Homing guidance

Homing guidance systems use sensors within the missile to obtain guidance information from the target. Possible sensors include radar, infrared sensors, or light sensors. Homing guidance does not usually require communication with a ground station or other launch platform.^[1]

An AGM-65E Maverick laser-guided missile

Proportional navigation

Main article: Proportional navigation

Proportional navigation (also known as "PN" or "Pro-Nav") is a guidance principle (analogous to proportional control) used in some form or another by most homing air target missiles.^[2] It is based on the fact that two objects are on a collision course when the direction of their direct line of sight does not change. PN dictates that the missile velocity vector should rotate at a rate proportional to the rotation rate of the line of sight (line of sight rate or LOS-rate) and in the same direction.

Radar homing

Active homing

Main article: active radar homing

Active homing uses a radar system on the missile to provide a guidance signal. Typically, electronics in the missile keep the radar pointed directly at the target, and the missile then looks at this "angle" of its own centerline to guide itself. Radar resolution is based on the size of the antenna, so in a smaller missile these systems are useful for attacking only large targets, ships or large bombers for instance. Active radar systems remain in widespread use in anti-shipping missiles, and in "fire-and-forget" air-to-air missile systems such as the AIM-120 AMRAAM and R-77.

Semi-active homing

Main article: Semi-active radar homing

Semi-active homing systems combine a passive radar receiver on the missile with a separate targeting radar that "illuminates" the target. Since the missile is typically being launched after the target was detected using a powerful radar system, it makes sense to use that same radar system to track the target, thereby avoiding problems with resolution or power, and reducing the weight of the missile. Semi-active radar homing (SARH) is by far the most common "all weather" guidance solution for anti-aircraft systems, both ground- and air-launched.^[3]

It has the disadvantage for air-launched systems that the launch aircraft must keep moving towards the target in order to maintain radar and guidance lock. This has the potential to bring the aircraft within range of shorter-ranged IR-guided (infrared-guided) missile systems. It is an important consideration now that "all aspect" IR missiles are capable of "kills" from head on, something which did not prevail in the early days of guided missiles. For ships and mobile or fixed ground-based systems, this is irrelevant as the speed (and often size) of the launch platform precludes "running away" from the target or opening the range so as to make the enemy attack fail.

SALH is similar to SARH but uses a laser as a signal. Another difference is that most laser-guided weapons employ turret-mounted laser designators which increase the launching aircraft's ability to maneuver after launch. How much maneuvering can be done by the guiding aircraft depends on the turret field of view and the system's ability to maintain a lock-on while maneuvering. As most air-launched, laser-guided munitions are employed against surface targets the designator providing the guidance to the missile need not be the launching aircraft; designation can be provided by another aircraft or by a completely separate source (frequently troops on the ground equipped with the appropriate laser designator).

Passive homing

See also: passive radar and anti-radiation missile

Infrared homing is a passive system that homes in on the heat generated by the target. Typically used in the anti-aircraft role to track the heat of jet engines, it has also been used in the anti-vehicle role with some success. This means of guidance is sometimes also referred to as "heat seeking".^[3]

Contrast seekers use a video camera, typically black and white, to image a field of view in front of the missile, which is presented to the operator. When launched, the electronics in the missile look for the spot on the image where the contrast changes the fastest, both vertically and horizontally, and then attempts to keep that spot at a constant location in its view. Contrast seekers have been used for air-to-ground missiles, including the AGM-65 Maverick, because most ground targets can be distinguished only by visual means. However they rely on there being strong contrast changes to track, and even traditional camouflage can render them unable to "lock on".

Retransmission homing

Main article: Track-via-missile

Retransmission homing, also called "track-via-missile" or "TVM", is a hybrid between command guidance, semi-active radar homing and active radar homing. The missile picks up radiation broadcast by the tracking radar which bounces off the target and relays it to the tracking station, which relays commands back to the missile.