Missile lock-on refers to a scenario where the guidance system for a missile

Missile

Though a missile may be any thrown or launched object, it colloquially almost always refers to a self-propelled guided weapon system.-Etymology:The word missile comes from the Latin verb mittere, meaning "to send"...

 (or its associated launch vehicle) can accurately track a target, and a fire-control system

Fire-control system

A fire-control system is a number of components working together, usually a gun data computer, a director, and radar, which is designed to assist a weapon system in hitting its target. It performs the same task as a human gunner firing a weapon, but attempts to do so faster and more...

 can calculate the required flightpath for the missile to hit the target. Missile lock-on is typically indicated to the pilot or missile operator via an audible tone, a head-up display

Head-Up Display

A head-up display or heads-up display is any transparent display that presents data without requiring users to look away from their usual viewpoints...

 or a helmet-mounted display

Helmet mounted display

A helmet mounted display is a device used in some modern aircraft, especially combat aircraft. HMDs project information similar to that of head-up displays on an aircrew’s visor or reticle, thereby allowing him to obtain situational awareness and/or cue weapons systems to the direction his head...

.

Semi-active radar homing

With a semi-active radar homing

Semi-active radar homing

Semi-active radar homing, or SARH, is a common type of missile guidance system, perhaps the most common type for longer-range air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is only a passive detector of a radar signal – provided by an external ...

 system, the launch platform acquires the target with its search radar

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

. The missile is then powered up while the launch platform's illuminator radar "lights up" the target for it. The illuminator is a radar transmitter with a narrow, focused beam that may be separate from the search radar and that can be directed at a target using information from the search radar. When the passive radar

Passive radar

Passive radar systems encompass a class of radar systems that detect and track objects by processing reflections from non-cooperative sources of illumination in the environment, such as commercial broadcast and communications signals...

 of the missile's guidance system is able to "see" the radar return from the target, missile lock-on is achieved and the weapon is ready to be launched.

Detection by the target

The subject of a lock-on may become aware of the fact that it is being actively targeted by virtue of the electro-magnetic emissions of the tracking system, notably the illuminator. This condition will present a heightened threat to the target, as it indicates that a missile may be about to be fired at it.

See also

• Fire-control radar
  Fire-control radar
  A fire-control radar is a radar which is designed specifically to provide information to a fire-control system in order to calculate a firing solution...
  
  
• Missile guidance
  Missile guidance
  Missile guidance refers to a variety of methods of guiding a missile or a guided bomb to its intended target. The missile's target accuracy is a critical factor for its effectiveness...
  
  
• Air-to-air missile
  Air-to-air missile
  An air-to-air missile is a missile fired from an aircraft for the purpose of destroying another aircraft. AAMs are typically powered by one or more rocket motors, usually solid fuelled but sometimes liquid fuelled...
  
  
• Surface-to-air missile
  Surface-to-air missile
  A surface-to-air missile or ground-to-air missile is a missile designed to be launched from the ground to destroy aircraft or other missiles...
  
  
• Radar jamming and deception
  Radar jamming and deception
  Radar jamming and deception is the intentional emission of radio frequency signals to interfere with the operation of a radar by saturating its receiver with noise or false information...