultraviolet imaging detectors making them substantially harder to jam as they can lock on to lower heat generation sources, such as aircraft landing lights.
Civilian aircraft at low altitude making their final approach into airports are virtually defenceless targets against MANPADS and may not survive a direct hit to an engine or fuel tank. Although various techniques have been investigated to enhance the protection of aircraft against this threat, none are currently satisfactory.
For example, the lower exhaust gas temperature of double-flux civilian engines may prevent some earlier generation missiles from locking on as the infrared emissivity is low compared with military engines. When a civilian aircraft is landing and there is no engine thrust, the exhaust gas temperature falls below 400 °C. However, the aircraft will be a target during the take-off phase when the engines are providing maximum thrust. In addition, transport aircraft carrying a full load of passengers and fuel are not allowed to perform evasive manoeuvres.
Another way to improve protection against MANPADs is through flare cartridges, which can be mounted on some VIP or head-of-states transport aircraft, but there is a question mark over the fire risk associated with this approach. The transport aircraft can be on a parking lot in close proximity to a refuelling aircraft and the high temperature of the cartridges creates the risk of starting a fire accidentally.
Onboard laser jammers, similar to the ones used on military platforms, can be mounted on civilian jetliners. These systems have shown reasonable efficiency but have high maintenance and operation costs. At present, no commercial airline company has sufficient operational margin to afford this option.
Ground-based high-power microwave sources are also a feasible solution but this raises huge questions over electromagnetic compatibility. It may kill the missile, but what if it also kills the aircraft electronics? Such microwave systems are suitable if you fly a 1965 Boeing 707 but definitively not on a fly-by-wire recent platform.
Femtosecond solution
All of these reasons have motivated the study of a ground-based solution that takes advantage of the properties of femtosecond lasers. A good solution has to be ground-based for cost reasons, has to damage the missile with minimal side effects and avoid residues from falling from the sky. The ability to jam a missile and make it fly in a pre-determined area is an ideal solution.
Femtosecond lasers emit pulses that reach tremendously high peak powers and moderate energies. Such ultrafast pulsed lasers exhibit a very interesting effect: even at low energy operation, the electric field generated at the focal spot of the beam reaches high levels. For example, 100 mJ, 100 fs pulses generate a peak power of 1 TW and an electric field in excess of hundreds of megavolts in the focal spot.
When propagating in the air, such laser beams create filaments of plasma that can propagate over large distances, which are ideal for disrupting a missile's infrared guidance system. If this sort of beam is directed towards a missile guidance system, it will create several desirable effects. Firstly, the intense electric field will damage the missile's optics through an avalanche ionization effect and will cause the optical material to break down. Secondly, the electric field will ionize the surrounding air and block the missile's field-of-view. Finally, the plasma generated in the air acts as a flare for the missile's infrared sensor.
How to protect an airport
Our airport protection concept relies on truck-mounted lasers that are deployed on the extremities of each runway to safeguard departing and arriving aircraft. Each truck is equipped with a femtosecond laser and a rotary turret to cover the appropriate angular sector.
For example, the laser would be a chirped-pulse amplification system based on a Ti:sapphire lasing media, with a repetition rate of 10 or 100 Hz. Several terawatts of peak power at 800 nm are required for efficient long filament production in the air and the laser has to be equipped with a spectral phase-shaper to provide air propagation dispersion compensation.
Such laser jammers can be spread every 5-10 km on the ground to protect a complete access corridor and as they are truck-mounted, reconfiguration of the protected airspace is quite easy.
The system has two modes of operation. In preventive mode, the laser is permanently firing and protects a pre-determined zone in the airspace. A simple example would be to create a plasma shield below the aircraft's flight path. There is also the option to change the trajectory of the plasma and divert the missile.
In defensive mode, the laser is on standby and is triggered by a missile launch detector. Such an alert would also give the coordinates of the launch point allowing the laser to be fired into the missile's estimated trajectory. The missile launch detector is either a classical rocket plume optical detector or a microwave Doppler radar system that leads to a much better false alarm rate.
Practical alternative
This solution is suitable for protecting civilian airports as well as military installations and has several key advantages. The jamming concept is ground-based, mobile, and requires only a moderate power source. It is a practical alternative to aircraft-mounted laser jammers - a solution that no commercial airline can afford today.
It also benefits from the inherent properties of femtosecond lasers that effectively damage the optics used in the missile and disrupt its guidance system. The ground-based laser jammer can effectively tow the missile away from the airport area. Not only can the system be activated very quickly and jam missiles with a short time-of-flight, it also produces no chemical or mechanical waste.
With a flash of light in the air and the sound of thunder, terrorist-launched MANPADS can be successfully diverted from their targets.
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