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Homing guidance is a form of missile guidance that uses sensors within the missile to seek its 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]
Active radar homing uses a radar system on the missile to provide a guidance signal. [3] 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-ship missiles, and in "fire-and-forget" air-to-air missile systems such as the AIM-120 AMRAAM and R-77.
Semi-active radar homing (SARH) systems combine a passive radar receiver on the missile with a separate targeting radar that "illuminates" the target. [3] 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. SARH is by far the most common "all weather" guidance solution for anti-aircraft systems, both ground- and air-launched.
Since the targeting radar is separate from the missile, its angle to the target may mean that little energy is reflected to the missile, which can potentially result in a miss. [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.
Track-via-missile, also called retransmission guidance, uses elements of command guidance and semi-active radar homing. As in SARH, the target is illuminated by the tracking radar, and the missile detects this radiation, while being controlled by commands from the tracking station. [4]
Anti-radiation missiles passively home in on radio emission by a target, such as radar waves. These are usually used against radar systems, but they can be designed for use against any radio source, such as jammers or communication radios. [5]
Infrared homing passively tracks infrared radiation emitted 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".
Laser guidance, also known as semi-active laser homing, homes in on the illumination of a target by a laser designator separate from the missile or bomb. It is generally used against ground targets. The laser designator may be operated from the ground, or aboard an aircraft, such as in a targeting pod. It does not necessarily need to be aboard the aircraft that launched the missile or bomb.
Regardless of how a missile obtains target information, it must steer in order to hit the target. A guidance law is what determines how the missile steers. Guidance laws are designed not to exceed the structural or aerodynamic limits of the missiles they control, and may optimize for considerations such as reducing time of flight. [6]
In proportional navigation, also known as collision homing, the missile steers to reduce the rate of change in line of sight (LOS) to its target. It is based on the fact that two objects are on a collision course when the direction of the LOS between them does not change. Proportional navigation is the most commonly used missile guidance law. It is effective against moving targets and simple to implement, requiring only angle sensors to the target. It has been adapted into a variety of guidance laws to improve its flexibility, for example, against accelerating targets. [4] [6]
In pursuit guidance, the missile flies directly along the LOS to the target at all times. This form of guidance was common in early missiles [6] but is now considered impractical. During the last stage of flight, this form of guidance often requires impossibly high acceleration to hit maneuvering targets. It also often results in a tail-chase with the target, meaning that it must be considerably faster than the target to be effective. The best application of pursuit guidance is against slow-moving targets or on a head-on course. [4]
