Infrared Countermeasures

ON NOVEMBER 28, 2002, JUST MOMENTS AFTER TAKEOFF FROM MOMBASA, KENYA, Israeli vacationers felt their chartered Arkia Airlines Boeing 757 shudder as it flew through the wakes of two Soviet-designed SA-7 missiles that were meant to bring it down. Passengers hardly noticed the bump, but as the cockpit crew watched the white contrails arc away, they instantly grasped the situation.

For five hours the crew said nothing. Only when the flight was minutes from Tel Aviv were the passengers informed that terrorists had fired on the aircraft from an area around Moi International Airport. The relieved passengers broke into celebration, but the press began to wonder how two SAMs could miss such a large, slow target.

One theory is that poorly trained gunmen fired the shoulder-launched missiles from the wrong distance or at the wrong angle. It’s possible that the two heat-seekers may have locked on to a glint of sunlight or a wing’s edge that was too thin to hit, and there’s a good chance that their decades-old batteries were nearly dead. But there may have been other factors at work that affected the two missiles.

A few secretive Arkia, U.S., and Israeli officials know whether the airliner carried infrared countermeasures. (One passenger’s report of a small explosion near the wing suggests that the 757 may have dispensed flares as decoys.) Arkia, the Federal Aviation Administration, other airlines, and aircraft manufacturers don’t encourage discussion of current or planned safeguards against shoulder-fired man-portable air defense systems (manpads) like the SA series and the U.S.-made Stinger.

Congress recently began working on legislation to explore the best anti-missile measures and equip the U.S. jetliner fleet. Recently the Transportation Security Administration began surveying major airports to assess the risk at each. Some airports have expanded their boundary patrolling areas. But what if, despite preventive measures, a missile is launched?

Flares are one effective and simple way to deflect heat-seeking missiles, but military fleets rely far more on infrared jamming systems. Safer over populated areas than burning flares and perhaps a bit more effective, jamming devices deflect manpads by exploiting the way that the missiles track their targets.

To lock on to a target, a manpad gunner must physically point the system at an aircraft as the missile’s seeker passively searches for the most powerful source of infrared radiation in its limited view—usually a heat source such as a jet exhaust nozzle or heat plume. “Since their primary job is to stay locked on,” says Darrell Lamm, chief of Georgia Tech Research Institute’s Threat Analysis and Countermeasures Branch, heat seekers’ fields of view “are usually small to prevent distraction from competing sources.”

“We usually use the simile of looking through a soda straw,” says Jack Pledger, Northrop Grumman’s infrared countermeasures marketing director.

Seeker optics magnify infrared signatures emitted by distant aircraft; keeping track of the target is another matter altogether. The SA-7 and knockoffs of it use simple spin scanners, but the Stinger and more recent designs use conical scanning systems.

In con-scan arrangements, portions of light collected by a Cassegrainian primary mirror are reflected by a secondary mirror through a chopper reticle and onto an infrared detector . The secondary mirror swivels about the missile’s roll axis (manpad missiles, like bullets, spin in flight for stability) and must make a full revolution for the detector to be exposed to the seeker’s entire view of the sky. After infrared energy is focused by the secondary mirror, it passes through the chopper reticle, a disc with a rotor-like paint scheme—opaque blades alternate with transparent slots. The reticle chops the infrared energy into a series of “ons” and “offs” that help determine pointing error—the difference between the missile’s current trajectory and an intercept course.

When a target is directly lined up with the missile’s optics, it traces a perfectly centered circle through the reticle and onto the detector in time with its reflection off the spinning secondary mirror. To the signal processor linked to the detector, the target’s path will translate into ons and offs of equal duration as it is chopped into pulses by the reticle’s fan pattern. The circular path of an off-target signature , on the other hand, will result in waveforms of varying widths ; the more off-center the signature, the more pronounced the variations.

The guidance system tracks the section of the sky where the secondary mirror is sweeping. Programs in the system pair this information with the waveforms and move the fins to steer the missile. As Paul Handwerker, director of BAE Systems business development for countermeasures, explains, “The missile is always, constantly working to put the target back in the center of the reticle.”

Jammers blind the missile to the target aircraft by bathing the seeker head with intense infrared radiation that washes out the aircraft’s own signature. The comparison of the jamming energy to the aircraft’s energy is commonly expressed as the J-to-S ratio. J is the strength of the jamming signal and S is the strength of an aircraft’s signature. “You need to have a larger J than the S to be effective,” says Pledger. “You need to put out more energy than the signature of the target to effectively jam the missile.”

By pulsing energy like a powerful strobe light, jamming devices fool missile guidance systems by projecting extra ons and offs into a missile’s infrared detector , breaking up the target processing. Once a manpad’s lock has been broken and the missile has overshot the target, the seeker’s field of view is too small for the missile to reacquire the target.

Different types of jamming countermeasures trade power and effective range with other factors. Omni-directional lamps operate continuously and create entire hemispheres of protective jamming, though their signal at any given point in the sky is relatively weak. And because they send out a constant torrent of energy, they use a lot of electrical power. Directed systems are more effective and more energy efficient, using focused beams directed at a particular sector of the sky or lasers pointed at the seeker head. Pledger says directed beams can have a J-to-S ratio of between 2:1 and 50:1, while lasers can be 300 to 2,000 times more powerful than their host aircraft’s signature. But the pointing feature of directed systems requires complex missile detection hardware. Multiple threats and highly maneuverable supersonic missiles are a challenge for directed systems.

The cost to equip the world’s airliners makes it unlikely to happen anytime soon, but some comfort can be found in the fact that airliners can keep flying after losing an engine.