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A brief glossary of terms used in space-based ballistic missile defense systems

Please refer to a page one story today on ''War in Space, Part 2.'' AIRBORNE OPTICAL ADJUNCT: A set of sensors designed to detect, track, and discriminate an incoming warhead. The sensors are typically optical or infrared devices flown in aircraft stationed above clouds.

BOOST PHASE: The portion of a missile's flight during which the payload is accelerated by large rocket motors.

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DIRECTED ENERGY: Energy in the form of particle or laser beams that can be sent long distances at nearly the speed of light.

ELECTROMAGNETIC GUN: A gun based on the idea of an open solenoid. The projectile is accelerated by electromagnetic forces rather than by an explosion.

HYPERVELOCITY GUN: A gun that can accelerate projectiles to 5 kilometers per second (1,118 miles per hour) or more; for example, an electromagnetic (or rail) gun.

INFRARED SENSOR: A sensor to detect the infrared radiation from objects such as a missile reentry vehicle.

KINETIC ENERGY: The energy from the momentum of an object.

LASER: A device that amplifies light, and emits it in a narrow, highly concentrated beam.

LEAKAGE: The percentage of warheads that get through a defensive system intact and operational.

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MIDCOURSE PHASE: The period of a warhead's flight after it has been dispensed from the post-boost vehicle (''bus'') until it reenters the atmosphere.

PARTICLE BEAM: A type of directed energy device that emits a stream of atoms or subatomic particles (electrons, protons, or neutrons) accelerated to nearly the speed of light.

PENETRATION AIDS: Methods to defeat defenses by camouflage, deception, decoys , and countermeasures.

SIGNAL PROCESSING: A computer system's capability to organize the raw data received from many different sources.

TERMINAL PHASE: The final phase of a ballistic missile trajectory, during which warheads and penetration aids reenter the atmosphere.

THREAT CLOUDS: Dense concentrations of both threatening and nonthreatening objects. The defense must distinguish between them.