Washington ? Technology to thwart shoulder-fired missile attacks already exists and is in use on some military and commercial aircraft.

But questions of cost, effectiveness and safety likely will determine whether Americans will soon be flying more safely in unfriendly skies.

The foiled plot to smuggle a shoulder-fired missile into the United States for use against civilian airliners is likely to boost stalled federal legislation to install countermeasures on commercial aircraft.

Existing antimissile technology falls into four categories:

• An infrared detection system using lasers to jam the missile’s guidance system.
• A “lamp-based” system that uses heat transmitters to confuse heat-seeking missile sensors.
• Phosphorous flares that deploy to divert a heat-seeking missile.
• Thin, metallic “chaff” strips that deploy to confuse radar-guidance systems

Two companies owned by the Israeli government, Israeli Aircraft Industries (IAI) and Rafael, are offering to develop civilian versions of their military systems.

IAI’s Elta Systems division offers “Flight Guard” technology using phosphorous flares. The system is in use on 150 aircraft, including military airplanes and commercial aircraft used by heads of state and corporate executives. IAI spokesman Marvin Klemow declined to identify the commercial customers except to say none was based in the United States.

Flight Guard uses six sensors that provide 360-degree coverage. The sensors detect a missile, confirm a threat and deploy phosphorous flares.

The time of greatest threat from a shoulder-fired missile is described by experts as the 30 minutes after takeoff and the 30 minutes before landing. Most such missiles are effective up to about 15,000 feet.

To effectively control the launch area from which a missile could effectively be targeted against a plane, about 300 square miles surrounding each runway would have to be patrolled, experts said.