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Homing guidance

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Homing guidance is the most common form of guidance used in anti-air missiles today. Three primary forms of guidance fall under the homing guidance umbrella--semi active, active, and passive. We will discuss each of these in turn, as well as a more unusual form called retransmission or track-via-missile homing.


Homing guidance








Homing guidance

Semi-Active Homing Guidance

A semi-active system is similar to command guidance since the missile relies on an external source to illuminate the target. The energy reflected by this target is intercepted by a receiver on the missile. The difference between command guidance and semi-active homing is that the missile has an onboard computer in this case. The computer uses the energy collected by its radar receiver to determine the target's relative trajectory and send correcting commands to control surfaces so that the missile will intercept the target.


Semi-active homing guidance












Semi-active homing guidance

The example shown above illustrates the guidance method used on an air-to-air missile like Sparrow. This missile relies on radar energy transmitted by the launch aircraft to track and home in on the target. This system is also sometimes referred to as bistatic meaning that the radar waves that intercept the target and those reflected back to the missile are at different angles to one another.

However, it should be noted that semi-active guidance is used by other types of seekers besides radar. Laser-guided weapons like the Paveway series can also be considered semi-active weapons because the laser energy these bombs track as they steer towards a target is supplied by an external source. The source could be a laser designation pod on the launch aircraft, on a second aircraft, or aimed by a soldier on the ground.

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 ground-to-air missile systems. The name refers to the missile itself being a passive detector, while an off board radar provides a signal for the missile guidance system to "listen to" when it reflects off the target.

The basic concept of SARH is that almost all detection and tracking systems consist of a radar system In addition, the resolution of a radar is strongly related to the physical size of the antenna, in the small nose cone of a missile there isn't enough room to provide the sort of accuracy needed for guidance. Instead the larger radar dish on the ground or launch aircraft will provide the needed signal and tracking logic, and the missile simply has to listen to the signal and point itself in the right direction.

Contrast this with beam riding systems, in which the radar is pointed at the target and the missile keeps itself centered in the beam by listening to the signal at the rear of the missile body. In the SARH system the missile listens for the reflected signal at the nose, and is still responsible for providing some sort of "lead" guidance. The advantages are twofold. One is that a radar signal is "fan shaped" growing larger, and therefore less accurate, with distance. This means that the beam riding system is not terribly accurate at long ranges, while SARH is largely independent of range and grows more accurate as it approaches the target -- the "source" of the signal it listens for. Another addition is that a beam riding system must accurately track the target at high speeds, typically requiring one radar for tracking and another "tighter" beam for guidance. The SARH system needs only one radar set to a wider pattern.

SARH systems use continuous-wave radar for guidance. Even though most modern fighter radars are pulse Doppler sets, most have a CW function to guide radar missiles. A few Soviet aircraft, such as some versions of the MiG-23 and MiG-27, used an auxiliary guidance pod or aerial to provide a CW signal.

SARH missiles require the tracking radar to lock on to the target and then illuminate it for the entire duration of the missile's flight.


Active Homing Guidance

Active homing works just like semi-active except that the tracking energy is now both transmitted by and received by the missile itself. No external source is needed. It is for this reason that active homing missiles are often called "fire-and-forget" because the launch aircraft does not need to continue illuminating the target after the missile is launched.


Active homing guidance
Active homing guidance

Active homing missiles typically use radar seekers to track their target. These seekers are also sometimes called monostatic because, unlike semi-active guidance, the transmitted and reflected waves are at the same angle with respect to the line of sight between the missile and target. Examples of active homing missiles include the AMRAAM air-to-air and Exocet anti-ship missiles.




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Last modified: 10/12/05.