The systems that cool the threat from heat-seeking missiles.
- By Sam Goldberg
- Air & Space magazine, July 2003
(Page 2 of 2)
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.